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FISH

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.

WHERE DO FISH LIVE?

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.

TROPICAL FISH

Most fish that live on tropical coral reefs are colorful and have striking markings. Coral reefs are very crowded places. Scientists think that the bright colors and patterns may help fish to recognize others of their own kind.

HOW DO SHARKS HUNT?

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.

ARE THERE ANY PARASITIC FISH?

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.

FISH CLASSIFICATION

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.

GILLS

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.

HOW DO FISH BREATHE UNDER WATER?

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.

DO ANY FISH LIVE ON LAND?

No fish live on land, but some can survive out of water for years. For example, when the pool or lake where they live dries up, lungfish can survive while buried in the mud, extracting oxygen from the air.

MUDSKIPPER

Mudskippers come out onto muddy shores to graze algae. They keep their gills puffed out with water, returning every so often for a refill. Mudskippers wriggle over the mud using their pectoral fins.

BIOLUMINESCENCE

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.

WHAT IS THE PURPOSE OF BIOLUMINESCENCE?

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.

FIND OUT MORE

Cnidarians
Fishing
Oceans

Copyright © 2007 Dorling Kindersley

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