Ocean saltwater covers more than three-quarters of Earth's surface; lakes, rivers, streams, ponds, canals, swamps, marshes, and other forms of freshwater cover vast expanses of the planet's surface as well. One of the most successful groups of animals that have evolved to fill all these habitats are fish.

There are two types of fish: a small group with skeletons made of cartilage (a bonelike elastic tissue less rigid than true bone) and an enormous group with skeletons made of bone (like that found in humans). Cartilaginous fish include the sharks, skates, rays, and dogfish. The remainder—more than 25,000 species (more than all other species of vertebrates combined)—are known as bony fish.

All fish are cold-blooded, meaning they do not have a constant body temperature but take on the temperature of the surrounding water. The majority of fish species have bodies that are streamlined; their bodies are covered with tiny, smooth scales that offer no resistance to a fish's movement through water. The scales themselves are covered with a slimy coating that further reduces friction. Additionally, a fish's external appendages (fins) have been reduced to produce minimal resistance to the water as they propel the fish through it. Fins fall into two categories: vertical fins, which occur individually, and paired fins. Examples of the vertical fins are a dorsal fin that runs down the middle of a fish's back and the anal fin that runs along its underside. Examples of paired fins are those that appear on either side of a fish's upper body, below and behind its eyes.

A diver swimming with a stingray in the Grand Cayman Islands. (Reproduced by permission of The Stock Market.)
The Stock Market

The form, size, and number of fins varies considerably according to an individual species' habitat and requirements. In fast-swimming species, such as the tuna or mackerel, the dorsal and anal fins have thin, sharp shapes that reduce friction. In puffer or porcupine fish, by contrast, the fins are greatly reduced—for use in short paddling movements. Other species, such as eels, have lost almost all traces of external fins and swim instead by rhythmic movements of their muscular bodies.

Another important adaptation made by fish is their swim bladder. The swim bladder is a chamber filled with air that allows a fish to remain at the same level in water while expending very little energy.

Fish breathe through structures known as gills. When a fish takes in water through its mouth, the flaps that cover its gills are closed. When the fish closes its mouth, the flaps open and water is expelled through the gills. In this process, oxygen dissolved in the water is absorbed into the fish's bloodstream.

Bony fish are either carnivorous (meat-eating), herbivorous (planteating), or both. And fish are, of course, one of the world's most popular foods. In island nations and countries with long coastlines, fish are a major part of the diet. They are also a healthful food since they are high in protein and low in fat content.

Cartilaginous fish

The cartilaginous fish—whose skeletons are made of cartilage—include both sharks and rays. An intriguing characteristic of sharks is the presence of tiny primitive teeth on their skin. These denticles are similar in some ways to human teeth, although much smaller in size. Thus the texture of a shark's skin is similar to that of fine sandpaper. Human swimmers can be badly cut by coming into contact with the skin of a shark. The skin of a ray, on the other hand, is entirely smooth except for the back or upper tail surface, where denticles have developed into large, strong spines.

The jaw teeth of both sharks and rays are, in fact, modified denticles. These teeth are lost when they become worn and are replaced by rows of new teeth from the space behind them. In some species of sharks, the jaw looks like an assembly line, with new teeth filling spaces immediately.

Like bony fish, both sharks and rays breathe through gills. They also have an opening called a spiracle on both sides of the head behind the eye. The spiracle allows water to flow through the gills without taking in large amounts of mud and sand. This adaptation is especially useful for rays, which often bury in the sand, and for sharks, which often rest on the ocean bottom. Unlike the bony fish, sharks and rays do not possess a swim bladder.

Cartilaginous fish are predatory: they feed on other animals, from zooplankton to shellfish to whales. And they themselves are sought after by humans as a food source. Shark meat, once marketed under the pseudonyms of flake and steakfish, is now popular worldwide. Shark fins have long been popular in Asia. Rays are considered delicacies in Great Britain and France, and thornback rays and flapper skates are often sold as sea trout.

[ See also Coelacanth ]

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