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• Cartilaginous fish

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...