Aquatic animals are animals that live constantly under water as well as those that swim on its surface or plunge beneath it for their food.
Aquatic Members of the Animal Kingdom
Aquatic animals are found at almost every level of the animal kingdom. Most crustaceans are aquatic, but a few, such as the wood louse and the sand crab, are modified for life ashore. There are both aquatic and terrestrial forms among the mollusks, and many other mollusks illustrate a structural transition from life in the water to life on land.
The ascidians, or tunicates, are exclusively marine. Although the fishes are almost completely aquatic, a few, especially the air-breathing lung fishes, have a limited capability for life out of the water.
The transition from an aquatic habitat to a land habitat is seen in the individual life histories of the amphibians. In the frog, for example, the tadpole, or larval, form is fishlike with gills, but the adult frog has lungs and no gills. The black salamander of the Alps, whose life is entirely terrestrial, has no tadpole stage, but there is a gilled stage in the adult.
Among reptiles, there are so many aquatic forms such as turtles, lizards, snakes, and crocodiles. The absence of gill respiration in these forms, however, marks a progressive general adaptation to terrestrial life.
Whereas terrestrial amphibians, such as the tree frog, seek water to rear their young, which are gill-breathing tadpoles, reptiles such as the sea turtle, which are aquatic, return to the land to lay their eggs. In both cases, the place in which the young are reared indicates the ancestral habitat of the adult.
Although birds occupy an aerial habitat, the structure of some, such as the penguin, has become adapted to a type of life that is almost exclusively aquatic.
Some of the land mammals have returned to the water. Familiar examples are the whale, the sea cow, and the seal.
Structural Adaptation to Aquatic Life
Animals in the water are subjected to environmental influences that are different from those on shore. Consequently, aquatic animals differ from terrestrial animals in modes of motion, form, respiration, and body temperature. There is also variation in the aquatic habitat itself -in composition, currents, pressure, food, and oxygen. Because of these differences, aquatic animals have a great diversity in structure.
Modes of motion vary among aquatic animals from the swimming bell of the jellyfish, which contracts and expands in the tide, to the paddling of the frog and duck. These modes of motion are both adaptations to and necessary results of aquatic life. Similarly, the smooth and frequently fishlike form, especially of locomotive water animals, is a very noticeable adaptive result of the condition of aquatic life.
Respiration is by gills in aquatic animals that have never left the aquatic environment. The blood is usually oxygenated by traveling through vessels spread out in feathery gills that trap the oxygen dissolved in the water.
In terrestrial animal forms that have reverted to an aquatic habitat, respiration is by lungs and is accomplished at the surface of the water. (In some isolated cases of insects and spiders, the air is entangled in their hairs and conveyed into their submerged homes.)
Transitional methods of respiration can be observed in larval insects, crustaceans, and fishes on land with a minimum of water about their gills, and also in the air- and water-breathing fishes.
The body temperature of aquatic animals is not much higher than that of the surrounding medium, and these animals often survive even the freezing of the water. Warm-blooded vertebrates that have returned to an aquatic habitat, however, have various modifications, such as thick fur or plumage, waterproof varnish, or blubber, that serve as protection against the cold.