There are numerous animals that are equally at home in the water or on land. Some animals, including many species of crabs and amphibians, live primarily in the water but can walk on land when required. Others, such as beavers, crocodiles and most otters are semi-aquatic, meaning they are largely terrestrial but spend a lot of time in the water. Some amphibious and semi-aquatic animals can breathe underwater while others can only breathe on land.
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Amphibians such as frogs and newts usually begin their life cycles as larvae living entirely in the water. Many species continue to live in and around fresh water ecosystems on reaching adulthood, while others become mostly or entirely terrestrial. Adult amphibians have primitive lungs that allow them to breathe air and most can also take in oxygen through their skin. This is known as cutaneous respiration and requires the skin to be kept moist.
Crocodiles and alligators spend a lot of time in the water but will often venture onto land to bask in the sun. They will also build nests out of the water in which to lay eggs. Crocodiles can move on land using a slithering "belly crawl." They can also extend their legs into a more erect posture, allowing them to walk or run on land.
Some mammals, such as dolphins and whales, spend their entire lives in the water. Semi-aquatic mammals such as otters and beavers spend much of their time in the water but sleep, breed and breathe on land. Otters are skilled aquatic hunters while beavers famously build dams in order to create permanent ponds. They will then build lodges within the ponds, which have underwater entrances but are relatively warm and dry inside.
Most crabs are primarily aquatic, spending most of their time in the water. They use gills to extract oxygen from the water, in a similar way to fish. They can also survive for long periods on land and can draw oxygen from the air if they keep their gills moist. Some crabs live primarily on land but most terrestrial crabs must still migrate to release their larvae into the ocean.
Mudskippers are amphibious fish that can use their front or pectoral fins to walk on land. Like all fish, mudskippers have gills that they use to extract oxygen from the water. They also have a cavity that can store water, keeping their gills wet and effectively acting like a scuba diver's air cylinders in reverse. They can also take oxygen through their skin in a similar way to an amphibian. The skin must be kept moist for this cutaneous respiration to be effective. For this reason they are usually found in humid or inter-tidal habitats.
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