An aquatic animal is any animal, whether vertebrate or invertebrate, that lives in a body of water for all or most of its lifetime.Biology Online Dictionary: "Aquatic" Aquatic animals generally conduct aquatic respiration by extracting dissolved oxygen in water via specialised respiratory organs called gills, through the skin or across enteral mucosae, although some are secondarily aquatic animals (e.g. marine reptiles and marine mammals) evolved from terrestrial ancestors that re-adapted to aquatic environments, in which case they actually use lungs to breathe air and are essentially holding their breath when living in water. Some species of gastropod mollusc, such as the eastern emerald sea slug, are even capable of kleptoplastic photosynthesis via endosymbiosis with ingested yellow-green algae.
Almost all aquatic animals reproduce in water, either oviparously or viviparously, and many species routinely migrate between different water bodies during their life cycle. Some animals have fully aquatic life stages (typically as eggs and larvae), while as adults they become terrestrial or semi-aquatic after undergoing metamorphosis. Such examples include amphibians such as frogs, many flying insects such as mosquitoes, mayflies, dragonflies, damselflies and caddisflies, as well as some species of cephalopod molluscs such as the algae octopus (whose larvae are completely planktonic, but adults are highly terrestrial).
Aquatic animals are a diverse polyphyletic group based purely on the natural environments they inhabit, and many morphological and behavioral similarities among them are the result of convergent evolution. They are distinct from terrestrial and semi-aquatic animals, who can survive away from water bodies, while aquatic animals often die of dehydration or hypoxia after prolonged removal out of water due to either gill failure or compressive asphyxia by their own body weight (as in the case of whale beaching). Along with aquatic plants, algae and microbes, aquatic animals form the food webs of various marine, brackish and freshwater aquatic ecosystems.
Description
The term "aquatic animal" can be applied to residents of either freshwater or saltwater environments. However, the adjective "marine" is most commonly used for animals that live in seawater (i.e. oceans and shallow seas) or sometimes brackish water (i.e. estuaries, lagoons, tidal rivers, etc), and "brine" is used for halophilic species that can tolerate hypersaline waterbodies.
Aquatic animals can be separated into four main ecological groups according to their positions within the water column. Neustons ("floaters"), more specifically the zooneustons, inhabit the surface ecosystem and use buoyancy and surface tension to stay at the water surface, sometimes with appendages hanging from the underside for foraging (e.g. Portuguese man o' war, chondrophores and the buoy barnacle). They only move around via passive locomotion, meaning they have vagility but no motility. Planktons ("drifters"), more specifically the metazoan zooplanktons, are suspended within the water column with no motility (most aquatic larvae) or limited motility (e.g. jellyfish, salps, larvaceans, and escape responses of copepods), causing them to be mostly carried by water currents. Nektons ("swimmers") have active motility that are strong enough to propel and overcome the influence of water currents. These are the aquatic animals most familiar to the common knowledge, as their movements are obvious on the macroscopic scale and the cultivation and harvesting of their biomass is most important to humans as seafoods. Nektons often have powerful tails, paddle/fan-shaped appendages with large wetted surfaces (e.g. fins, flippers or webbed feet) and/or jet propulsion (in the case of cephalopods) to achieve aquatic locomotion. Benthos ("bottom dwellers") inhabit the benthic zone at the floor of water bodies, which include both shallow sea (coastal, littoral and neritic) and deep sea communities. These animals include sessile organisms (e.g. sponges, sea anemones, corals, sea pens, sea lilies and sea squirts, some of which are reef-builders crucial to the biodiversity of marine ecosystems), sedentary filter feeders (e.g. bivalve molluscs, brachiopods, acorn worms, lancelets) and ambush predators (e.g. flatfishes and bobbit worms, who often burrow or camouflage within the marine sediment), and more actively moving bottom feeders who swim (e.g. demersal fishes) and crawl around (e.g. decapod crustaceans, marine chelicerates, octopus, most non-bivalvian molluscs, echinoderms etc.). Many benthic animals are algivores, detrivores and scavengers who are important basal consumers and intermediate recyclers in the marine nitrogen cycle.
Aquatic animals (especially freshwater animals) are often of special concern to conservationists because of the fragility of their environments. Aquatic animals are subject to pressure from overfishing/hunting, destructive fishing, water pollution, acidification, climate change and competition from invasive species. Many aquatic ecosystems are at risk of habitat destruction/fragmentation, which puts aquatic animals at risk as well. Aquatic animals play an important role in the world. The biodiversity of aquatic animals provide food, energy, and even jobs.
Freshwater aquatic animals Fresh water creates a hypotonic environment for aquatic organisms. This is problematic for organisms with pervious skins and gills, whose cell membranes may rupture if excess water is not excreted. Some protists accomplish this using contractile vacuoles, while freshwater fish excrete excess water via the kidney. Although most aquatic organisms have a limited ability to regulate their osmotic balance and therefore can only live within a narrow range of salinity, diadromous fish have the ability to migrate between fresh and saline water bodies. During these migrations they undergo changes to adapt to the surroundings of the changed salinities; these processes are hormonally controlled. The European eel (Anguilla anguilla) uses the hormone prolactin, while in salmon (Salmo salar) the hormone cortisol plays a key role during this process.
Freshwater molluscs include freshwater snails and freshwater bivalves. Freshwater crustaceans include freshwater shrimps, crabs, crayfish and copepods.
Air-breathing aquatic animals In addition to water-breathing animals (e.g. fish, most molluscs, etc.), the term "aquatic animal" can be applied to air-breathing secondarily aquatic tetrapods who descended from terrestrial vertebrates and have evolved and fully adapted to aquatic life. The most proliferative extant group are the marine mammals, such as cetaceans (whales, dolphins and porpoises, with some freshwater species) and sirenians (dugongs and manatees), who are too evolved for aquatic life to survive on land at all (where they will die of beaching), as well as the highly aquatically adapted but land-dwelling pinnipeds (true seals, eared seals and the walrus). The term "aquatic mammal" is also applied to riparian mammals like the river otter (Lontra canadensis) and beavers (family Castoridae), although they are technically semiaquatic or amphibious. Unlike the more common gill-bearing aquatic animals, these air-breathing animals have lungs (which are homologous to the swim bladders in bony fish) and need to surface periodically to change breaths, but their ranges are not restricted by oxygen saturation in water, although salinity changes can still affect their physiology to an extent.
There are also reptilian animals that are highly evolved for life in water, although most extant aquatic reptiles, including crocodilians, turtles, water snakes and the marine iguana, are technically semi-aquatic rather than fully aquatic, and most of them only inhabit freshwater ecosystems. Marine reptiles were once a dominant group of ocean predators that altered the marine fauna during the Mesozoic, with some clades such as ichthyosaurs evolving to become very fish-like, although most of them died out during the Cretaceous-Paleogene extinction event and now only the sea turtles (the only remaining descendants of the Mesozoic marine reptiles) and sea snakes (which only evolved during the Cenozoic) remain fully aquatic in saltwater ecosystems.
Amphibians, while still requiring access to water to inhabit, are separated into their own ecological classification. The majority of amphibians — except the order Gymnophiona (caecilians), which are mainly terrestrial burrowers — have a fully aquatic larval form known as tadpoles, but those from the order Anura (frogs and toads) and some of the order Urodela (salamanders) will metamorphosize into lung-bearing and sometimes skin-breathing terrestrial adults, and most of them may return to the water to breed. Axolotl, a Mexican salamander that retains its larval external gills into adulthood, is the only extant amphibian that remains fully aquatic throughout the entire life cycle.
Certain amphibious fish also evolved to breathe air to survive oxygen-deprived waters, such as lungfishes, mudskippers, labyrinth fishes, bichirs, arapaima and walking catfish. Their abilities to breathe atmospheric oxygen are achieved via skin-breathing, enteral respiration, or specialized gill organs such as the labyrinth organ and even primitive lungs (lungfish and bichirs).
Most molluscs have gills, while some freshwater gastropods (e.g. Planorbidae) have evolved pallial lungs and some amphibious species (e.g. Ampullariidae) have both. Many species of octopus have cutaneous respiration that allows them to survive out of water at the intertidal zones, with at least one species (Abdopus aculeatus) being routinely terrestrial hunting crabs among the tidal pools of rocky shores.
Importance Environmental Aquatic animals play an important role for the environment as indicator species, as they are particularly sensitive to deterioration in water quality and climate change. Biodiversity of aquatic animals is also an important factor for the sustainability of aquatic ecosystems as it reflects the food web status and the carrying capacity of the local habitats. Many migratory aquatic animals, predominantly forage fish (such as sardines) and euryhaline fish (such as salmon), are keystone species that accumulate and transfer biomass between marine, freshwater and even to terrestrial ecosystems.
Importance to humans
As a food source
Aquatic animals are important to humans as a source of food (i.e. seafood) and as raw material for fodders (e.g. feeder fish and fish meal), pharmaceuticals (e.g. fish oil, krill oil, cytarabine and bryostatin) and various industrial chemicals (e.g. chitin and bioplastics, formerly also whale oil). The harvesting of aquatic animals, especially finfish, shellfish and inkfish, provides direct and indirect employment to the livelihood of over 500 million people in developing countries, and both the fishing industry and aquaculture make up a major component of the primary sector of the economy. In 2024 aquaculture produced 103 million tonnes of aquatic animals, an all-time record, equivalent to 53% of global aquatic animal output, and over 59% of aquatic animal foods for human consumption.
World catches of aquatic animals in inland waters recorded a new high of 12 million tonnes in 2024, an increase of 5.9% compared to the average of the previous three years. The top five producers of aquatic animals in inland waters (India, Bangladesh, China, Myanmar and Uganda) accounted for 51% of the total. The three major freshwater species groups were “carps, barbels and other cyprinids” (2 million tonnes), “tilapias and other cichlids” (1 million tonnes) and “shads” (0.3 million tonnes). However, these figures may be underestimated as many countries continue to face major difficulties collecting accurate data for inland fisheries.