Notes on the phenomenon of Adaptation in Aquatic Habitat

Aquatic habitats may be either freshwater like lakes, ponds, rivers, spring or saline (salt water) like oceans. A wide variety of organisms lives in these habitats. Sudden change in water temperature is not observed because water absorbs and loses heat gradually. Over 99.9% of the surface water is available as freshwater for various hum an needs. The aquatic habitat offers a variety of physicochemical factors. These include penetration of light (transparency), availability oxygen, resistance to motion (viscosity), pressure fluctuations, nutrients, etc.

To adjust to the prevailing conditions, aquatic organisms have some of these features.

1. They have special structures like floats, air cavities, air sacs which help them to float in water at the desired depth.

2. For movement, animals have special appendages, for example, fishes have streamlined body and fins. Plants do not have these special structures as they are either fixed or drift along with the water current.

3. Animals develop special coverings like shells, cuticle, waxy coating, scales, etc. which prevent rotting and decay.

4. Animals have the ability to feed in water.

5. For respiration and exchange of gases animals have developed special structures such as gills in fishes.

The plants which live in water are known as hydrophytes. Hydrophytes are mainly of the following types.

1. Free-floating: Some hydrophytes float freely on the water surface. They are not attached to the bottom soil. Common examples are Eichhornta, Pistia, Lemna, Wolffia

2. Rooted and Free-floating: In some hydrop-hytes like Nelumba, Nymphaea, the roots are fixed in mud and leaves remain floating due to long petiole.

3. Submerged and Rooted: Some aquatic plants like Hydrila, Vallisneria remain submerged in water and are rotted in soil. In Vallisneria, the stem is tuberous which bears ribbon-like leaves while Hydrila has a long stem with small leaves on the nodes.

4. Submerged and Floating Some hydrophytes like Ceratophytllum remain completely submerged in water and are not rooted in the mud. They have long stems and small pointed leaves.

5. Emergent and Rooted Some hydrophytes like Ranunculus. Typha grow in shallow water. Their roots are completely under water fixed in the soil while the stems are partly or completely exposed to air.

Adaptation in hydrophytes (Aquatic Plants)

The roots in hydrophytes become less significant due to the availability of plenty of water. Either they have no roots or have poorly developed roots. Root hairs and root caps are absent. Air cavities (aerenchyma) are found in the roots.

The stem is spongy, long and flexible. It floats horizontally in free-floating forms. The stem of the water lily or lotus plant (Nelumbo) is a rhizome which contains large boles for air.

Floating leaves are large, flat, circular or oval which float on water. Submerged leaves are thin, long, liner or ribbon-like. Waxy coating is found on leaves, as in lotus which does not allow it to shrink or get destroyed. They have long flexible and mucilage-covered petioles. Narrow and long leaves of Vallisneria provide least resistance to flowing water.

Take a lotus plant. Cut its stalk with the help of a blade. You will see large tunnels and holes. If you press the stalk, what do you see? Observe and think why these holes are there.

Keep you hand or foot immersed in wager for about half an hour. Your skin will wrinkle. The water lily plant (or a lotus plant) does not wrinkle while if remains in water throughout its life. This is because of its waterproof, oily leaf. You may compare this waterproof layer with the scales of fish.

Adaptation in Hydrocoles (Aquatic Animals)

If you look at the structure of fish, you will find many structural features which reflect their adaptations to aquatic mode of life.

1. It has a streamlined body and is spindle-shaped. It has laterally compressed head, body and tail.

2. The entire body is covered with waterproof scales. There is a mucous coating on the scales which reduces water tension.

3. At the sides and posteriorly there are fins which help in swimming faster. The tail functions like a rudder and helps in changing direction while swimming.

4. Gills are present on the lateral sides. These are respiratory organs capable of oxygen uptake from water.

5. Fishes have an air bladder which is a thin-walled, gas-filled sac lying dorsal two the alimentary canal. It helps in respiration, sound production and stabilizes any fluctuation in pressure.

Other aquatic animals also have some modifications. In frogs and ducks, locomotion is achieved by webs between gingers. Whales (aquatic mammals) do not have or skin glands like sweat and oil glands. The body of whale is streamlined in a reverse direction with a short rounded front and a rapidly tapering posterior part with horizontal fin. Such shape helps them in locomotion in water. A thick layer of fat called blubber in present underneath their skin which helps in floatation.