Fish are aquatic animals with an inner skeleton, including skull, ribs, and backbone. Most fish have bony skeletons, but shark and ray skeletons are made of rubbery cartilage. Fish extract oxygen from the water using GILLS, and swim using their tail and fins. A fish’s skin is covered with tough scales.
Superbly adapted to life in water, fish are found throughout the world’s oceans, from warm tropical seas to icy polar waters. Some fish dwell near the surface. Others live in the depths, where some use BIOLUMINESCENCE. Fish are also found in freshwater habitats such as rivers, lakes, and swamps.
Predatory sharks detect prey with the aid of keen sensors, which can pick up tiny traces of blood from several miles away. They home in on victims using electrosensors that detect tiny charges given off by the prey’s muscles. At close range, sharks use their eyesight to target prey.
Lampreys are parasites. They attach themselves to larger creatures using their suckerlike mouth and drink their blood. Lamprey saliva contains a natural anticoagulant that prevents a victim’s blood from clotting, so that the lamprey can continue to feed.
|There are over 26,000 species of fish—more than half of all the world’s vertebrates. Fish divide into three major groups.|
|The first group, and by far the largest, contains the bony fish. There are more than 25,000 species alive today.|
|The second group contains the 600 species of cartilaginous fish—sharks and rays.|
|The smallest group, with about 60 species, is also the most primitive. Its members, the hagfish and lampreys, have skeletons but no jaws.|
Like all animals, fish need a constant supply of oxygen to survive. They do not breathe air but extract dissolved oxygen from the water using their gills—feathery organs located behind the eyes and supplied with many tiny blood vessels.
Water containing dissolved oxygen is drawn in through the fish’s mouth, to pass over four or five sets of gills on either side of the head. The gill arches hold delicate, flaplike membranes with very thin walls. Oxygen passes through these membranes into the fish’s bloodstream, to be distributed around the body.
Little light from the surface reaches the twilight zone in the ocean depths below 660 ft (200 m). However, over 1,000 species of fish that live there are bioluminescent—able to produce their own natural light.
Bioluminescence has several uses. Deep-sea anglerfish dangle a glowing lure in front of their jaws to attract prey. Other species use light to identify mates. A few even use it for camouflage—lights on the underside of the body help fish blend in with the small amount of light filtering down. Many fish nearer the surface have light-colored bellies for the same reason.