Fish come in a wide variety of colors, shapes and sizes - there's the largest fish, the 60+ foot long whale shark, popular seafood fish such as cod and tuna, and completely different-looking animals such as seahorses, sea dragons and pipefish. In all, about 20,000 species of marine fish have been identified.
Fish swim by flexing their bodies, forming waves of contractions along their muscles. These waves push water backward and move the fish forward.
One of the most notable features of fish are their fins - many fish have a dorsal fin and anal fin (near the tail, on the underside of the fish) that provide stability. They may also have pectoral and pelvic (ventral) fins to help with propulsion and steering. They also have a caudal fin, or tail.
Most fish have scales covered with a slimy mucus that helps protect them. They also have gills for breathing - the fish inhales water through its mouth, which passes over the gills, where hemoglobin in the fish's blood absorbs oxygen.
Fish may also have a lateral line system, which detects movement in the water, and a swim bladder, which the fish uses for buoyancy. Learn more about fish anatomy.
The fishes are divided into two superclasses: Gnathostomata, or vertebrates with jaws, and the Agnatha, or jawless fishes.
- Class Elasmobranchii, the elasmobranchs: sharks and rays, who have a skeleton made of cartilage
- Class Actinopterygii, the ray-finned fishes: fish with skeletons made of bone, and spines in their fins (e.g., cod, bass, clownfish/anemonefish, seahorses)
- Class Holocephali, the chimaeras
- Class Sarcopterygii, the lobe-finned fish, the coelacanth and lungfishes.
- Class Cephalaspidomorphi, the lampreys
- Class Myxini, the hagfishes