Aquatic animal

"Aquatic animals" redirects here. For the QI episode, see List of QI episodes.

Longfin sculpin (Jordania zonope)

Sperm whales, an example of air-breathing aquatic animals.

An aquatic animal is any animal, whether vertebrate or invertebrate, that lives in bodies of water for all or most of its lifetime. Aquatic animals generally conduct gas exchange in water by extracting dissolved oxygen via specialised respiratory organs called gills, through the skin or across enteral mucosae, although some are evolved from terrestrial ancestors that re-adapted to aquatic environments (e.g. marine reptiles and marine mammals), 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).

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 four main types of aquatic animals: neustons, planktons, nektons and benthos

The term aquatic can be applied to animals that live in either fresh water or salt water. However, the adjective marine is most commonly used for animals that live in saltwater, i.e. in oceans, seas, etc.

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, destructive fishing, marine pollution, hunting, and climate change. Many habitats are at risk 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.

Fresh water creates a hypotonic environment for aquatic organisms. This is problematic for some organisms with pervious skins or with gill membranes, whose cell membranes may burst 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 water 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 crabs and crayfish.

Air-breathing aquatic animal

In addition to water-breathing animals, e.g., fish, most mollusks, etc., the term "aquatic animal" can be applied to air-breathing marine mammals such as those in the orders Cetacea (whales, dolphins and porpoises) and Sirenia (dugongs and manatees), which cannot survive on land at all, 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.

Amphibians, while still requiring access to water to inhabit, are separated into their own ecological classification. The majority of amphibians (class Amphibia) 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.

Certain amphibious fish also evolved to breathe air to survive oxygen-deprived waters, such as lungfishes, mudskippers, labyrinth fishes, Arapaima (family Osteoglossidae) and walking catfish.

Most molluscs have gills, while some freshwater gastropods have evolved lungs (e.g. Planorbidae) and some amphibious ones have both (e.g. Ampullariidae). 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

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). They also have cultural significance in human societies by serving as the subjects of arts, literature and heraldry, as well as providing educational and recreational values in the form of aquaria and oceanaria.

See also

• Aquatic
• Aquatic ecosystem
• Aquatic locomotion
• Aquatic mammal
• Aquatic plant
• Freshwater snail
• Marine biology
• Marine invertebrates
• Marine mammal
• Terrestrial animal
• Terrestrial ecosystem
• Terrestrial locomotion
• Terrestrial plant
• Wetland – Land area that is permanently, or seasonally saturated with water
• Wetland indicator status
• Zoology