Marine vertebrates arevertebrates that live inmarine environments, which includesaltwater fish (includingpelagic,coral anddeep sea fish) and marinetetrapods (primarilymarine mammals andmarine reptiles, as well assemiaquaticclades such asseabirds). As asubphylum ofchordates, all vertebrates have evolved avertebral column (backbone) based around the embryonicnotochord (which becomes theintervertebral discs), forming thecorestructural support of aninternal skeleton, and also serves to enclose and protect thespinal cord.
Compared to othermarine animals, marine vertebrates are distinctly morenektonic, and theiraquatic locomotions rely mainly onpropulsion by thetail and pairedappendages such asfins,flippers andwebbedlimbs. Marine vertebrates also have a far morecentralizednervous system than marineinvertebrates, with most of the higher functionscephalized and monopolized by thebrain; and most of them have evolvedmyelinatedcentral andperipheral nerve system, which increasesconduction speeds significantly. The combination of endoskeleton (which allows much larger body sizes for the sameskeletalmass) and a more robust and efficient nervous system (which enables more acuteperception and more sophisticatedmotor control) gives vertebrates much quickerbody reactivity andbehavioraladaptability, which have led to marine vertebrates dominating most of thehigher-levelniches in themarine ecosystems.
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Fish fall into two main groups:fish with bony internal skeletons andfish with cartilaginous internal skeletons.Fish anatomy andphysiology generally includes atwo-chambered heart,eyes adapted to seeing underwater, and a skin protected byscales andmucous. They typically breathe by extracting oxygen from water throughgills. Fish usefins to propel and stabilise themselves in the water. Over 33,000 species of fish have been described as of 2017,[1] of which about 20,000 are marine fish.[2]
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.[3] 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,[4] only 18 species of lampreys are actually parasitic.[5] Together hagfish and lampreys are the sister group to vertebrates. Living hagfish remain similar to hagfish from around 300 million years ago.[6] The lampreys are a very ancient lineage of vertebrates, though their exact relationship tohagfishes andjawed vertebrates is still a matter of dispute.[7] Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys,[8] and so also are vertebrates in amonophyletic sense. Others consider them a sister group of vertebrates in the common taxon of craniata.[9]
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.
Cartilaginous fish, such assharks andrays, havejaws and skeletons made ofcartilage rather thanbone.Megalodon is an extinct species of shark that lived about 28 to 1.5 Ma. It looked much like a stocky version of thegreat white shark, but was much larger with fossil lengths reaching 20.3 metres (67 ft).[10] Found in all oceans[11] it was one of the largest and most powerful predators in vertebrate history,[10] and probably had a profound impact on marine life.[12] TheGreenland shark has the longest known lifespan of all vertebrates, about 400 years.[13]
![The manta ray, largest ray in the world, has been targeted by fisheries and is now vulnerable.[14]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aManta_ray_-_Chura-umi_Aquarium.jpg)
![Sawfish are rays with long rostrums resembling a saw. All are now endangered or critically endangered[15]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aPristis_clavata_2.jpg)
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Bony fish havejaws and skeletons made ofbone rather thancartilage. About 90% of the world's fish species are bony fish. 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. Lobe fins have the form of fleshylobes supported by bony stalks which extend from the body.[16] 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 rest of the modern 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.
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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 toliving on land.[17] This change from abody plan for breathing and navigating in gravity-neutral water to a body plan with mechanisms enabling the animal to breathe in air without dehydrating and move on land is one of the most profound evolutionary changes known.[18][19] Tetrapods can be divided into four classes:amphibians,reptiles,birds andmammals.
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[20] whereas other returns occurred as recently as theCenozoic, as in cetaceans,pinnipeds,[21] and severalmodern amphibians.[22]
Amphibians (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.[23] There have been reports, however, of amphibians invading marine waters, such as a Black Sea invasion by the natural hybridPelophylax esculentus reported in 2010.[24]
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[25] — 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.[26][27][28][29]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.[30]
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.[31][32] Unlike land snakes, sea snakes have evolved flattened tails which help them swim.[30]
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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.[33] The biologistStephen Jay Gould said the ichthyosaur was his favourite example ofconvergent evolution.[34] 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.[35]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.[36] 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 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 extinctichthyosaur.[37]
![Endangered blue whale, largest animal ever[38]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAnim1754_-_Flickr_-_NOAA_Photo_Library.jpg)
![Bottlenose dolphin, highest encephalization of any animal after humans[39]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aTursiops_truncatus_01.jpg)
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