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Marine life,sea life orocean life is the collectiveecological communities that encompass allaquatic animals,plants,algae,fungi,protists,single-celledmicroorganisms and associatedviruses living in thesaline water ofmarine habitats, either thesea water ofmarginal seas andoceans, or thebrackish water ofcoastalwetlands,lagoons,estuaries andinland seas. As of 2023[update], more than 242,000 marinespecies have been documented, and perhaps two million marine species are yet to be documented. An average of 2,332 new species per year are being described.[2][3] Marine life is studied scientifically in bothmarine biology and inbiological oceanography.
By volume, oceans provide about 90% of the living space onEarth,[4] and served as thecradle of life and vital biotic sanctuaries throughoutEarth's geological history. Theearliest known life forms evolved asanaerobicprokaryotes (archaea andbacteria) in theArchean oceans around thedeep seahydrothermal vents, beforephotoautotrophs appeared and allowed themicrobial mats to expand intoshallow water marine environments. TheGreat Oxygenation Event of the earlyProterozoic significantly altered themarine chemistry, which likely caused a widespread anaerobeextinction event but also led to theevolution ofeukaryotes throughsymbiogenesis between surviving anaerobes andaerobes.Complex life eventually arose out of marine eukaryotes during theNeoproterozoic, and which culminated in a largeevolutionary radiation event ofmostly sessile macrofaunae known as theAvalon Explosion. This was followed in the earlyPhanerozoic by a more prominent radiation event known as theCambrian Explosion, where actively movingeumetazoan became prevalent. These marine life also expanded intofresh waters, where fungi andgreen algae that were washed ashore ontoriparian areas started to take hold later during theOrdivician beforerapidly expanding inland during theSilurian andDevonian, paving the way forterrestrial ecosystems to develop.
Today, marine species range in size from the microscopicphytoplankton, which can be as small as 0.02–micrometers; to hugecetaceans like theblue whale, which can reach 33 m (108 ft) in length.[5][6] Marine microorganisms have been variously estimated as constituting about 70%[7] or about 90%[8][1] of the total marinebiomass.Marine primary producers, mainlycyanobacteria andchloroplasticalgae,produce oxygen andsequester carbon viaphotosynthesis, which generate enormous biomass and significantly influence theatmospheric chemistry.Migratory species, such asoceanodromous andanadromousfish, also create biomass andbiological energy transfer between different regions of Earth, with many serving askeystone species of various ecosystems. At a fundamental level, marine life affects the nature of the planet, and in part, shape and protect shorelines, and some marine organisms (e.g.corals) even help create newland via accumulatedreef-building.
Marine life can be roughly grouped intoautotrophs andheterotrophs according to their roles within thefood web: the former include photosynthetic and the much rarerchemosyntheticorganisms (chemoautotrophs) that can convertinorganic molecules intoorganic compounds using energy fromsunlight orexothermicoxidation, such as cyanobacteria,iron-oxidizing bacteria, algae (seaweeds and variousmicroalgae) andseagrass; the latter include all the rest that mustfeed on other organisms to acquirenutrients and energy, which include animals, fungi, protists and non-photosynthetic microorganisms. Marine animals are further informally divided intomarine vertebrates and marineinvertebrates, both of which arepolyphyletic groupings with the former including allsaltwater fish,marine mammals,marine reptiles andseabirds, and the latter include all that are not consideredvertebrates. Generally, marine vertebrates are much morenektonic andmetabolically demanding ofoxygen and nutrients, often suffering distress or evenmass deaths (a.k.a. "fish kills") duringanoxic events, while marine invertebrates are a lot morehypoxia-tolerant and exhibit a wide range of morphological and physiological modifications to survive inpoorly oxygenated waters.
There is no life without water.[9] It has been described as theuniversal solvent for its ability todissolve many substances,[10][11] and as thesolvent of life.[12] Water is the only common substance to exist as asolid, liquid, andgas under conditions normal to life on Earth.[13] TheNobel Prize winnerAlbert Szent-Györgyi referred to water as themater und matrix: the mother and womb of life.[14]
The abundance of surface water on Earth is a unique feature in theSolar System. Earth'shydrosphere consists chiefly of the oceans but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 metres (6,600 ft). The deepest underwater location isChallenger Deep of theMariana Trench in thePacific Ocean, having a depth of 10,900 metres (6.8 mi).[note 1][15]
Conventionally, the planet is divided into five separate oceans, but these oceans all connect into a singleworld ocean.[16] The mass of this world ocean is 1.35×1018 metric tons or about 1/4400 of Earth's total mass. The world ocean covers an area of3.618×108 km2 with a mean depth of3682 m, resulting in an estimated volume of1.332×109 km3.[17] If all of Earth's crustal surface was at the same elevation as a smooth sphere, the depth of the resulting world ocean would be about 2.7 kilometres (1.7 mi).[18][19]
About 97.5% of the water on Earth issaline; the remaining 2.5% isfresh water. Most fresh water – about 69% – is present as ice inice caps andglaciers.[20] The average salinity of Earth's oceans is about 35 grams (1.2 oz) of salt per kilogram of seawater (3.5% salt).[21] Most of the salt in the ocean comes from the weathering and erosion of rocks on land.[22] Some salts are released fromvolcanic activity or extracted from cooligneous rocks.[23]
The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[24] Sea water has an important influence on the world's climate, with the oceans acting as a largeheat reservoir.[25] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as theEl Niño-Southern Oscillation.[26]
Altogether the ocean occupies 71 percent of the world surface,[4] averaging nearly 3.7 kilometres (2.3 mi) in depth.[27] By volume, the ocean provides about 90 percent of the living space on the planet.[4] The science fiction writerArthur C. Clarke has pointed out it would be more appropriate to refer to planet Earth as planet Ocean.[28][29]
However, water is found elsewhere in the Solar System.Europa, one of the moons orbitingJupiter, is slightly smaller than theEarth's Moon. There is a strong possibility a large saltwater ocean exists beneath its ice surface.[30] It has been estimated the outer crust of solid ice is about 10–30 km (6–19 mi) thick and the liquid ocean underneath is about 100 km (60 mi) deep.[31] This would make Europa's ocean over twice the volume of the Earth's ocean. There has been speculation Europa's oceancould support life,[32][33] and could be capable of supporting multicellularmicroorganisms ifhydrothermal vents are active on the ocean floor.[34]Enceladus, a small icy moon of Saturn, also has what appears to bean underground ocean which actively vents warm water from the moon's surface.[35]
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TheEarth is about 4.54 billion years old.[36][37][38] The earliest undisputed evidence oflife on Earth dates from at least 3.5 billion years ago,[39][40] during theEoarchean era after a geologicalcrust started to solidify following the earlier moltenHadean Eon.Microbial matfossils have been found in 3.48 billion-year-oldsandstone inWestern Australia.[41][42] Other early physical evidence of abiogenic substance isgraphite in 3.7 billion-year-oldmetasedimentary rocks discovered inWestern Greenland[43] as well as "remains ofbiotic life" found in 4.1 billion-year-old rocks in Western Australia.[44][45] According to one of the researchers, "If life arose relatively quickly on Earth … then it could be common in theuniverse."[44]
All organisms on Earth are descended from acommon ancestor or ancestralgene pool.[46][47] Highly energetic chemistry is thought to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later thelast common ancestor of all life existed.[48] The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions.[49] The beginning of life may have included self-replicating molecules such asRNA[50] and the assembly of simple cells.[51] In 2016 scientists reported a set of 355genes from thelast universal common ancestor (LUCA) of alllife, including microorganisms, living onEarth.[52]
Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.[53] The common descent of organisms was first deduced from four simple facts about organisms: First, they have geographic distributions that cannot be explained by local adaptation. Second, the diversity of life is not a set of unique organisms, but organisms that sharemorphological similarities. Third, vestigial traits with no clear purpose resemble functional ancestral traits and finally, that organisms can be classified using these similarities into a hierarchy of nested groups—similar to a family tree.[54] However, modern research has suggested that, due tohorizontal gene transfer, this "tree of life" may be more complicated than a simple branching tree since some genes have spread independently between distantly related species.[55][56]
Past species have also left records of their evolutionary history. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record.[57] By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry.
More recently, evidence for common descent has come from the study of biochemical similarities between organisms. For example, all living cells use the same basic set ofnucleotides andamino acids.[59] The development ofmolecular genetics has revealed the record of evolution left in organisms' genomes: dating when species diverged through themolecular clock produced by mutations.[60] For example, these DNA sequence comparisons have revealed that humans and chimpanzees share 98% of their genomes and analyzing the few areas where they differ helps shed light on when the common ancestor of these species existed.[61]
Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.[62][63] No obvious changes inmorphology or cellular organization occurred in these organisms over the next few billion years.[64] The eukaryotic cells emerged between 1.6 and 2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association calledendosymbiosis.[65][66] The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria orhydrogenosomes.[67] Another engulfment ofcyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.[68]
The history of life was that of theunicellular eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in theEdiacaran period.[62][69] Theevolution of multicellularity occurred in multiple independent events, in organisms as diverse assponges,brown algae,cyanobacteria,slime moulds andmyxobacteria.[70] In 2016 scientists reported that, about 800 million years ago, a minor genetic change in a single molecule calledGK-PID may have allowed organisms to go from a single cell organism to one of many cells.[71]
Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over a span of about 10 million years, in an event called theCambrian explosion. Here, the majority oftypes of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.[72] Various triggers for the Cambrian explosion have been proposed, including the accumulation ofoxygen in theatmosphere from photosynthesis.[73]
About 500 million years ago, plants and fungi started colonizing the land. Evidence for the appearance of the firstland plants occurs in theOrdovician, around450 million years ago, in the form of fossil spores.[74] Land plants began to diversify in theLate Silurian, from around430 million years ago.[75] The colonization of the land by plants was soon followed byarthropods and other animals.[76]Insects were particularly successful and even today make up the majority of animal species.[77]Amphibians first appeared around 364 million years ago, followed by earlyamniotes andbirds around 155 million years ago (both from "reptile"-like lineages),mammals around 129 million years ago,homininae around 10 million years ago andmodern humans around 250,000 years ago.[78][79][80] However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes.[81]
Estimates on the number of Earth's currentspecies range from 10 million to 14 million,[82] of which about 1.2 million have been documented and over 86 percent have not yet been described.[83]
Microorganisms make up about 70% of themarine biomass.[7] Amicroorganism, or microbe, is amicroscopicorganism too small to be recognized with the naked eye. It can besingle-celled[84] ormulticellular. Microorganisms are diverse and include allbacteria andarchaea, mostprotozoa such asalgae,fungi, and certain microscopic animals such asrotifers.
Manymacroscopic animals andplants have microscopicjuvenile stages. Some microbiologists also classifyviruses (andviroids) as microorganisms, but others consider these as nonliving.[85][86]
Microorganisms are crucial to nutrient recycling inecosystems as they act asdecomposers. Some microorganisms arepathogenic, causing disease and even death in plants and animals.[87] As inhabitants of the largest environment on Earth, microbial marine systems drive changes in every global system. Microbes are responsible for virtually all thephotosynthesis that occurs in the ocean, as well as the cycling ofcarbon,nitrogen,phosphorus, othernutrients and trace elements.[88]
Microscopic life undersea is diverse and still poorly understood, such as for the role ofviruses in marine ecosystems.[89] Most marine viruses arebacteriophages, which are harmless to plants and animals, but are essential to the regulation of saltwater and freshwater ecosystems.[90]: 5 They infect and destroy bacteria in aquatic microbial communities, and are the most important mechanism ofrecycling carbon in the marine environment. The organic molecules released from the dead bacterial cells stimulate fresh bacterial and algal growth.[90]: 593 Viral activity may also contribute to thebiological pump, the process wherebycarbon issequestered in the deep ocean.[91]
A stream of airborne microorganisms circles the planet above weather systems but below commercial air lanes.[92] Some peripatetic microorganisms are swept up from terrestrial dust storms, but most originate from marine microorganisms insea spray. In 2018, scientists reported that hundreds of millions of viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.[93][94]
Microscopic organisms live throughout thebiosphere. The mass ofprokaryote microorganisms — which includes bacteria and archaea, but not the nucleatedeukaryote microorganisms — may be as much as 0.8 trillion tons of carbon (of the total biospheremass, estimated at between 1 and 4 trillion tons).[95] Single-celledbarophilic marine microbes have been found at a depth of 10,900 m (35,800 ft) in theMariana Trench, the deepest spot in the Earth's oceans.[96][97] Microorganisms live inside rocks 580 m (1,900 ft) below the sea floor under 2,590 m (8,500 ft) of ocean off the coast of the northwestern United States,[96][98] as well as 2,400 m (7,900 ft; 1.5 mi) beneath the seabed off Japan.[99] The greatest known temperature at which microbial life can exist is 122 °C (252 °F) (Methanopyrus kandleri).[100] In 2014, scientists confirmed the existence of microorganisms living 800 m (2,600 ft) below the ice ofAntarctica.[101][102] According to one researcher, "You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."[96]
Viruses are smallinfectious agents that do not have their ownmetabolism and canreplicate only inside the livingcells of otherorganisms.[103] Viruses can infect all types oflife forms, fromanimals andplants tomicroorganisms, includingbacteria andarchaea.[104] The linear size of the average virus is about one one-hundredth that of the averagebacterium. Most viruses cannot be seen with anoptical microscope soelectron microscopes are used instead.[105]
Viruses are found wherever there is life and have probably existed since living cells first evolved.[106] The origin of viruses is unclear because they do not form fossils, somolecular techniques have been used to compare the DNA or RNA of viruses and are a useful means of investigating how they arise.[107]
Viruses are now recognized as ancient and as having origins that pre-date the divergence of life into thethree domains.[108] But the origins of viruses in theevolutionary history of life are unclear: some may haveevolved fromplasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means ofhorizontal gene transfer, which increasesgenetic diversity.[109]
Opinions differ on whether viruses are a form oflife or organic structures that interact with living organisms.[110] They are considered by some to be a life form, because they carry genetic material, reproduce by creating multiple copies of themselves through self-assembly, and evolve throughnatural selection. However they lack key characteristics such as a cellular structure generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as replicators[110] and as "organisms at the edge of life".[111]
Bacteriophages, often just calledphages, are viruses thatparasite bacteria and archaea.Marine phages parasite marine bacteria and archaea, such ascyanobacteria.[112] They are a common and diverse group of viruses and are the most abundant biological entity in marine environments, because their hosts, bacteria, are typically the numerically dominant cellular life in the sea. Generally there are about 1 million to 10 million viruses in each mL of seawater, or about ten times more double-stranded DNA viruses than there are cellular organisms,[113][114] although estimates of viral abundance in seawater can vary over a wide range.[115][116]Tailed bacteriophages appear to dominate marine ecosystems in number and diversity of organisms.[112] Bacteriophages belonging to the familiesCorticoviridae,[117]Inoviridae[118] andMicroviridae[119] are also known to infect diverse marine bacteria.
Microorganisms make up about 70% of the marine biomass.[7] It is estimated viruses kill 20% of this biomass each day and that there are 15 times as many viruses in the oceans as there are bacteria and archaea. Viruses are the main agents responsible for the rapid destruction of harmfulalgal blooms,[114] which often kill other marine life.[120]The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.[91]
There are alsoarchaeal viruses which replicate withinarchaea: these are double-stranded DNA viruses with unusual and sometimes unique shapes.[121][122] These viruses have been studied in most detail in thethermophilic archaea, particularly the ordersSulfolobales andThermoproteales.[123]
Viruses are an important natural means oftransferring genes between different species, which increasesgenetic diversity and drives evolution.[109] It is thought that viruses played a central role in the early evolution, before the diversification of bacteria, archaea and eukaryotes, at the time of thelast universal common ancestor of life on Earth.[124] Viruses are still one of the largest reservoirs of unexplored genetic diversity on Earth.[91]
Bacteria constitute a largedomain ofprokaryoticmicroorganisms. Typically a fewmicrometers in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear onEarth, and are present in most of itshabitats. Bacteria inhabit soil, water,acidic hot springs,radioactive waste,[125] and the deep portions ofEarth's crust. Bacteria also live insymbiotic andparasitic relationships with plants and animals.
Once regarded asplants constituting the classSchizomycetes, bacteria are now classified asprokaryotes. Unlike cells of animals and othereukaryotes, bacterial cells do not contain anucleus and rarely harbormembrane-boundorganelles. Although the termbacteria traditionally included all prokaryotes, thescientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms thatevolved from an ancient common ancestor. Theseevolutionary domains are calledBacteria andArchaea.[126]
The ancestors of modern bacteria were unicellular microorganisms that were thefirst forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life.[64][127] Although bacterialfossils exist, such asstromatolites, their lack of distinctivemorphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species. However, gene sequences can be used to reconstruct the bacterialphylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage.[128] Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria intoendosymbiotic associations with the ancestors of eukaryotic cells, which were themselves possibly related to theArchaea.[66][65] This involved the engulfment by proto-eukaryotic cells ofalphaproteobacterial symbionts to form eithermitochondria orhydrogenosomes, which are still found in all known Eukarya. Later on, some eukaryotes that already contained mitochondria also engulfed cyanobacterial-like organisms. This led to the formation ofchloroplasts in algae and plants. There are also some algae that originated from even later endosymbiotic events. Here, eukaryotes engulfed a eukaryotic algae that developed into a "second-generation" plastid.[129][130] This is known assecondary endosymbiosis.
![The chloroplasts of glaucophytes have a peptidoglycan layer, evidence suggesting their endosymbiotic origin from cyanobacteria.[131]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aGlaucocystis_sp.jpg)
The largest known bacterium, the marineThiomargarita namibiensis, can be visible to the naked eye and sometimes attains 0.75 mm (750 μm).[132][133]
Thearchaea (Greek forancient[134]) constitute adomain andkingdom ofsingle-celledmicroorganisms. These microbes areprokaryotes, meaning they have nocell nucleus or any other membrane-boundorganelles in their cells.
Archaea were initially classified asbacteria, but this classification is outdated.[135] Archaeal cells have unique properties separating them from the other two domains of life,Bacteria andEukaryota. The Archaea are further divided into multiple recognizedphyla. Classification is difficult because the majority have not been isolated in the laboratory and have only been detected by analysis of theirnucleic acids in samples from their environment.
Archaea and bacteria are generally similar in size and shape, although a few archaea have very strange shapes, such as the flat and square-shaped cells ofHaloquadratum walsbyi.[136] Despite this morphological similarity to bacteria, archaea possessgenes and severalmetabolic pathways that are more closely related to those of eukaryotes, notably theenzymes involved intranscription andtranslation. Other aspects of archaeal biochemistry are unique, such as their reliance onether lipids in theircell membranes, such asarchaeols. Archaea use more energy sources than eukaryotes: these range fromorganic compounds, such as sugars, toammonia,metal ions or evenhydrogen gas. Salt-tolerant archaea (theHaloarchaea) use sunlight as an energy source, and other species of archaeafix carbon; however, unlike plants andcyanobacteria, no known species of archaea does both. Archaeareproduce asexually bybinary fission,fragmentation, orbudding; unlike bacteria and eukaryotes, no known species formsspores.
Archaea are particularly numerous in the oceans, and the archaea inplankton may be one of the most abundant groups of organisms on the planet. Archaea are a major part of Earth's life and may play roles in both thecarbon cycle and thenitrogen cycle.
Protists are eukaryotes that cannot be classified as plants, fungi or animals. They are usually single-celled and microscopic. Life originated assingle-celled prokaryotes (bacteria andarchaea) and later evolved intomore complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. The termprotist came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics, because they areparaphyletic (lacking a common ancestor). Protists can be broadly divided into four groups depending on whether their nutrition is plant-like, animal-like, fungus-like,[137] or a mixture of these.[138]
Protists according to how they get food | |||||||
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| Type of protist | Description | Example | Other examples | ||||
| Plant-like | Autotrophic protists that make their own food without needing to consume other organisms, usually by using photosynthesis |  | Red algae,Cyanidium sp. | Green algae,brown algae,diatoms and somedinoflagellates. Plant-like protists are important components of phytoplanktondiscussed below. | |||
| Animal-like | Heterotrophic protists that get their food consuming other organisms |  | Radiolarian protist as drawn byHaeckel | Foraminiferans, and some marineamoebae,ciliates andflagellates. | |||
| Fungus-like | Saprotrophic protists that get their food from the remains of organisms that have broken down and decayed |  | Marineslime nets form labyrinthine networks of tubes in which amoeba without pseudopods can travel | Marine lichen | |||
| Mixotropes | Various | Mixotrophic andosmotrophic protists that get their food from a combination of the above | _(cropped).jpg) | Euglena mutabilis, a photosyntheticflagellate | Many marine mixotropes are found among protists, including among ciliates,Rhizaria and dinoflagellates[139] | ||
Getting to know our single-celled ancestors -MicroCosmosProtists are highly diverse organisms currently organized into 18 phyla, but are not easy to classify.[141][142] Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting a large number of eukaryotic microbial communities have yet to be discovered.[143][144] There has been little research onmixotrophic protists, but recent studies in marine environments found mixotrophic protests contribute a significant part of the protistbiomass.[139]
![Diatoms are a major algae group generating about 20% of world oxygen production.[145]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aDiatoms_through_the_microscope.jpg)
![Diatoms have glass like cell walls made of silica and called frustules.[146]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aDiatom_algae_Amphora_sp.jpg)
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In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organized. Plants, animals and fungi are usuallymulti-celled and are typicallymacroscopic. Most protists are single-celled and microscopic. But there are exceptions. Some single-celled marine protists are macroscopic. Some marine slime molds have unique life cycles that involve switching between unicellular,colonial, and multicellular forms.[147] Other marine protist are neither single-celled nor microscopic, such asseaweed.
![The xenophyophore, another single-celled foraminiferan, lives in abyssal zones. It has a giant shell up to 20 cm (7.9 in) across.[148]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aXenophyophore.jpg)
Protists have been described as a taxonomic grab bag where anything that doesn't fit into one of the mainbiological kingdoms can be placed.[149] Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms.[150][151] This more constrained definition excludesseaweeds andslime molds.[152]
| External videos | |
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 Copepods: The Diatom-Devouring King of Plankton -Journey to the Microcosmos |
As juveniles, animals develop from microscopic stages, which can includespores,eggs andlarvae. At least one microscopic animal group, theparasiticcnidarianMyxozoa, is unicellular in its adult form, and includes marine species. Other adult marinemicroanimals are multicellular. Microscopic adultarthropods are more commonly found inland in freshwater, but there are marine species as well. Microscopic adult marinecrustaceans include somecopepods,cladocera andtardigrades (water bears). Some marinenematodes androtifers are also too small to be recognized with the naked eye, as are manyloricifera, including the recently discoveredanaerobic species that spend their lives in ananoxic environment.[153][154] Copepods contribute more to thesecondary productivity andcarbon sink of the world oceans than any other group of organisms.[155][156] Whilemites are not normally thought of as marine organisms, most species of the familyHalacaridae live in the sea.[157]
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Over 1500 species offungi are known from marine environments.[158] These are parasitic onmarine algae or animals, or aresaprobes feeding on dead organic matter from algae, corals, protozoan cysts, sea grasses, wood and other substrata.[159] Spores of many species have special appendages which facilitate attachment to the substratum.[160] Marine fungi can also be found insea foam and aroundhydrothermal areas of the ocean.[161] A diverse range of unusual secondarymetabolites is produced by marine fungi.[162]
Mycoplankton aresaprotropic members of theplankton communities ofmarine andfreshwaterecosystems.[163][164] They are composed offilamentous free-livingfungi and yeasts associated with planktonic particles orphytoplankton.[165] Similar tobacterioplankton, these aquatic fungi play a significant role inheterotrophicmineralization andnutrient cycling.[166] Mycoplankton can be up to 20 mm in diameter and over 50 mm in length.[167]
A typical milliliter of seawater contains about 103 to 104 fungal cells.[168] This number is greater in coastal ecosystems andestuaries due to nutritional runoff from terrestrial communities. A higher diversity of mycoplankton is found around coasts and in surface waters down to 1000 meters, with avertical profile that depends on how abundantphytoplankton is.[169][170] This profile changes between seasons due to changes in nutrient availability.[171] Marine fungi survive in a constant oxygen deficient environment, and therefore depend on oxygen diffusion byturbulence and oxygen generated byphotosynthetic organisms.[172]
Marine fungi can be classified as:[172]
Lichens aremutualistic associations between a fungus, usually anascomycete, and an alga or acyanobacterium. Several lichens are found in marine environments.[173] Many more occur in thesplash zone, where they occupy different vertical zones depending on how tolerant they are to submersion.[174] Some lichens live a long time; one species has been dated at 8,600 years.[175] However their lifespan is difficult to measure because what defines the same lichen is not precise.[176] Lichens grow by vegetatively breaking off a piece, which may or may not be defined as the same lichen, and two lichens of different ages can merge, raising the issue of whether it is the same lichen.[176]Thesea snailLittoraria irrorata damages plants ofSpartina in the sea marshes where it lives, which enables spores of intertidal ascomycetous fungi to colonize the plant. The snail then eats the fungal growth in preference to the grass itself.[177]
According to fossil records, fungi date back to the lateProterozoic era 900–570 million years ago. Fossil marine lichens 600 million years old have been discovered in China.[178] It has been hypothesized that mycoplankton evolved from terrestrial fungi, likely in thePaleozoic era (390 million years ago).[179]
The earliestanimals were marineinvertebrates, that is,vertebrates came later. Animals aremulticellulareukaryotes,[note 2] and are distinguished from plants, algae, and fungi by lackingcell walls.[180]Marine invertebrates are animals that inhabit amarine environment apart from the vertebrate members of thechordate phylum; invertebrates lack avertebral column. Some have evolved ashell or a hardexoskeleton.
The earliest animal fossils may belong to the genusDickinsonia,[181] 571 million to 541 million years ago.[182] IndividualDickinsonia typically resemble a bilaterally symmetrical ribbed oval. They kept growing until they were covered with sediment or otherwise killed,[183] and spent most of their lives with their bodies firmly anchored to the sediment.[184] Theirtaxonomic affinities are presently unknown, but their mode of growth is consistent with abilaterian affinity.[185]
Apart fromDickinsonia, the earliest widely accepted animal fossils are the rather modern-lookingcnidarians (the group that includescoral,jellyfish,sea anemones andHydra), possibly from around580 Ma[186] TheEdiacara biota, which flourished for the last 40 million years before the start of theCambrian,[187] were the first animals more than a very few centimeters long. LikeDickinsonia, many were flat with a "quilted" appearance, and seemed so strange that there was a proposal to classify them as a separatekingdom,Vendozoa.[188] Others, however, have been interpreted as earlymolluscs (Kimberella[189][190]),echinoderms (Arkarua[191]), andarthropods (Spriggina,[192]Parvancorina[193]). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans. However, there seems little doubt thatKimberella was at least atriploblastic bilaterian animal, in other words, an animal significantly more complex than the cnidarians.[194]
Small shelly fauna are a very mixed collection of fossils found between the Late Ediacaran andMiddle Cambrian periods. The earliest,Cloudina, shows signs of successful defense against predation and may indicate the start of anevolutionary arms race. Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates,"Halkieria andMicrodictyon, were eventually identified when more complete specimens were found in Cambrianlagerstätten that preserved soft-bodied animals.[195]
Invertebrates are grouped into differentphyla. Informally phyla can be thought of as a way of grouping organisms according to theirbody plan.[196][197]: 33 A body plan refers to a blueprint which describes the shape ormorphology of an organism, such as itssymmetry,segmentation and the disposition of itsappendages. The idea of body plans originated withvertebrates, which were grouped into one phylum. But the vertebrate body plan is only one of many, and invertebrates consist of many phyla or body plans. The history of the discovery of body plans can be seen as a movement from a worldview centered on vertebrates, to seeing the vertebrates as one body plan among many. Among the pioneeringzoologists, Linnaeus identified two body plans outside the vertebrates; Cuvier identified three; and Haeckel had four, as well as the Protista with eight more, for a total of twelve. For comparison, the number of phyla recognized by modern zoologists hasrisen to 35.[197]
Historically body plans were thought of as having evolved rapidly during theCambrian explosion,[200] but a more nuanced understanding of animal evolution suggests a gradual development of body plans throughout the earlyPalaeozoic and beyond.[201] More generally a phylum can be defined in two ways: as described above, as a group of organisms with a certain degree of morphological or developmental similarity (thephenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (thephylogenetic definition).[201]
In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage ofPrecambrian animal fossils.[195] A re-analysis of fossils from theBurgess Shale lagerstätte increased interest in the issue when it revealed animals, such asOpabinia, which did not fit into any knownphylum. At the time these were interpreted as evidence that the modern phyla had evolved very rapidly in the Cambrian explosion and that the Burgess Shale's "weird wonders" showed that the Early Cambrian was a uniquely experimental period of animal evolution.[202] Later discoveries of similar animals and the development of new theoretical approaches led to the conclusion that many of the "weird wonders" were evolutionary "aunts" or "cousins" of modern groups[203]—for example thatOpabinia was a member of thelobopods, a group which includes the ancestors of the arthropods, and that it may have been closely related to the moderntardigrades.[204] Nevertheless, there is still much debate about whether the Cambrian explosion was really explosive and, if so, how and why it happened and why it appears unique in the history of animals.[205]
Thedeepest-branching animals — the earliest animals that appeared during evolution — are marine non-vertebrate organisms. The earliest animal phyla are thePorifera,Ctenophora,Placozoa andCnidaria. No member of theseclades exhibit body plans withbilateral symmetry.
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Sponges are animals of thephylumPorifera (from Modern Latin forbearing pores[209]). They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-likemesohyl sandwiched between two thin layers ofcells. They have non-specialized cells that cantransform into other types and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not havenervous,digestive orcirculatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes.
Sponges are similar to other animals in that they aremulticellular,heterotrophic, lackcell walls and producesperm cells. Unlike other animals, they lack truetissues andorgans, and have nobody symmetry. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where it deposits nutrients, and leaves through a hole called theosculum. Many sponges have internal skeletons ofspongin and/or spicules ofcalcium carbonate orsilicon dioxide. All sponges aresessile aquatic animals. Although there are freshwater species, the great majority are marine (salt water) species, ranging from tidal zones to depths exceeding 8,800 m (5.5 mi). Some sponges live to great ages; there is evidence of the deep-sea glass spongeMonorhaphis chuni living about 11,000 years.[210][211]
While most of the approximately 5,000–10,000 known species feed onbacteria and other food particles in the water, some hostphotosynthesizing micro-organisms asendosymbionts and these alliances often produce more food and oxygen than they consume. A few species of sponge that live in food-poor environments have becomecarnivores that prey mainly on smallcrustaceans.[212]
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Linnaeus mistakenly identified sponges as plants in the orderAlgae.[213] For a long time thereafter sponges were assigned to a separate subkingdom,Parazoa (meaningbeside the animals).[214] They are now classified as aparaphyleticphylum from which the higher animals have evolved.[215]
Ctenophores (from Greek forcarrying a comb), commonly known as comb jellies, are a phylum that live worldwide in marine waters. They are the largest non-colonial animals to swim with the help ofcilia (hairs or combs).[216] Coastal species need to be tough enough to withstand waves and swirling sediment, but some oceanic species are so fragile and transparent that it is very difficult to capture them intact for study.[217] In the past ctenophores were thought to have only a modest presence in the ocean, but it is now known they are often significant and even dominant parts of the planktonic biomass.[218]: 269
The phylum has about 150 known species with a wide range of body forms. Sizes range from a fewmillimeters to 1.5 m (4 ft 11 in).Cydippids are egg-shaped with their cilia arranged in eight radial comb rows, and deploy retractable tentacles for capturing prey. The benthicplatyctenids are generally combless and flat. The coastalberoids have gaping mouths and lack tentacles. Most adult ctenophores prey on microscopic larvae androtifers and smallcrustaceans but beroids prey on other ctenophores.
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Early writers combined ctenophores withcnidarians. Ctenophores resemble cnidarians in relying on water flow through the body cavity for both digestion and respiration, as well as in having a decentralizednerve net rather than a brain. Also like cnidarians, the bodies of ctenophores consist of a mass of jelly, with one layer ofcells on the outside and another lining the internal cavity. In ctenophores, however, these layers are two cells deep, while those in cnidarians are only a single cell deep. While cnidarians exhibitradial symmetry, ctenophores have two anal canals which exhibitbiradial symmetry (half-turn rotational symmetry).[219] The position of the ctenophores in the evolutionary family tree of animals has long been debated, and the majority view at present, based onmolecular phylogenetics, is that cnidarians andbilaterians are more closely related to each other than either is to ctenophores.[218]: 222
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| Iridescent red ctenophore —EVNautilus |
Placozoa (from Greek forflat animals) have the simplest structure of all animals. They are abasal form of free-living (non-parasitic)multicellular organism[220] that do not yet have a common name.[221] They live in marine environments and form a phylum containing so far only three described species, of which the first, the classicalTrichoplax adhaerens, was discovered in 1883.[222] Two more species have been discovered since 2017,[223][224] and genetic methods indicate this phylum has a further 100 to 200undescribed species.[225]
Trichoplax is a small, flattened, animal about one mm across and usually about 25 μm thick. Like theamoebae they superficially resemble, they continually change their external shape. In addition, spherical phases occasionally form which may facilitate movement.Trichoplax lacks tissues and organs. There is no manifest body symmetry, so it is not possible to distinguish anterior from posterior or left from right. It is made up of a few thousand cells of six types in three distinct layers.[226] The outer layer of simpleepithelial cells bearcilia which the animal uses to help it creep along the seafloor.[227]Trichoplax feed by engulfing and absorbing food particles – mainly microbes and organic detritus – with their underside.
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Cnidarians (from Greek fornettle) are distinguished by the presence ofstinging cells, specialized cells that they use mainly for capturing prey. Cnidarians includecorals,sea anemones,jellyfish andhydrozoans. They form aphylum containing over 10,000[228]species ofanimals found exclusively in aquatic (mainly marine) environments. Their bodies consist ofmesoglea, a non-living jelly-like substance, sandwiched between two layers ofepithelium that are mostly onecell thick. They have two basic body forms: swimmingmedusae andsessilepolyps, both of which areradially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a singleorifice and body cavity that are used for digestion andrespiration.
Fossil cnidarians have been found in rocks formed about580 million years ago. Fossils of cnidarians that do not buildmineralized structures are rare. Scientists currently think cnidarians,ctenophores andbilaterians are more closely related tocalcareous sponges than these are to othersponges, and thatanthozoans are the evolutionary "aunts" or "sisters" of other cnidarians, and the most closely related to bilaterians.
Cnidarians are the simplest animals in which the cells are organized into tissues.[229] Thestarlet sea anemone is used as amodel organism in research.[230] It is easy to care for in the laboratory and aprotocol has been developed which can yield large numbers of embryos on a daily basis.[231] There is a remarkable degree of similarity in the gene sequence conservation and complexity between the sea anemone and vertebrates.[231] In particular, genes concerned in the formation of the head in vertebrates are also present in the anemone.[232][233]
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![Porpita porpita consists of a colony of hydroids[234]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aPorpita_porpita.jpg)
![Lion's mane jellyfish, largest known jellyfish[235]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aLargelionsmanejellyfish.jpg)
![Turritopsis dohrnii achieves biological immortality by transferring its cells back to childhood.[236][237]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aTurritopsis_dohrnii_(cropped).jpg)
![The sea wasp is the most lethal jellyfish in the world.[238]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aChironex_fleckeri_(sea_wasp).jpg)
Some of the earliestbilaterians were wormlike, and the originalbilaterian may have been a bottom dwelling worm with a single body opening.[239] A bilaterian body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, acoelom or pseudocoelom.[a] Animals with this bilaterally symmetricbody plan have a head (anterior) end and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.[240][241]
Having a front end means that this part of the body encounters stimuli, such as food, favoringcephalisation, the development of a head withsense organs and a mouth.[242] The body stretches back from the head, and many bilaterians have a combination of circularmuscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body;[241] these enable soft-bodied animals with ahydrostatic skeleton to move byperistalsis.[243] They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primarylarvae which swim withcilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), and certainparasitic worms have extremely simplified body structures.[240][241]
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Protostomes (fromGreek forfirst mouth) are asuperphylum ofanimals. It is a sister clade of thedeuterostomes (from Greek forsecond mouth), with which it forms theNephrozoa clade. Protostomes are distinguished from deuterostomes by the way theirembryos develop. In protostomes the first opening that develops becomes themouth, while in deuterostomes it becomes the anus.[245][246]
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Worms (Old English forserpents) form a number of phyla. Different groups of marine worms are related only distantly, so they are found in several differentphyla such as theAnnelida (segmented worms),Chaetognatha (arrow worms),Phoronida (horseshoe worms), andHemichordata. All worms, apart from the Hemichordata, are protostomes. The Hemichordata are deuterostomes and are discussed in their own section below.
The typical body plan of a worm involves long cylindrical tube-like bodies and nolimbs.Marine worms vary in size from microscopic to over 1 metre (3.3 ft) in length for some marine polychaete worms (bristle worms)[247] and up to 58 metres (190 ft) for the marine nemertean worm (bootlace worm).[248] Some marine worms occupy a small variety ofparasitic niches, living inside the bodies of other animals, while others live more freely in the marine environment or by burrowing underground. Many of these worms have specialized tentacles used for exchanging oxygen and carbon dioxide and also may be used for reproduction. Some marine worms aretube worms, such as thegiant tube worm which lives in waters near underwatervolcanoes and can withstand temperatures up to 90 degreesCelsius.Platyhelminthes (flatworms) form another worm phylum which includes a class of parasitic tapeworms. The marine tapewormPolygonoporus giganticus, found in the gut ofsperm whales, can grow to over 30 m (100 ft).[249][250]
Nematodes (roundworms) constitute a further worm phylum with tubulardigestive systems and an opening at both ends.[251][252] Over 25,000 nematode species have been described,[253][254] of which more than half are parasitic. It has been estimated that another million are beyond our current knowledge.[255] They are ubiquitous in marine, freshwater and terrestrial environments, where they often outnumber other animals in both individual and species counts. They are found in every part of the Earth'slithosphere, from the top of mountains to the bottom ofoceanic trenches.[256] By count they represent 90% of all animals on theocean floor.[257] Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on Earth, their diversity of life cycles, and their presence at various trophic levels point at an important role in many ecosystems.[258]
Molluscs (Latin forsoft) form aphylum with about 85,000extant recognizedspecies.[261] They are the largestmarine phylum in terms of species count, containing about 23% of all the named marineorganisms.[262] Molluscs have more varied forms than other invertebrate phyla. They are highly diverse, not just in size and inanatomical structure, but also in behavior and in habitat.
The mollusc phylum is divided into 9 or 10taxonomicclasses. These classes includegastropods,bivalves andcephalopods, as well as other lesser-known but distinctive classes.Gastropods with protective shells are referred to assnails, whereas gastropods without protective shells are referred to asslugs.Gastropods are by far the most numerous molluscs in terms of species.[263]Bivalves includeclams,oysters,cockles,mussels,scallops, and numerous otherfamilies. There are about 8,000 marine bivalves species (includingbrackish water andestuarine species). A deep seaocean quahog clam has been reported ashaving lived 507 years[264] making it the longest recorded life of all animals apart fromcolonial animals, or near-colonial animals likesponges.[210]
Cephalopods includeoctopus,squid andcuttlefish. About 800 living species of marine cephalopods have been identified,[265] and an estimated 11,000 extincttaxa have been described.[266] They are found in all oceans, but there are no fully freshwater cephalopods.[267]
![The nautilus is a living fossil little changed since it evolved 500 million years ago as one of the first cephalopods.[268][269][270]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aNautilus_belauensis_from_Palau.jpg)
Molluscs have such diverse shapes that many textbooks base their descriptions of molluscan anatomy on a generalized orhypothetical ancestral mollusc. This generalized mollusc is unsegmented andbilaterally symmetrical with an underside consisting of a single muscularfoot. Beyond that it has three further key features. Firstly, it has a muscular cloak called amantle covering its viscera and containing a significant cavity used for breathing andexcretion. Ashell secreted by the mantle covers the upper surface. Secondly (apart from bivalves) it has a rasping tongue called aradula used for feeding. Thirdly, it has anervous system including a complex digestive system using microscopic, muscle-powered hairs calledcilia to exudemucus. The generalized mollusc has two pairednerve cords (three in bivalves). Thebrain, in species that have one, encircles theesophagus. Most molluscs haveeyes and all have sensors detecting chemicals, vibrations, and touch.[271][272]
Good evidence exists for the appearance of marine gastropods,cephalopods and bivalves in theCambrian period538.8 to 486.85 million years ago.
Arthropods (Greek forjointed feet) have anexoskeleton (externalskeleton), asegmented body, and jointedappendages (paired appendages). They form aphylum which includesinsects,arachnids,myriapods, andcrustaceans. Arthropods are characterized by their jointed limbs andcuticle made ofchitin, often mineralized withcalcium carbonate. The arthropodbody plan consists ofsegments, each with a pair ofappendages. The rigid cuticle inhibits growth, so arthropods replace it periodically bymoulting. Their versatility has enabled them to become the most species-rich members of allecological guilds in most environments.
The evolutionary ancestry of arthropods dates back to theCambrian period and is generally regarded asmonophyletic. However,basal relationships of arthropods with extinct phyla such aslobopodians have recently been debated.[275][276]
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![Fossil trilobite. Trilobites first appeared about 521 Ma. They were highly successful and were found everywhere in the ocean for 270 Ma.[277]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aCheirurus_ingricus.png)
![The largest known arthropod, the sea scorpion Jaekelopterus rhenaniae, has been found in estuarine strata from about 390 Ma. It was up to 2.5 m (8.2 ft) long.[278][279]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aJaekelopterus_rhenaniae_reconstruction.jpg)
![Xiphosurans, the group including modern Horseshoe crabs appeared around 480 Ma.[280]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aLimulus_(cropped).jpg)
Extant marine arthropods range in size from the microscopiccrustaceanStygotantulus to theJapanese spider crab. Arthropods' primary internal cavity is ahemocoel, which accommodates their internalorgans, and through which theirhaemolymph - analogue ofblood - circulates; they haveopen circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Theirnervous system is "ladder-like", with pairedventralnerve cords running through all segments and forming pairedganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and theirbrains are formed by fusion of the ganglia of these segments and encircle theesophagus. Therespiratory andexcretory systems of arthropods vary, depending as much on their environment as on thesubphylum to which they belong.
![Mantis shrimp have the most advanced eyes in the animal kingdom,[281] and smash prey by swinging their club-like raptorial claws.[282]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aOdontodactylus_scyllarus_2.png)
Arthropod vision relies on various combinations ofcompound eyes and pigment-pitocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the manysetae (bristles) that project through their cuticles. Arthropod methods of reproduction are diverse: terrestrial species use some form ofinternal fertilization while marine species lay eggs using either internal orexternal fertilization. Arthropod hatchlings vary from miniature adults to grubs that lack jointed limbs and eventually undergo a totalmetamorphosis to produce the adult form.
Indeuterostomes the first opening that develops in the growing embryo becomes theanus, while in protostomes it becomes the mouth. Deuterostomes form asuperphylum ofanimals and are the sister clade of theprotostomes.[245][246] It is once considered that the earliest known deuterostomes areSaccorhytus fossils from about 540 million years ago.[283] However, another study considered thatSaccorhytus is more likely to be anecdysozoan.[284]
| [[#Bilateral invertebrate animals|]] deuterostomes | |
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Echinoderms (Greek forspiny skin) is a phylum which contains only marine invertebrates. The phylum contains about 7000 livingspecies,[285] making it the second-largest grouping ofdeuterostomes, after thechordates.
Adult echinoderms are recognizable by theirradial symmetry (usually five-point) and includestarfish,sea urchins,sand dollars, andsea cucumbers, as well as thesea lilies.[286] Echinoderms are found at every ocean depth, from theintertidal zone to theabyssal zone. They are unique among animals in having bilateral symmetry at the larval stage, but five-fold symmetry (pentamerism, a special type of radial symmetry) as adults.[287]
Echinoderms are important both biologically and geologically. Biologically, there are few other groupings so abundant in thebiotic desert of thedeep sea, as well as shallower oceans. Most echinoderms are able toregenerate tissue, organs, limbs, andreproduce asexually; in some cases, they can undergo complete regeneration from a single limb. Geologically, the value of echinoderms is in theirossifiedskeletons, which are major contributors to manylimestone formations, and can provide valuable clues as to the geological environment. They were the most used species in regenerative research in the 19th and 20th centuries.
![The ochre sea star was the first keystone predator to be studied. They limit mussels which can overwhelm intertidal communities.[288]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aOchre_sea_star.jpg)
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It is held by some scientists that the radiation of echinoderms was responsible for theMesozoic Marine Revolution. Aside from the hard-to-classifyArkarua (aPrecambrian animal with echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of theCambrian.
Hemichordates form a sister phylum to theechinoderms. They are solitary worm-shaped organisms rarely seen by humans because of their lifestyle. They include two main groups, theacorn worms and thePterobranchia. Pterobranchia form a class containing about 30 species of small worm-shaped animals that live in secreted tubes on the ocean floor. Acorn worms form a class containing about 111 species that generally live in U-shaped burrows on the seabed, from the shoreline to a depth of 3000 meters. The worms lie there with the proboscis sticking out of one opening in the burrow, subsisting as deposit feeders or suspension feeders. It is supposed the ancestors of acorn worms used to live in tubes like their relatives, the Pterobranchia, but eventually started to live a safer and more sheltered existence in sediment burrows.[292] Some of these worms may grow to be very long; one particular species may reach a length of 2.5 meters (8 ft 2 in), although most acorn worms are much smaller.
Acorn worms are more highly specialized and advanced than other worm-like organisms. They have a circulatory system with a heart that also functions as a kidney. Acorn worms have gill-like structures they use for breathing, similar to the gills of fish. Therefore, acorn worms are sometimes said to be a link between classical invertebrates andvertebrates. Acorn worms continually form new gill slits as they grow in size, and some older individuals have more than a hundred on each side. Each slit consists of a branchial chamber opening to the pharynx through a U-shaped cleft. Cilia push water through the slits, maintaining a constant flow, just as in fish.[293] Some acorn worms also have a postanal tail which may be homologous to the post-anal tail of vertebrates.
The three-section body plan of the acorn worm is no longer present in the vertebrates, except in the anatomy of the frontal neural tube, later developed into a brain divided into three parts. This means some of the original anatomy of the early chordate ancestors is still present in vertebrates even if it is not always visible. One theory is the three-part body originated from an early common ancestor of the deuterostomes, and maybe even from a common bilateral ancestor of both deuterostomes and protostomes. Studies have shown the gene expression in the embryo share three of the same signaling centers that shape the brains of all vertebrates, but instead of taking part in the formation of their neural system,[294] they are controlling the development of the different body regions.[295]
Thechordate phylum has three subphyla, one of which is thevertebrates (see below). The other two subphyla are marine invertebrates: thetunicates (salps andsea squirts) and thecephalochordates (such aslancelets). Invertebrate chordates are close relatives to vertebrates. In particular, there has been discussion about how closely some extinct marine species, such asPikaiidae,Palaeospondylus,Zhongxiniscus andVetulicolia, might relate ancestrally to vertebrates.
![The lancelet, a small translucent fish-like cephalochordate, is one of the closest living invertebrate relative of the vertebrates.[297][298]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAmphioxus.png)
![Tunicates, like these fluorescent-colored sea squirts, may provide clues to vertebrate and therefore human ancestry.[299]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAscidian_(Rhopalaea_Crassa)_(4_cm).png)
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Vertebrates (Latin forjoints of the spine) are asubphylum ofchordates. They are chordates that have avertebral column (backbone). The vertebral column provides the central support structure for aninternal skeleton which gives shape, support, and protection to the body and can provide a means of anchoring fins or limbs to the body. The vertebral column also serves to house and protect thespinal cord that lies within the vertebral column.
Marine vertebrates can be divided into marinefish and marinetetrapods.
Fish typically breathe by extracting oxygen from water throughgills and have a skin protected byscales andmucous. They usefins to propel and stabilise themselves in the water, and usually have atwo-chambered heart andeyes well adapted to seeing underwater, as well as othersensory systems. Over 33,000 species of fish have been described as of 2017,[300] of which about 20,000 are marine fish.[301]
| ← vertebrates | |
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Early fish had nojaws. Most went extinct when they were outcompeted by jawed fish (below), but two groups survived:hagfish andlampreys. Hagfish form a class of about 20 species ofeel-shaped,slime-producing marine fish. They are the only known living animals that have askull but novertebral column.Lampreys form a superclass containing 38 known extant species ofjawless fish.[302] The adult lamprey is characterized by a toothed, funnel-like sucking mouth. Although they are well known for boring into the flesh of other fish tosuck their blood,[303] only 18 species of lampreys are actually parasitic.[304] Together hagfish and lampreys are the sister group to vertebrates. Living hagfish remain similar to hagfish from around 300 million years ago.[305] The lampreys are a very ancient lineage of vertebrates, though their exact relationship tohagfishes andjawed vertebrates is still a matter of dispute.[306] Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys,[307] and so also are vertebrates in amonophyletic sense. Others consider them a sister group of vertebrates in the common taxon of craniata.[308]
TheTully monster is an extinct genus of soft-bodied bilaterians that lived in tropical estuaries about 300 million years ago. Since 2016 there has been controversy over whether this animal was a vertebrate or an invertebrate.[309][310] In 2020 researchers found "strong evidence" that the Tully monster was a vertebrate, and was ajawless fish in the lineage of thelamprey,[311][312] while in 2023 other researchers found 3D fossils scans did not support those conclusions.[313]
Pteraspidomorphi is an extinctclass of early jawless fish ancestral to jawed vertebrates. The few characteristics they share with the latter are now considered as primitive for allvertebrates.
Around the start of theDevonian, fish started appearing with a deep remodelling of the vertebrate skull that resulted in ajaw.[314]All vertebrate jaws, including the human jaw, have evolved from these early fish jaws. The appearance of the early vertebrate jaw has been described as "perhaps the most profound and radical evolutionary step in vertebrate history".[315][316] Jaws make it possible to capture, hold, and chew prey.Fish without jaws had more difficulty surviving than fish with jaws, and most jawless fish became extinct during the Triassic period.
Jawed fish fall into two main groups:fish with bony internal skeletons andfish with cartilaginous internal skeletons. Cartilaginous fish, such assharks andrays, have jaws and skeletons made ofcartilage rather thanbone.Megalodon is an extinct species of shark that lived about 28 to 1.5 Ma. It may looked much like a stocky version of thegreat white shark, but was much larger with estimated lengths reaching 20.3 metres (67 ft).[317] Found in all oceans[318] it was one of the largest and most powerful predators in vertebrate history,[317] and probably had a profound impact on marine life.[319] TheGreenland shark has the longest known lifespan of all vertebrates, about 400 years.[320] Some sharks such as the great white are partially warm blooded and give live birth. Themanta ray, largest ray in the world, has been targeted by fisheries and is nowvulnerable.[321]
![Sawfish, rays with long rostrums resembling a saw. All species are now endangered.[322]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aPristis_clavata_2.jpg)
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Bony fish have jaws and skeletons made ofbone rather thancartilage. Bony fish also have hard, bony plates calledoperculum which help them respire and protect their gills, and they often possess aswim bladder which they use for better control of their buoyancy. Bony fish can be further divided into those withlobe fins and those withray fins. The approximate dates in the phylogenetic tree are from Near et al., 2012[323] and Zhu et al., 2009.[324]
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Lobe fins have the form of fleshylobes supported by bony stalks which extend from the body.[325]Guiyu oneiros, the earliest-known bony fish, lived during the LateSilurian 419 million years ago. It has the combination of bothray-finned and lobe-finned features, although analysis of the totality of its features place it closer to lobe-finned fish.[324] Lobe fins evolved into the legs of the first tetrapod land vertebrates, so by extension an early ancestor of humans was a lobe-finned fish. Apart from the coelacanths and the lungfishes, lobe-finned fishes are now extinct.
The remaining bony fish have ray fins. These are made of webs of skin supported by bony or horny spines (rays) which can be erected to control the fin stiffness.
About 96% of all modern fish species are teleosts,[328] of which about 14,000 are marine species.[329] Teleosts can be distinguished from other bony fish by their possession of ahomocercal tail, a tail where the upper half mirrors the lower half.[330] Another difference lies in their jaw bones – teleosts have modifications in the jaw musculature which make it possible for them toprotrude their jaws. This enables them tograb prey anddraw it into their mouth.[330] In general, teleosts tend to be quicker and more flexible than more basal bony fishes. Their skeletal structure has evolved towards greater lightness. While teleost bones are wellcalcified, they are constructed from a scaffolding of struts, rather than the densecancellous bones ofholostean fish.[331]
Teleosts are found in almost allmarine habitats.[332] They have enormousdiversity, and range in size from adultgobies 8mm long[333] toocean sunfish weighing over 2,000 kg.[334] The following images show something of the diversity in the shape and colour of modern marine teleosts...
Nearly half of all extant vertebrate species are teleosts.[335]
Atetrapod (Greek forfour feet) is a vertebrate withlimbs (feet). Tetrapods evolved from ancientlobe-finned fishes about 400 million years ago during theDevonian Period when their earliest ancestors emerged from the sea and adapted to living on land.[336] This change from a body plan for breathing and navigating in gravity-neutral water to a body plan with mechanisms enabling the animal to breath in air without dehydrating and move on land is one of the most profound evolutionary changes known.[337][338] Tetrapods can be divided into four classes:amphibians,reptiles,birds andmammals.
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Marine tetrapods are tetrapods that returned from land back to the sea again. The first returns to the ocean may have occurred as early as theCarboniferous Period[339] whereas other returns occurred as recently as theCenozoic, as in cetaceans,pinnipeds,[340] and severalmodern amphibians.[341]Amphibians (from Greek forboth kinds of life) live part of their life in water and part on land. They mostly require fresh water to reproduce. A few inhabit brackish water, but there are no true marine amphibians.[342] There have been reports, however, of amphibians invading marine waters, such as a Black Sea invasion by the natural hybridPelophylax esculentus reported in 2010.[343]
Reptiles (Late Latin forcreeping orcrawling) do not have an aquatic larval stage, and in this way are unlike amphibians. Most reptiles are oviparous, although several species of squamates areviviparous, as were some extinct aquatic clades[344] — the fetus develops within the mother, contained in aplacenta rather than aneggshell. Asamniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed theirfetuses through various forms of placenta analogous to those ofmammals, with some providing initial care for their hatchlings.
Some reptiles are more closely related tobirds than other reptiles, and many scientists prefer to make Reptilia a monophyletic group which includes the birds.[345][346][347][348]Extant non-avian reptiles which inhabit or frequent the sea includesea turtles,sea snakes,terrapins, themarine iguana, and thesaltwater crocodile. Currently, of the approximately 12,000 extantreptile species and sub-species, only about 100 of are classed as marine reptiles.[349]
Except for some sea snakes, most extant marine reptiles areoviparous and need to return to land to lay their eggs. Apart from sea turtles, the species usually spend most of their lives on or near land rather than in the ocean. Sea snakes generally prefer shallow waters nearby land, around islands, especially waters that are somewhat sheltered, as well as near estuaries.[350][351] Unlike land snakes, sea snakes have evolved flattened tails which help them swim.[352]
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Someextinct marine reptiles, such asichthyosaurs, evolved to beviviparous and had no requirement to return to land.Ichthyosaurs resembled dolphins. They first appeared about 245 million years ago and disappeared about 90 million years ago. The terrestrial ancestor of the ichthyosaur had no features already on its back or tail that might have helped along the evolutionary process. Yet the ichthyosaur developed adorsal andtail fin which improved its ability to swim.[353] The biologistStephen Jay Gould said the ichthyosaur was his favourite example ofconvergent evolution.[354] The earliest marine reptiles arose in thePermian. During theMesozoic many groups of reptiles became adapted to life in the seas, includingichthyosaurs,plesiosaurs,mosasaurs,nothosaurs,placodonts,sea turtles,thalattosaurs andthalattosuchians. Marine reptiles were less numerous aftermass extinction at the end of theCretaceous.
Marine birds areadapted to life within themarine environment. They are often calledseabirds. While marine birds vary greatly in lifestyle, behaviour and physiology, they often exhibit strikingconvergent evolution, as the same environmental problems and feedingniches have resulted in similar adaptations. Examples includealbatross,penguins,gannets, andauks.
In general, marine birds live longer,breed later and have fewer young than terrestrial birds do, but they invest a great deal of time in their young. Mostspecies nest incolonies, which can vary in size from a few dozen birds to millions. Many species are famous for undertaking long annualmigrations, crossing theequator or circumnavigating the Earth in some cases. They feed both at the ocean's surface and below it, and even feed on each other. Marine birds can be highlypelagic, coastal, or in some cases spend a part of the year away from the sea entirely. Some marine birds plummet from heights, plunging through the water leaving vapour-like trails, similar to that of fighter planes.[355]Gannets plunge into the water at up to 100 kilometres per hour (60 mph). They have air sacs under their skin in their face and chest which act likebubble-wrap, cushioning the impact with the water.
The first marine birds evolved in theCretaceousperiod, and modern marine bird families emerged in thePaleogene.
Mammals (from Latin forbreast) are characterised by the presence ofmammary glands which infemales producemilk for feeding (nursing) their young. There are about 130 living and recently extinct marinemammal species such asseals,dolphins,whales,manatees,sea otters andpolar bears.[356] They do not represent a distinct taxon or systematic grouping, but are instead unified by their reliance on the marine environment for feeding. Both cetaceans and sirenians are fully aquatic and therefore are obligate water dwellers. Seals and sea-lions are semiaquatic; they spend the majority of their time in the water, but need to return to land for important activities such asmating,breeding andmolting. In contrast, both otters and the polar bear are much less adapted to aquatic living. Their diet varies considerably as well: some may eatzooplankton; others may eat fish, squid, shellfish, and sea-grass; and a few may eat other mammals.
In a process ofconvergent evolution, marine mammals, especially cetaceans such as dolphins and whales, redeveloped theirbody plan to parallel the streamlinedfusiform body plan ofpelagic fish. Front legs becameflippers and back legs disappeared, adorsal fin reappeared and the tail morphed into a powerful horizontalfluke. This body plan is an adaptation to being an active predator in a highdrag environment. A parallel convergence occurred with the now extinct marine reptileichthyosaur.[357]
![Endangered blue whale, the largest living animal[358]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aBluewhale2_noaa.jpg)
![The bottlenose dolphin has the highest encephalization of any animal after humans[359]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aTursiops_truncatus_01.jpg)
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Primary producers are theautotroph organisms that make their own food instead of eating other organisms. This means primary producers become the starting point in thefood chain forheterotroph organisms that do eat other organisms. Some marine primary producers are specialised bacteria and archaea which arechemotrophs, making their own food by gathering aroundhydrothermal vents andcold seeps and usingchemosynthesis. However most marineprimary production comes from organisms which usephotosynthesis on the carbon dioxide dissolved in the water. This process uses energy from sunlight to convert water andcarbon dioxide[360]: 186–187 into sugars that can be used both as a source of chemical energy and of organic molecules that are used in the structural components of cells.[360]: 1242 Marine primary producers are important because they underpin almost all marine animal life by generating most of theoxygen and food that provide other organisms with the chemical energy they need to exist.
The principal marine primary producers arecyanobacteria,algae and marine plants. Theoxygen released as a by-product of photosynthesis is needed bynearly all living things to carry outcellular respiration. In addition, primary producers are influential in the globalcarbon andwater cycles. They stabilize coastal areas and can provide habitats for marine animals. The termdivision has been traditionally used instead ofphylum when discussing primary producers, but theInternational Code of Nomenclature for algae, fungi, and plants now accepts both terms as equivalents.[361]
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Cyanobacteria were the first organisms to evolve an ability to turn sunlight into chemical energy. They form a phylum (division) of bacteria which range from unicellular tofilamentous and includecolonial species. They are found almost everywhere on earth: in damp soil, in both freshwater and marine environments, and even on Antarctic rocks.[362] In particular, some species occur as drifting cells floating in the ocean, and as such were amongst the first of thephytoplankton.
The first primary producers that used photosynthesis were oceanic cyanobacteria about 2.3 billion years ago.[363][364] The release of molecularoxygen by cyanobacteria as a by-product of photosynthesis induced global changes in the Earth's environment. Because oxygen was toxic to most life on Earth at the time, this led to the near-extinction ofoxygen-intolerant organisms, adramatic change which redirected the evolution of the major animal and plant species.[365]
The tiny marine cyanobacteriumProchlorococcus, discovered in 1986, forms today part of the base of the oceanfood chain and accounts for much of the photosynthesis of the open ocean[366] and an estimated 20% of the oxygen in the Earth's atmosphere.[367] It is possibly the most plentiful genus on Earth: a single millilitre of surface seawater may contain 100,000 cells or more.[368]
Originally, biologists classified cyanobacteria as algae, and referred to it as "blue-green algae". The more recent view is that cyanobacteria are bacteria, and hence are not even in the sameKingdom as algae. Most authorities today exclude allprokaryotes, and hence cyanobacteria from the definition of algae.[369][370]
Algae is an informal term for a widespread and diverse group of photosyntheticprotists which are not necessarily closely related and are thuspolyphyletic. Marine algae can be divided into six groups:
Unlike higher plants, algae lack roots, stems, or leaves. They can be classified by size asmicroalgae ormacroalgae.
Microalgae are the microscopic types of algae, not visible to the naked eye. They are mostlyunicellular species which exist as individuals or in chains or groups, though some aremulticellular. Microalgae are important components of the marine protists (discussed above), as well as the phytoplankton (discussed below). They are verydiverse. It has been estimated there are 200,000-800,000 species of which about 50,000 species have been described.[379] Depending on the species, their sizes range from a few micrometers (μm) to a few hundred micrometers. They are specially adapted to an environment dominated by viscous forces.
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![Chlorella vulgaris, a common green microalgae, in endosymbiosis with a ciliate[380]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3a%25D0%2598%25D0%25BD%25D1%2584%25D1%2583%25D0%25B7%25D0%25BE%25D1%2580%25D0%25B8%25D0%25B8_Ophridium_versatile.jpg)
Macroalgae are the larger,multicellular and more visible types of algae, commonly calledseaweeds. Seaweeds usually grow in shallow coastal waters where they are anchored to the seafloor by aholdfast. Seaweed that becomes adrift can wash up on beaches.Kelp is a large brown seaweed that forms large underwaterforests covering about 25% of the world coastlines.[381] They are among the most productive and dynamic ecosystems on Earth.[382] SomeSargassum seaweeds are planktonic (free-floating). Like microalgae, macroalgae (seaweeds) are technicallymarine protists since they are not true plants.
![The unicellular bubble algae lives in tidal zones. It can have a 4 cm diameter.[383]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aVentricaria_ventricosa.JPG)
![Killer algae are single-celled organisms, but look like ferns and grow stalks up to 80 cm long.[384]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aCaulerpaTaxifolia.jpg)
Unicellular organisms are usually microscopic, less than one tenth of a millimeter long. There are exceptions.Mermaid's wineglass, a genus of subtropicalgreen algae, is single-celled but remarkably large and complex in form with a single large nucleus, making it a model organism for studyingcell biology.[385] Another single celled algae,Caulerpa taxifolia, has the appearance of a vascular plant including "leaves" arranged neatly up stalks like a fern. Selective breeding in aquariums to produce hardier strains resulted in an accidental release into the Mediterranean where it has become aninvasive species known colloquially askiller algae.[386]
Back in theSilurian, some phytoplankton evolved intored,brown andgreen algae. These algae then invaded the land and started evolving into the land plants we know today. Later, in theCretaceous, some of these land plants returned to the sea as marine plants, such asmangroves andseagrasses.[387]
Marine plants can be found inintertidal zones and shallow waters, such asseagrasses likeeelgrass andturtle grass,Thalassia. These plants have adapted to the high salinity of the ocean environment. Plant life can also flourish in the brackish waters ofestuaries, wheremangroves orcordgrass orbeach grassbeach grass might grow.
![Sea dragons camouflaged to look like floating seaweed live in kelp forests and seagrass meadows.[388]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aLeafy_Sea_Dragon_SA.jpg)
The total world area of mangrove forests was estimated in 2010 as 134,257 square kilometres (51,837 sq mi) (based on satellite data).[389][390] The total world area of seagrass meadows is more difficult to determine, but was conservatively estimated in 2003 as 177,000 square kilometres (68,000 sq mi).[391]
Mangroves and seagrasses provide important nursery habitats for marine life, acting as hiding and foraging places for larval and juvenile forms of larger fish and invertebrates.[392]
Plankton (from Greek forwanderers) are a diverse group of organisms that live in thewater column of large bodies of water but cannot swim against a current. As a result, they wander or drift with the currents.[393] Plankton are defined by theirecological niche, not by anyphylogenetic ortaxonomic classification. They are a crucial source of food for many marine animals, fromforage fish towhales. Plankton can be divided into a plant-like component and an animal component.
Phytoplankton are the plant-like components of the plankton community ("phyto" comes from the Greek forplant). They areautotrophic (self-feeding), meaning they generate their own food and do not need to consume other organisms.
Phytoplankton consist mainly of microscopic photosyntheticeukaryotes which inhabit the upper sunlit layer in all oceans. They need sunlight so they can photosynthesize. Most phytoplankton are single-celled algae, but other phytoplankton are bacteria and some areprotists.[394] Phytoplankton groups includecyanobacteria (above),diatoms, various other types ofalgae (red, green, brown, and yellow-green),dinoflagellates,euglenoids,coccolithophorids,cryptomonads,chrysophytes,chlorophytes,prasinophytes, andsilicoflagellates. They form the base of theprimary production that drives the oceanfood web, and account for half of the current global primary production, more than the terrestrial forests.[395]
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Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek foranimal). They areheterotrophic (other-feeding), meaning they cannot produce their own food and must consume instead other plants or animals as food. In particular, this means they eat phytoplankton.
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Zooplankton are generally larger than phytoplankton, mostly still microscopic but some can be seen with the naked eye. Manyprotozoans (single-celledprotists that prey on other microscopic life) are zooplankton, includingzooflagellates,foraminiferans,radiolarians and somedinoflagellates. Other dinoflagellates aremixotrophic and could also be classified as phytoplankton; the distinction between plants and animals often breaks down in very small organisms. Other zooplankton include pelagiccnidarians,ctenophores,molluscs,arthropods andtunicates, as well as planktonicarrow worms andbristle worms.
Radiolarians are unicellularprotists with elaborate silica shells
Microzooplankton: major grazers of the plankton
Larger zooplankton can be predatory on smaller zooplankton.
Macrozooplankton
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![Tomopteris, a planktonic segmented worm with unusual yellow bioluminescence[398]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aTomopteriskils.jpg)
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| Venus Girdle -Youtube |
Many marine animals begin life as zooplankton in the form of eggs or larvae, before they develop into adults. These aremeroplanktic, that is, they are planktonic for only part of their life.
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Dinoflagellates are oftenmixotrophic or live insymbiosis with other organisms.
Some dinoflagellates arebioluminescent. At night, ocean water can light up internally andsparkle with blue light because of these dinoflagellates.[400][401] Bioluminescent dinoflagellates possessscintillons, individualcytoplasmic bodies which containdinoflagellate luciferase, the main enzyme involved in the luminescence. The luminescence, sometimes calledthe phosphorescence of the sea, occurs as brief (0.1 sec) blue flashes or sparks when individual scintillons are stimulated, usually by mechanical disturbances from, for example, a boat or a swimmer or surf.[402]
Compared to terrestrial environments, marine environments have biomass pyramids which are inverted at the base. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. This happens because the ocean's primary producers are tiny phytoplankton which tend to ber-strategists that grow and reproduce rapidly, so a small mass can have a fast rate of primary production. In contrast, terrestrial primary producers, such as mature forests, are oftenK-strategists that grow and reproduce slowly, so a much larger mass is needed to achieve the same rate of primary production.
Because of this inversion, it is the zooplankton that make up most of the marine animalbiomass. Asprimary consumers, they are the crucial link between the primary producers (mainly phytoplankton) and the rest of the marine food web (secondary consumers).[403]
If phytoplankton dies before it is eaten, it descends through theeuphotic zone as part of themarine snow and settles into the depths of sea. In this way, phytoplankton sequester about 2 billion tons of carbon dioxide into the ocean each year, causing the ocean to become a sink of carbon dioxide holding about 90% of all sequestered carbon.[404]
In 2010 researchers found whales carry nutrients from the depths of the ocean back to the surface using a process they called thewhale pump.[405] Whales feed at deeper levels in the ocean wherekrill is found, but return regularly to the surface to breathe. There whalesdefecate a liquid rich in nitrogen and iron. Instead of sinking, the liquid stays at the surface wherephytoplankton consume it. In the Gulf of Maine the whale pump provides more nitrogen than the rivers.[406]
Taken as a whole, the oceans form a single marine system where water – the "universal solvent"[407] – dissolves nutrients and substances containing elements such as oxygen, carbon, nitrogen and phosphorus. These substances are endlessly cycled and recycled, chemically combined and then broken down again, dissolved and then precipitated or evaporated, imported from and exported back to the land and the atmosphere and the ocean floor. Powered both by the biological activity of marine organisms and by the natural actions of the sun and tides and movements within the Earth's crust, these are themarine biogeochemical cycles.[408][409]
![Marine carbon cycle[410]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aOceanCarbonCycle.jpg)
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Sediments at the bottom of the ocean have two main origins, terrigenous and biogenous.Terrigenous sediments account for about 45% of the total marine sediment, and originate in the erosion ofrocks on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers.
Biogenous sediments account for the other 55% of the total sediment, and originate in the skeletal remains ofmarine protists (single-celled plankton and benthos organisms). Much smaller amounts of precipitated minerals and meteoric dust can also be present.Ooze, in the context of a marine sediment, does not refer to the consistency of the sediment but to its biological origin. The term ooze was originally used byJohn Murray, the "father of modern oceanography", who proposed the termradiolarian ooze for the silica deposits of radiolarian shells brought to the surface during theChallenger Expedition.[411] Abiogenic ooze is apelagic sediment containing at least 30 percent from the skeletal remains of marine organisms.
Main types of biogenic ooze | |||||||
|---|---|---|---|---|---|---|---|
| type | mineral forms | protist responsible | name of skeleton | description | |||
| Siliceous ooze | SiO2 quartz glass opal chert | diatoms |  | frustule | Individual diatoms range in size from 0.002 to 0.2 mm.[412] | ||
| radiolarians | .jpg) | skeleton | Radiolarians are protozoa with diameters typically between 0.1 and 0.2 mm that produce intricate mineral skeletons, usually made of silica | ||||
| Calcareous ooze | CaCO3 calcite aragonite limestone chalk | foraminiferans | | test | There are about 10,000 living species of foraminiferans,[413] usually under 1 mm in size. | ||
| coccolithophores |  | coccolith | Coccolithophores are spherical cells usually less than 0.1 mm across, enclosed by calcareous plates called coccoliths.[414] Coccoliths are importantmicrofossils. They are the largest global source of biogenic calcium carbonate, and make significant contributions to the global carbon cycle.[415] They are the main constituent of chalk deposits such as thewhite cliffs of Dover. | ||||
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Land interactions impact marine life in many ways. Coastlines typically havecontinental shelves extending some way from the shore. These provide extensive shallows sunlit down to the seafloor, allowing for photosynthesis and enabling habitats for seagrass meadows, coral reefs, kelp forests and otherbenthic life. Further from shore the continental shelf slopes towards deep water. Windblowing at the ocean surface ordeep ocean currents can result in cold and nutrient rich waters fromabyssal depths moving up thecontinental slopes. This can result inupwellings along the outer edges of continental shelves, providing conditions forphytoplankton blooms.
Water evaporated by the sun from the surface of the ocean can precipitate on land and eventually return to the ocean asrunoff or discharge from rivers, enriched with nutrients as well aspollutants. As rivers discharge intoestuaries,freshwater mixes withsaltwater and becomesbrackish. This provides another shallow water habitat wheremangrove forests andestuarine fish thrive. Overall, life in inland lakes can evolve with greater diversity than happens in the sea, because freshwater habitats are themselves diverse and compartmentalised in a way marine habitats are not. Some aquatic life, such assalmon andeels,migrate back and forth between freshwater and marine habitats. These migrations can result in exchanges of pathogens and have impacts on the way life evolves in the ocean.
Human activities affect marine life andmarine habitats throughoverfishing,pollution,acidification and the introduction ofinvasive species. These impactmarine ecosystems andfood webs and may result in consequences as yet unrecognised for thebiodiversity and continuation of marine life forms.[417]
Biodiversity is the result of over three billion years ofevolution. Until approximately 600 million years ago, all life consisted ofarchaea,bacteria,protozoans and similarsingle-celled organisms. The history of biodiversity during thePhanerozoic (the last 540 million years), starts with rapid growth during theCambrian explosion – a period during which nearly everyphylum ofmulticellular organisms first appeared. Over the next 400 million years or so, invertebrate diversity showed little overall trend and vertebrate diversity shows an overall exponential trend.[419]
However, more than 99 percent of all species that ever lived on Earth, amounting to over five billion species,[420] are estimated to beextinct.[421][422] These extinctions occur at an uneven rate. The dramatic rise in diversity has been marked by periodic, massive losses of diversity classified asmass extinction events.[419] Mass extinction events occur when life undergoes precipitous global declines. Most diversity andbiomass on earth is found among themicroorganisms, which are difficult to measure. Recorded extinction events are therefore based on the more easily observed changes in the diversity and abundance of largermulticellular organisms, rather than the total diversity and abundance of life.[423] Marine fossils are mostly used to measure extinction rates because of their superior fossil record andstratigraphic range compared to land organisms.
Based on thefossil record, thebackground rate of extinctions on Earth is about two to fivetaxonomicfamilies of marine animals every million years. TheGreat Oxygenation Event was perhaps the first major extinction event. Since theCambrian explosion fivemajor mass extinctions have significantly exceeded the background extinction rate.[424] The worst was thePermian-Triassic extinction event, 251 million years ago. One generally estimates that the Big Five mass extinctions of the Phanerozoic (the last 540 million years) wiped out more than 40% of marine genera and probably more than 70% of marine species.[425] The currentHolocene extinction caused by human activity, and now referred to as the "sixth extinction", may prove ultimately more devastating.
In order to perform research and enrich Marine Life knowledge, Scientists use various methods in-order to reach and explore the depths of the ocean. several Hi-tech instruments and vehicles are used for this purpose.[426]
Modified text was copied from this source, which is available under aCreative Commons Attribution 4.0 International License.Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. Usually organisms were grouped according to their morphological similarities as perceived by those early workers, and those groups were then grouped according to their similarities, and so on, to form a hierarchy.
Modified text was copied from this source, which is available under aCreative Commons Attribution 4.0 International LicenseTable 1: Numbers of threatened species by major groups of organisms (1996–2014)
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