Which animals live in water

"Which animals live in water?" — more than you might think, and they don't all live there the same way. Some, like seahorses, octopuses and sea turtles, spend their entire lives in the sea. Others are semi-aquatic: hippos, crocodiles, otters and platypuses feed and shelter in water but also haul out onto land. And then there are the water birds and amphibians — ducks, penguins and frogs — built for a life split between the two.

Here are 13 of the best-known animals that live in or around water, each with a clear photo and the facts that make it special — from a one-and-a-half-tonne hippo to a snail no bigger than an apple.

At a glance: 13 animals that live in water

Animal	Group	Where it lives
Hippopotamus	Mammal	Rivers & lakes; grazes on land
Crocodile	Reptile	Fresh or salt water
Sea otter	Mammal	Cold coastal seas
Platypus	Mammal	Freshwater streams
Duck (Mallard)	Bird	Ponds, lakes & rivers
Penguin	Bird	Southern seas & coasts
Frog	Amphibian	Fresh water & damp land
Water snail	Mollusc	Ponds, rivers & paddies
Seahorse	Fish	Shallow seas & seagrass
Octopus	Mollusc	Seas & oceans
Sea snake	Reptile	Warm seas
Sea turtle	Reptile	Open ocean
Dolphin	Mammal	Seas & a few rivers

Hippopotamus

The hippopotamus is Africa's river giant — the third-heaviest land animal after the elephant and rhino. It spends the hot day almost fully submerged in rivers and lakes, its eyes, ears and nostrils set high on the head so it can breathe and watch while the rest of it stays cool underwater, then comes out at night to graze on grass. Oddly for such a water-lover it cannot truly swim: it walks and pushes off along the bottom. It is also fiercely territorial and, taken together, hippos kill more people in Africa than most large predators. Read more about the hippopotamus.

Crocodile

Crocodiles are large semi-aquatic reptiles found across the tropics of Africa, Asia, the Americas and Australia. Some species live in salt water and others in fresh, but all are built the same way for a life in water — a powerful tail that drives them along, and eyes and nostrils on top of the head so they can lie almost invisible at the surface before an ambush. In India the crocodile you are most likely to meet is the broad-snouted mugger, or marsh crocodile. Several of the world's crocodile species are endangered.

Sea Otter

The sea otter is a marine mammal, part of the same family as weasels, badgers and minks. It lives in the cold coastal waters of the North Pacific and rarely comes ashore, diving to the seabed to forage and then floating on its back to eat — often cracking a shellfish open on a stone balanced on its chest, one of the few animals that uses a tool. With no blubber to keep it warm, it relies on the densest fur of any animal, and will wrap itself in kelp so it doesn't drift while it sleeps. It remains classed as endangered.

Platypus

The platypus is one of the strangest animals alive: a semi-aquatic, egg-laying mammal from the streams of eastern Australia and Tasmania, with a duck-like bill, webbed feet and a beaver-like tail. It hunts underwater with its eyes, ears and nostrils shut, feeling for worms, shellfish and insects using thousands of sensors in its bill that pick up the faint electrical signals of its prey. The males even carry venomous spurs on their hind ankles. When the first specimen reached Britain in 1799, scientists thought someone had stitched a hoax together.

Duck

Ducks are the water birds most of us know best, and the mallard is the classic wild duck of ponds, lakes, rivers and town parks right across the Northern Hemisphere — the ancestor of nearly all farmyard ducks. Waterproof, oil-coated feathers and webbed feet make them at home on the surface, where they "up-end," tipping tail-up to reach plants and creatures on the bottom. The drake wears the glossy bottle-green head; the female is a mottled brown. Ducklings can swim within a day of hatching. Read more about the mallard.

Penguin

Penguins are flightless seabirds that live almost entirely in the southern hemisphere; only the Galápagos penguin ranges north of the Equator. Their wings have become stiff flippers, so they are clumsy waddlers on land but superb in the sea, effectively "flying" underwater to chase fish and squid. The emperor penguin pictured here is the largest of all, standing over a metre tall and breeding through the brutal Antarctic winter. A dense coat of feathers over a layer of fat keeps the cold water out.

Frog

Frogs are amphibians, and they live a double life. The eggs and tadpoles are fully aquatic — a tadpole breathes through gills and swims with a tail — while most adults split their time between water and damp land, breathing partly through skin that has to stay moist. Powerful back legs serve for both leaping and swimming, and the eyes sit high on the head so a frog can watch above the surface while its body hangs in the water. Because their thin skin absorbs whatever is in the water, frogs are an early warning sign of pollution and a mark of a healthy wetland.

Water Snail

Freshwater snails are gastropod molluscs that live in ponds, rivers, lakes and paddy fields all over the world. Some breathe dissolved oxygen through a gill, while others carry a simple lung and rise to gulp air at the surface. The one pictured is the golden apple snail, a big, tough species that has become a serious pest of rice crops in many countries. Snails spend their time grazing algae and soft plants, and in turn feed fish, birds and other animals — a small but important link in freshwater food chains.

Seahorse

The seahorse is a small marine fish — there are around 46 species in the genus Hippocampus — and it barely looks like a fish at all, with its horse-like head, a grasping tail instead of a tail fin, and eyes that swivel independently. Lacking that fin, it swims slowly and upright, fluttering a tiny dorsal fin and anchoring to seagrass with its tail to avoid being swept away. Its most famous trick is a role reversal: the female passes her eggs to the male, who carries them in a belly pouch and gives birth to the young.

Octopus

The octopus is an eight-armed cephalopod mollusc and one of the most intelligent animals in the sea. Having no bones, its soft body can pour through a gap barely wider than its beak, and it can change colour and texture in an instant to melt into rock or coral. It has three hearts and blue, copper-based blood, jets a burst of water to shoot backwards when startled, and can screen its escape behind a cloud of ink. In captivity, octopuses solve puzzles, open jars and clearly recognise the people who feed them.

Sea Snake

Sea snakes are highly venomous marine snakes of the cobra family, and there are more than 60 species. A flattened, paddle-like tail and a body that is compressed side-to-side make them powerful swimmers, and many spend their whole lives at sea. True sea snakes even give birth in the water; the sea kraits, like the banded krait pictured, are the exception — they come ashore to rest, digest and lay their eggs. For all their potent venom they are shy, and would far rather slip away than bite. They range across the warm Indian and Pacific oceans.

Sea Turtle

Sea turtles are ancient marine reptiles that spend almost their entire lives at sea, their legs long since turned into flippers. They must surface to breathe, but can rest underwater for hours between breaths. Females come ashore only to nest, hauling up the beach to bury their eggs — remarkably, on the very beach where they themselves hatched. The green turtle pictured grazes on seagrass and algae. Most of the world's sea turtle species are endangered, threatened by fishing nets, plastic waste and the loss of the quiet beaches they need to breed.

Dolphin

Dolphins are intelligent marine mammals: they breathe air at the surface, yet spend their whole lives in the water, hunting fish by echolocation — firing out clicks and reading the echoes. Most live in the sea in large, social pods, but a few live only in fresh water. India's own Ganges river dolphin, the country's National Aquatic Animal, is one of them — nearly blind in the muddy Ganga and Brahmaputra, it "sees" its river entirely by sound. It is endangered, threatened by dams, pollution and fishing nets.

More animal guides

Water life doesn't stop at these thirteen. The polar bear is a powerful long-distance swimmer, and water birds like the common kingfisher and the greater flamingo spend their lives around lakes, rivers and shores. For more, browse our illustrated A–Z of animals and A–Z of birds, or the big picture guide to the A–Z of bird names.

Photos via Wikimedia Commons under free licences: Hippopotamus — Charles J. Sharp (CC BY-SA 4.0); Crocodile — Giles Laurent (CC BY-SA 4.0); Sea otter — Rennett Stowe (CC BY 2.0); Platypus — Charles J. Sharp (CC BY-SA 4.0); Duck — Richard Bartz (CC BY-SA 2.5); Penguin — Ian Duffy (CC BY 2.0); Frog — Richard Bartz (CC BY-SA 2.5); Water snail — FlyingBatt (CC BY-SA 4.0); Seahorse — Diego Delso (CC BY-SA 4.0); Octopus — Diego Delso (CC BY-SA 4.0); Sea snake — Diego Delso (CC BY-SA 4.0); Sea turtle — Charles J. Sharp (CC BY-SA 4.0); Dolphin — Kukil Gogoi (CC BY-SA 4.0).