The physical feature that helps fishes live in the water is "gills".
Just like us, fishes also need oxygen to survive. However, unlike us they do not have lungs. In case of animals (including human beings), lungs extract oxygen from the air we breathe in and supply it to our blood cells. In case of fishes, breathing air is not possible because they are under water. Fishes consume water and this water is expelled through the gills, but not before dissolved oxygen is extracted from it. The gills are specialized organs and are full of blood vessels. As the water is pushed out of fish's body (through gills), the dissolved oxygen mixes with the blood. And that is how fishes manage to get oxygen while living underwater.
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Fishes have following features which enable them to adapt to live in water.
- Fishes have a body contour which is spindle shaped and streamlined . This helps in movement by cutting through water easily. It has fins that helps it to change directions in its movement.
- Fish have slippery scales on their bodies that help prevent decay from water and also help in easy movement through water.
- Fish have gills which contain capillaries (fine blood vessels) that take up dissolved oxygen in the water for respiration and release carbon dioxide. A flap called operculum on each side of the head covers the gills, and allows air to enter the gills.
- Fishes have a hollow outgrowth of the alimentary canal, called air bladder or swim bladder which functions as an organ of flotation and accessory respiratory organ.The swim bladder works a little like a hot air balloon. The more oxygen it contains, the higher a fish will suspend or float in the water
- Fishes have a special row of sense organs called as lateral line systems extending the whole length of the body, which gives them the ability to hear low-frequency sounds. A fish’s ears are located beneath the skin on either side of the head. The lateral line senses pressure of water and is used to detect movement and vibration in the water. Lateral lines serve an important role in detecting prey or predators.