The animals living in sea, rivers, lakes ponds, pools and ditches are known as aquatic animals. These are broadly divided in two types:
- Primary Aquatic Adaptations in Animals
- Secondary Aquatic Adaptations
Primary Aquatic Adaptations in Animals
Animals living and performing various initial activities essential for life in water are called aquatic animals. Aquatic environment includes such water bodies as ditches, ponds, lakes, streams, rivers and seas. Fishes have been derived form aquatic ancestors. These animals always live in water and have following aquatic adaptations:
- Body Contour - The body of the fishes is generally spindle or boat shaped with compressed head and a typical snout. This provides very little resistance while swimming in water.
- Presence of Fins - The fish bears two paired fins (pectoral fins and pelvic fins). and the median fins. The pectoral fins and dorsal fin act as balancers or stabilizers and the caudal fin gives a forward push to the body.
- Musculature - Body muscles in aquatic forms are arranged in the form of bundles separated by myccommata. Their contraction produces lateral undulations of the body.
- Skin - The skin in these animals is fully adapted to aquatic environment. It either possesses abundant mucous glands which secrete mucus or the scales surrounded the entire body surface except on fins and orifices. These mucous glands or scales provide a sort shield to prevent direct diffusion of water from in or out of the general body of the fishes.
- Lateral Line System - The presence of lateral-line system in fishes is directly linked with their aquatic mode of life. Special cells neuromasts in lateral- line system act as rheoreceptos. This system helps in balancing the animal and perceiving the objects in the surrounding.
- Gills - Most fishes respire by means of the gills they develop. Fishes can conveniently take dissolved oxygen from water with the help of gills rather than depending on lungs which are main respiratory organs of terrestrial animals. The gills in fishes are constantly bathed with water. The oxygen dissolved in water diffuses into the blood flowing regularly in these organs.
- Air bladder - Most of the bony fishes possess swim bladder or air bladder that serves as a hydrostatic organ. With the help of a swim bladder a fish can float at any level in the water. In some fishes, it is also associated with the production of sound.
- Nictitating Membrane - Nictating membrane is found in all aquatic vertebrates specially fish to protect the eyes from water and to make the vision perfect in the liquid environment.
- Osmoregulation - Fishes have the ability to regulate the contents of water inside their body at desired level irrespective of the concentration of salts in the aquatic medium where they live. This is a unique feature suited to aquatic mode of life.
Secondary Aquatic Adaptations
The secondary aquatic animals live permanently in water but most of them are amphibious in nature. They come out of water to deposit their eggs on the earth. Secondary aquatic animals belong to Catacea, Sirenia, Pinnipedia and Ichthyosauria. They exhibit following adaptations to live in water -
1. Body Contour - These animals also possess a stream-lined body. The neck constriction disappears and the tail enlarges. IN cetacean and sirenians even the traces of limbs and external ears have disappeared.
2. Shortening of Neck - IN aquatic animals the neck is shortened and has lost the power of mobility. IN wales cervical vertebrae are coalesced to form a solid compressed mass of bone.
3. Integument - The skin of secondary aquatic animals is smooth and naked, since the scales, hairs or other exoskeletal structures are lost. The armour is absent in Ichthyosarus. In Ceataceans and Sirenians even the sweat glands and sebaceous glands are absent. Below the skin is found a thick layer of fat, called blubber, It provides buoyancy and reduces loss of that from the body.
4. Limbs - In aquatic amphibians, the hind limbs act as organs of propulsion and are longer than the forelimbs. Their digits are webbed in amphibians ant he aquatic birds (duck, geese, etc.). In mammals (Cetacea and Sirenia) the hind limbs are lost. The Forelimbs are modified into paddles or flippers.
5. Fins - The forelimbs are modified into flippers, dorsal fin is present in killer whale, It is in the form of a fold integument. The tail is flattened with a caudal find, which is laterally expanded and blobbed in cetaceans and rounded or rhomboidal in sirenians. The storks of caudal find propel the body forward.
6. Skeleton - The skeleton of different parts of the body is modified to cape with the aquatic environment. The bones in general are light and spongy. In whales their interstices are filled with oil.
- Skull - The is greatly modified. The cranium is shortened and dorsoventrally flattened. The facial portion is produced into an elongated and slender snout or rostrum (Porpoises and Ichthyosaurs).
- Vertebral column - The resignation of vertebral column is lacking in aquatic animals. In Ichthysoaurs vertebrae are simple and amphicoelous (as in fishes). Additional articulations like zgapophyses become reduced. The neck region is short. In cetaceans the cervical vertebrate are coalesced. The sacral portion of vertebral column is reduced because it does not support the hind limbs as in terrestrial forms. Chest tends to be cylindrical. The ribs are single headed and articulate with the centrum of vertebrae.
- Limb Bones - The limb bones are short and compact and free movement of individual limb bones is lacking. Articulation between the bones is firm. In cetaceans the manus is extremely modified. Hyperphalangy is observed in few forms. The hindlimbs are absent but their remnants are embedded in the flesh.
8. Teeth - In aquatic mammals, the power of mastication is lost except in sea-cows and walruses. Therefore, the coronoid process of mandible is reduced. The teeth are simplified and greater in number. Dentition is homodont in Whales but in Sperm whales teeth are present only on one jaw are replaced by ballen plates in Ballen whales.
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