aquatic

Living in water. The aquatic environment has several advantages for organisms, and all life on Earth originated in the early oceans. Dehydration is almost impossible, temperatures usually remain stable, and the density of water provides physical support.

Life forms that cannot exist out of water, amphibians that take to the water on occasions, animals that are also perfectly at home on land, and insects that spend a stage of their life cycle in water can all be described as aquatic. Aquatic plants are known as hydrophytes.

Invertebrates, fish, and reptiles Most of the lower invertebrates are truly aquatic; sea anemones, jellyfish, many of the annelids, even earthworms, and crustacea such as crabs and lobsters, cuttlefish, and molluscs such as mussels and cockles (with the exception of some gastropod molluscs, for example, landsnails), would perish if deprived of their aquatic environment. Among vertebrates, fish are exclusively formed for inhabiting a fluid medium; they breathe by gills, are covered with scales, their form is elongated and compressed, the eye is suited to the dense medium of the water, they balance themselves on fins, and the laterally compressed tail serves as a paddle. Some reptiles, the crocodiles and turtles, may be regarded as truly aquatic; they breathe air, however, and come on land to lay their eggs.

Aquatic birds and mammals Nearly all ‘aquatic’ birds have webbed, oared, or lobated feet, among them: grebes, auks, puffins, razorbills, geese, ducks, pelicans, gannets, and gulls; the penguin is as awkward on land as a seal, and as much at home in the sea. Mammals include the whales and porpoises which are truly aquatic, having a smooth and oily skin, a layer of insulating blubber covering the internal viscera, and are constructed to permit a long cessation of respiration while submerged; walruses and seals obtain their food in the sea, but breed and rest on rocks; their forelimbs are formed into paddles, the hind limbs are placed far back and are also paddles or oars, and every part of their internal structure is saturated with oil. Other aquatic mammals, for instance, the otter, are usually only web-footed and visit the water for prey.

Aquatic insects A few insects have aquatic immature stages, which usually live in fresh water, sometimes in brackish water. In certain orders there are species where all stages (egg, larva, and adult) live in the water. Members of 11 out of the 29 orders of the class Insecta have some aquatic stages. The order Collembola has two species in which all stages are aquatic: Hydropodura aquatica and Isotoma palustris. Both can be found on surface waters in Britain. Three orders, Ephemeroptera (mayflies), Odonata (dragonflies), and Plecoptera (stoneflies), have aquatic larvae, but the adults are terrestrial. In the orders Neuroptera (alder flies), Trichoptera (caddis flies), Lepidoptera (butterflies and moths), and Diptera (true flies), some species, but not all, have aquatic larvae. In Hemiptera (bugs), and Coleoptera (beetles), some members, for example, the water bugs, spend all stages of the life cycle in the water. Hymenoptera, the social insect order which includes the ants and bees, has some aquatic species of ichneumon fly: immature stages of Agriotypus are aquatic, and adults of Caraphractus and Prestwitchia.