The Complete Anatomy of a Fish

Drawing of the anatomy of osteichthyes
De Agostini Picture Library/Getty Images

Fish come in many shapes, colors and sizes. There are thought to be over 20,000 species of marine fish. But all bony fish (fish that have a bony skeleton, as opposed to sharks and rays, whose skeletons are made of cartilage) have the same basic body plan. 

Piscine Body Parts

In general, fish have the same vertebrate body as all vertebrates. This includes a notochord, head, tail, and rudimentary vertebrae. Most often, the fish body is fusiform, so it is fast-moving, but it can also be known as filiform (eel-shaped) or vermiform (worm-shaped). Fish are either depressed and flat, or compressed to be laterally thin.

Fins

Fish have several types of fins, and they may have stiff rays or spines inside of them that keep them upright. Here are the types of fish fins and where they are located:

  • Dorsal fin: This fin is on the fish's back.
  • Anal fin: This fin is located near the tail, on the underside of the fish.
  • Pectoral fins: This fin is on each side of the fish, near its head.
  • Pelvic fins: This fin is found on each side of the fish, on the underside near its head.
  • Caudal fin: This is the tail.

Depending on where they are located, a fish's fins may be used for stability and hydrodynamics (the dorsal fin and anal fin), propulsion (the caudal fin), or steering with occasional propulsion (the pectoral fins).

Scales

Most fish have scales covered with a slimy mucus that helps protect them. There are different scale types:

  • Ctenoid scales: Have a rough, comb-like edge
  • Cycloid scales: Have a smooth edge
  • Ganoid scales: Thick and made of bone covered with an enamel-like substance
  • Placoid scales: Like modified teeth, they give the skin of elasmobranchs a rough feel.

Gills

Fish have gills for breathing. They inhale water through their mouths, then close their mouths and force water out over the gills. Here, hemoglobin in blood circulating in the gills absorbs dissolved oxygen in the water. The gills have a gill cover, or operculum, through which the water flows out.

Swim Bladder

Many fish have a swim bladder, which is used for buoyancy. The swim bladder is a sac filled with gas that is located inside the fish. The fish can inflate or deflate the swim bladder so that it is neutrally buoyant in the water, allowing it to be at the optimal water depth.

Lateral Line System

Some fish have a lateral line system, a series of sensory cells that detect water currents and depth changes. In some fish, this lateral line is visible as a physical line that runs from behind the fish's gills to its tail.