Fish come in many shapes, colors and sizes - in fact, 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 skeleton is made of cartilage) have the same basic body plan. (Click here for a larger diagram showing fish body parts
Fish have several types of fins, and they may have stiff rays in them to keep them upright. Here are the types of fish fins and where they are located:
- Dorsal fin: on the fish's back
- Anal fin: near the tail, on the underside of the fish
- Pectoral fins: a fin on each side of the fish, near its head
- Pelvic fins: a fin on each side of the fish, on the underside near its head
- Caudal fin: the tail
Depending on where they are located, a fish's fins may be used for stability and hydrodynamics (e.g., the dorsal fin and anal fin), propulsion (e.g., caudal fin), and/or steering (e.g., the pectoral fins)
Fish have gills for breathing - the fish inhales water through its mouth, then closes the mouth, forcing water over the gills, where hemoglobin in the blood circulating in the gills absorbs dissolved oxygen in the water. The gills have a gill cover, or operculum, though which the water flows out.
Most fish have scales covered with a slimy mucus that helps protect them. There are different scale types:
- Ctenoid scales, which have a rough, comb-like edge
- Cycloid scales, which have a smooth edge
- Ganoid scales, which are thick and made of bone covered with an enamel-like substance
- Placoid scales, which are like modified teeth and give the skin of elasmobranchs a rough feel.
Lateral Line System:
Some fish have a lateral line system, which is a series of sensory cells that detect water currents and depth changes. In some fish, this lateral line is visible as a line that runs from behind the fish's gills to its tail.
Many fish have a swim bladder, which is used for buoyancy. The swim bladder is a sac filled with gas 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.