Water, dense and dim, shaped every part of a fish's body
Water is far denser than air, carries little dissolved oxygen and swallows light quickly. Those three facts explain much of how a fish is built, from its two-chambered heart and single loop of circulation to the lateral line along its flanks that feels tiny shifts in pressure nearby.
A fish body divides into head, trunk and tail, though the boundaries are often invisible from outside. Many are fusiform, the streamlined torpedo shape of fast swimmers, while others are eel-like, worm-like, tall and thin, or flattened from top to bottom. The skeleton is either cartilage or bone, built around a spine of light but strong vertebrae. Ribs attach to it, yet there are no limbs, and apart from the tail fin, the fins have no direct link to the backbone, held in place only by muscle.
Senses are tuned to life underwater. Fish eyes see only locally, and there is an inner ear but neither a middle nor an outer one. Low-frequency vibrations register through the lateral line, a row of sense organs running down each side. Blood passes from the heart through the gills and on around the body in one circuit.
Two broad designs stand out. Sharks and rays keep many ancient features: cartilage skeletons, usually five pairs of gill slits, a mouth on the underside, separate placoid scales and a cloaca, but no swim bladder. They lay a few large, yolky eggs. Bony fish show more derived traits: gills covered by a protective flap, a mouth near the snout tip, overlapping scales and a swim bladder that holds them at a steady depth. Most scatter huge numbers of small eggs into open water.
Some odd cases test the rules. Sturgeon keep their notochord, the stiff rod found in all chordates, into adulthood rather than replacing it with vertebrae. Hagfish lack a true backbone but have a skull, and were once excluded from vertebrates; molecular studies since 1992 place them beside lampreys, making them vertebrates after all.
Source: Fish anatomy
