They have to breathe in oxygen that is dissolved in water or from the air above the water , using these structural adaptations:
Gills Animals that breathe in dissolved oxygen usually have gills . When oxygen-rich water passes over the gills, the dissolved oxygen is absorbed and carbon dioxide is given out.
Gill Chambers Some animals breathe with gills but they can also stay out of water for short periods of time. This is because they have gill chambers to store water for breathing on land.
Skin Some animals use their skin to take in oxygen that is dissolved in water. Such skins are usually thin and soft and must be kept wet all the time. Examples: aquatic worms such as tubifex worms and flatworms, amphibians such as frogs and toads.
Air Tube Many aquatic insects cannot take in dissolved oxygen. They have to take in oxygen from the air. They have air tubes or breathing tubes that extend from their bodies and stick out above the water surface.
Air Bubble Other aquatic insects have adapted wings, legs, or hairy bodies to trap air bubbles . In this way they carry their own supply of air and can dive into the water to hunt for food.
Special nostrils Mammals can only breathe in oxygen from the air through their lungs . Mammals that spend a lot of time in water have nostrils at the tip of their snouts.
Special nostrils When they want to breathe, they push their nostrils above the water surface. When the animals are underwater, they shut their nostrils to keep water out.
Blowhole Some aquatic mammals have special nostrils called blowholes on top of their heads. The blowholes enable them to breathe even when most of the body is underwater – only with the blowhole above the water surface.
Summary Adaptations for breathing underwater dugongs, crocodiles, dolphins, whales (dolphins' and whales' ‘nostrils’ (blowholes) are located on the top of their heads) Special nostrils that can be shut mosquito larvae, water scorpions Air tubes/Breathing tubes frogs, flatworms, amphibians Skin fishes, crabs, shrimps, tadpoles Gills Examples Breathing Organ
Many aquatic animals with lungs have special adaptations in their bodies, capable of tolerating higher concentrations of carbon dioxide in their blood than most other breathing animals. This enables them to use oxygen more efficiently and stay underwater longer.
Some animals like the diving beetle and water spider trap air under the wings or hairy body respectively. They carry these air bubbles underwater with them.