s,
planktons,
nektons and
benthos 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 ==