Anaquatic animal is anyanimal, whethervertebrate orinvertebrate, that lives in abody of water for all or most of itslifetime.[1] Aquatic animals generally conductaquatic respiration byextractingdissolved oxygen in water via specialisedrespiratoryorgans calledgills,through the skin oracross enteral mucosae, although some aresecondarily aquatic animals (e.g.marine reptiles andmarine mammals)evolved from terrestrial ancestors that re-adapted to aquatic environments, in which case they actually uselungs tobreatheair and are essentiallyholding their breath when living in water. Some species ofgastropodmollusc, such as theeastern emerald sea slug, are even capable ofkleptoplasticphotosynthesis viaendosymbiosis with ingestedyellow-green algae.
Almost all aquatic animalsreproduce in water, eitheroviparously orviviparously, and many species routinelymigrate between different water bodies during theirlife cycle. Some animals have fully aquatic life stages (typically aseggs andlarvae), while asadults they becometerrestrial orsemi-aquatic after undergoingmetamorphosis. Such examples includeamphibians such asfrogs, manyflying insects such asmosquitoes,mayflies,dragonflies,damselflies andcaddisflies, as well as some species ofcephalopodmolluscs such as thealgae octopus (whose larvae are completelyplanktonic, but adults are highly terrestrial).
Aquatic animals are a diversepolyphyletic group based purely on thenatural environments they inhabit, and many morphological and behavioral similarities among them are the result ofconvergent evolution. They are distinct from terrestrial and semi-aquatic animals, who can survive away from water bodies, while aquatic animals often die ofdehydration orhypoxia after prolonged removal out of water due to either gill failure or compressiveasphyxia by their own body weight (as in the case ofwhale beaching). Along withaquatic plants,algae andmicrobes, aquatic animals form thefood webs of variousmarine,brackish andfreshwateraquatic ecosystems.
The term aquatic can be applied to animals that live in eitherfresh water orsalt water. However, the adjectivemarine is most commonly used for animals that live in saltwater or sometimesbrackish water, i.e. inoceans,shallow seas,estuaries, etc.
Aquatic animals can be separated into four main groups according to their positions within thewater column.
Aquatic animals (especially freshwater animals) are often of special concern toconservationists because of the fragility of their environments. Aquatic animals are subject to pressure fromoverfishing/hunting,destructive fishing,water pollution,acidification,climate change andcompetition frominvasive species. Many aquatic ecosystems are at risk ofhabitat destruction/fragmentation, which puts aquatic animals at risk as well.[2] Aquatic animals play an important role in the world. The biodiversity of aquatic animals provide food, energy, and even jobs.[3]
Fresh water creates ahypotonic environment for aquatic organisms. This is problematic for organisms with pervious skins andgills, whosecell membranes may rupture if excess water is not excreted. Someprotists accomplish this usingcontractile vacuoles, whilefreshwater fish excrete excess water via thekidney.[4] Although most aquatic organisms have a limited ability to regulate theirosmotic balance and therefore can only live within a narrow range of salinity,diadromousfish have the ability tomigrate between fresh andsaline water bodies. During these migrations they undergo changes to adapt to the surroundings of the changed salinities; these processes are hormonally controlled. TheEuropean eel (Anguilla anguilla) uses thehormoneprolactin,[5] while insalmon (Salmo salar) the hormonecortisol plays a key role during this process.[6]
Freshwater molluscs includefreshwater snails andfreshwater bivalves. Freshwatercrustaceans includefreshwater shrimps,crabs,crayfish andcopepods.[7][8]
In addition to water-breathing animals (e.g.fish, mostmolluscs, etc.), the term "aquatic animal" can be applied to air-breathingsecondarily aquatic tetrapods who descended fromterrestrialvertebrates and have evolved and fullyadapted to aquatic life. The most proliferative extant group are themarine mammals, such ascetaceans (whales,dolphins andporpoises, with somefreshwater species) andsirenians (dugongs andmanatees), who aretoo evolved for aquatic life to survive on land at all (where they will die ofbeaching), as well as the highly aquatically adapted but land-dwellingpinnipeds (true seals,eared seals and thewalrus). The term "aquatic mammal" is also applied toriparian mammals like theriver otter (Lontra canadensis) andbeavers (familyCastoridae), although they are technicallysemiaquatic or amphibious.[9] Unlike the more commongill-bearing aquatic animals, these air-breathing animals havelungs (which arehomologous to theswim bladders inbony fish) and need to surface periodically to change breaths, but their ranges are not restricted byoxygen saturation in water, althoughsalinity changes can still affect their physiology to an extent.[citation needed]
There are alsoreptilian animals that are highly evolved for life in water, although most extant aquatic reptiles, includingcrocodilians,turtles,water snakes and themarine iguana, are technically semi-aquatic rather than fully aquatic, and most of them only inhabitfreshwater ecosystems.Marine reptiles were once a dominant group of ocean predators thataltered the marine fauna during theMesozoic, with some clades such asichthyosaurs evolving to become very fish-like, although most of them died out during theCretaceous-Paleogene extinction event and now only thesea turtles (the only remaining descendants of the Mesozoic marine reptiles) andsea snakes (which only evolved during theCenozoic) remain fully aquatic insaltwater ecosystems.[citation needed]
Amphibians, while still requiring access to water to inhabit, are separated into their own ecological classification. The majority of amphibians — except the orderGymnophiona (caecilians), which are mainly terrestrialburrowers — have a fully aquaticlarval form known astadpoles, but those from the orderAnura (frogs andtoads) and some of the orderUrodela (salamanders) willmetamorphosize intolung-bearing and sometimesskin-breathing terrestrial adults, and most of them may return to the water tobreed.Axolotl, a Mexican salamander that retains its larvalexternal gills into adulthood, is the only extant amphibian that remains fully aquatic throughout the entirelife cycle.[citation needed]
Certainamphibious fish also evolved to breathe air to surviveoxygen-deprived waters, such aslungfishes,mudskippers,labyrinth fishes,bichirs,arapaima andwalking catfish. Their abilities to breathe atmospheric oxygen are achieved via skin-breathing,enteral respiration, or specialized gill organs such as thelabyrinth organ and even primitive lungs (lungfish and bichirs).[citation needed]
Mostmolluscs havegills, while somefreshwater gastropods (e.g.Planorbidae) have evolvedpallial lungs and some amphibious species (e.g.Ampullariidae) have both.[9] Many species ofoctopus havecutaneous respiration that allows them to survive out of water at theintertidal zones, with at least one species (Abdopus aculeatus) being routinely terrestrial huntingcrabs among thetidal pools ofrocky shores.[citation needed]
Aquatic animals play an important role for theenvironment asindicator species, as they are particularly sensitive to deterioration inwater quality andclimate change.Biodiversity of aquatic animals is also an important factor for thesustainability ofaquatic ecosystems as it reflects thefood web status and thecarrying capacity of the localhabitats.[10] Manymigratory aquatic animals, predominantlyforage fish (such assardines) andeuryhalinefish (such assalmon), arekeystone species that accumulate and transferbiomass betweenmarine,freshwater and even toterrestrial ecosystems.
Aquatic animals are important tohumans as a source offood (i.e.seafood) and asraw material forfodders (e.g.feeder fish andfish meal),pharmaceuticals (e.g.fish oil,krill oil,cytarabine andbryostatin) and variousindustrial chemicals (e.g.chitin andbioplastics, formerly alsowhale oil). The harvesting of aquatic animals, especiallyfinfish,shellfish andinkfish, provides direct and indirectemployment to thelivelihood of over 500 million people indeveloping countries, and both thefishing industry andaquaculture make up a major component of theprimary sector of the economy.
TheUnited Nations Food and Agriculture Organization estimates that global consumption of aquatic animals in 2022 was 185 million tonnes (live weight equivalent), an increase of 4 percent from 2020. The value of the 2022 global trade was estimated atUSD 452 billion, comprising USD 157 billion for wildfisheries and USD 296 billion foraquaculture. Of the total 185 million tonnes of aquatic animals produced in 2022, about 164.6 million tonnes (89%) were destined for human consumption, equivalent to an estimated 20.7 kg per capita. The remaining 20.8 million tonnes were destined for non-food uses, to produce mainly fishmeal and fish oil. In 2022,China remained the major producer (36% of the total), followed byIndia (8%),Indonesia (7%),Vietnam (5%) andPeru (3%).[11]
Total fish production in 2016 reached an all-time high of 171 million tonnes, of which 88% was utilized for direct human consumption, resulting in a record-highper capita consumption of 20.3 kg (45 lb).[12] Since 1961 the annual global growth in fish consumption has been twice as high as population growth. While annual growth of aquaculture has declined in recent years, significant double-digit growth is still recorded in some countries, particularly inAfrica andAsia.[12]Overfishing anddestructive fishing practices fuelled by commercial incentives have reducedfish stocks beyond sustainable levels in many world regions, causing the fishery industry to maladaptivelyfishing down the food web.[13][14] It was estimated in 2014 that global fisheries were adding US$270 billion a year to globalGDP, but by full implementation ofsustainable fishing, that figure could rise by as much as US$50 billion.[15] UN Food and Agriculture Organization projects world production of aquatic animals to reach 205 million tonnes by 2032.[16]
Where sex-disaggregated data are available, approximately 24 percent of the total workforce were women; of these, 53 percent were employed in the sector on a full-time basis, a great improvement since 1995, when only 32 percent of women were employed full time.[16]
Aquatic animal are highlyperishable and several chemical and biological changes take place immediately after death; this can result inspoilage andfood safety risks if good handling and preservation practices are not applied all along the supply chain. These practices are based on temperature reduction (chilling andfreezing), heat treatment (canning,boiling andsmoking), reduction of available water (drying,salting and smoking) and changing of the storage environment (vacuum packing,modified atmosphere packaging and refrigeration). Aquatic animal products also require special facilities such as cold storage and refrigerated transport, and rapid delivery to consumers.[16]
In addition tocommercial andsubsistence fishing,recreational fishing is a popularpastime in bothdeveloped anddeveloping countries,[17] and themanufacturing,retail andservice sectors associated with recreational fishing have together conglomerated into a multibillion-dollar industry.[18] In 2014 alone, around 11 million saltwater sportfishing participants theUnited States generatedUSD$58 billion of retailrevenue (comparatively, commercial fishing generated USD$141 billion that same year).[19] In 2021, the total revenue of recreational fishing industry in the United States overtook those ofLockheed Martin,Intel,Chrysler andGoogle;[20] and together with personnelsalary (about USD$39.5 billion) and varioustolls andfees collected byfisheries management agencies (about USD$17 billion), contributed almost USD$129 billion to theGDP of the United States, roughly 1% of the national GDP and more than the economic sum of 17U.S. states.[20]
This article incorporates text from afree content work. Licensed under CC BY 4.0 (license statement/permission). Text taken fromThe State of World Fisheries and Aquaculture 2024, FAO.
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