Fish are mostly slim, streamlined animals that live in water. Many are covered in tiny shiny plates called scales. Most have bony skeletons and a backbone.
There are well over 21,000 species of fish, ranging from the 8 mm-long pygmy goby to the 12 m-long whale shark.
Fish are cold-blooded.
Fish breathe through gills – rows of feathery brushes inside each side of the fish’s head.
To get oxygen, fish gulp water in through their mouths and draw it over their gills.
Fish have fins for swimming, not limbs.
Most fish have a pectoral fin behind each gill and two pelvic fins below to the rear, as well as a dorsal fin on top of their body, an anal fin beneath, and a caudal (tail) fin.
Fish let gas in and out of their swim bladders to float at particular depths.
Some fish communicate by making sounds with their swim bladder. Catfish use them like bagpipes.
Angling (catching fish) is a popular pastime all around the world. The fish is hooked as it bites the lure or bait.
The arapaima lives in the swampy parts of tropical south America. It can breathe in the normal fish way using its gills or gulp down air. It can grow to a vast 3 m in length and weigh up to 200 kg.
Nearly 75% of all fish live in the seas and oceans.
The biggest, fastest swimming fish, such as swordfish and marlin, live near the surface of the open ocean, far from land. They often migrate vast distances to spawn (lay their eggs) or find food.
Many smaller fish live deeper down, including seabed-dwellers like eels and flatfish (such as plaice, turbot and flounders).
The blue-fin tuna can grow to a massive 4 m in length and 700 kg in weight. It lives in all seas and oceans hut moves around with the seasons.
Flying fish beat their tails so fast they are able to fly’away from predators.
Flatfish start life as normal-shaped fish. As they grow older, one eye slowly slides around the head to join the other. The pattern of scales also changes so that one side is the top and one side is the bottom.
Plaice lie on the seabed on their left side, while turbot lie on their right side. Some flounders lie on their left and some on their right. The upper side of a flatfish is usually camouflaged to help it to blend in with the sea bed. In the temperate waters of the Atlantic there are rich fishing grounds for fish such as herring. The swordfish can swim at up to 80 km/h. It uses its long spike to stab squid. The bluefin tuna can grow to as long as 3 m and weigh more than 500 kg. It is also a fast swimmer – one crossed the Atlantic in 199 days.
Flying fish can glide over the sea for 400 m and soar up to 6 m above the waves.
Deep-sea anglerfish live deep down in the ocean where it is pitch black. They lure prey into their mouths using a special fishing-rod-like fin spine with a light at its tip.
Anglerfish cannot find each other easily in the dark, so when a male meets a female he stays with her until mating time.
Hatchet fish have giant eyeballs that point upwards so they see prey from below as silhouettes against the surface.
The viperfish looks fearsome and is one of the larger predators of the ocean depths. Yet it is only 30 cm long. The general lack of food in the deep means animals are mostly small.
Viperfish shine in the dark, thousands of metres down, and look like a jet airliner at night, with rows of lights along their bodies.
Siphonophores are colonies of tiny creatures that live in the deep oceans. They string themselves together in lines 20 m long and glow – so they look like fairy lights.
The cirrate octopod looks like a jelly because its skin is 95% water – the water cannot be crushed by the intense pressure of the deep oceans where the octopod lives.
The weedy seadragon of Australia is a seahorse, but it looks just like a piece of flapping seaweed.
The sleeper shark lives in the freezing depths of the North Atlantic and Arctic Oceans. This shark is 6.5 m long, but very slow and sluggish.
Flashlight fish have light organs made by billions of bacteria which shine like Headlights. The fish can suddenly block off these lights and change direction in t he dark to confuse predators.
In the Arab-Israeli War of 1967 a shoal of flashlight fish was mistaken for enemy and blown right out of the water.