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Air-breathing aquatic animals
Not all animals that live in water take oxygen from the water. Sometimes the water is so still and warm that the animals in it come to the surface to breathe air. There are many aquatic animals which breathe air from the surface. These include many aquatic insects and the aquatic mammals (whales and dolphins).
The aquatic insects which breathe air must come to the surface from time to time to breathe. Some of them, such as the diving beetles and the water boatman, can carry a supply of air with them when they dive. They do this by trapping a bubble of air. The diving beetles trap a bubble under their wing cases and the water boatman (shown in the illustration above) has special hairs on its body to hold the bubble.
Whales and dolphins are mammals which live all their lives in water and yet they breathe air at the surface. These animals are highly specialized for life in water. Their limbs are like fishes' fins and their bodies have the same shape as a fish's body but they do not have gills.
Whales and dolphins breathe through their noses which are found on the top of their heads. They are called blow holes. Even though they breathe air they can dive for long periods and to great depths. The record is held by the sperm whale which can dive for up to 30 minutes at a time and as deep as 1000 metres. For comparison, a man with his lungs full of air can dive to about 20 metres for only two minutes.
Water spiders breathe air. They use their silk to make a spherical container under the water, attaching the silk to water plants. When this is done they collect air from the surface and place it in the silk container. They even lay their eggs in this trapped air bubble.
The elephant seal can reach a depth of 200 metres and stay at that depth for 30 minutes. The king penguin can dive to a depth of 265 metres. The Weddell seal can maintain a depth of 40 metres for 60 minutes.
Pelicans and other diving birds have water repellent feathers and extra air sacs in their lungs which tend to make them float. They are able to reach depths in water by diving at speed from the air. The surface rapidly, however. A cormorant's dive usually lasts less than 30 seconds.