The External Anatomy of a Fish

If you’re someone who’s going to care for your own fish instead of having a service maintain your tank, you need to know a little bit about fish anatomy. Here are some of the basic – and not so basic – external fish parts.

Remember that unlike humans, whose basic form as a species is upright, fish range much more in shape, and even in which body parts they have. But, generally speaking, they’re similar enough and easy to identify.

The Shapes of Fish

• Fusiform. Bullet-shaped, the most common form.
• Truncate. Shortened fusiform like a triggerfish or cowfish.
• Serpentine. Eel or snake-like.
• Attenuate. Long and thin like a needlefish.
• Compressed. Disk-like to bullet-shaped and very thin, like an angelfish or butterfly fish, or very flat, like a flounder.

Fins

Perhaps the most familiar parts of a fish are its fins. Fins are not much more than sheets of skin that hang from the fish and are supported by a structure of rays to offer a fish stability when jetting through the water. These rays are made from organic material that can be soft or hard. Throughout evolution, some fins have evolved into protective devices like spines, or into a bait decoy that lures the prey closer to a hungry mouth.
The Extras

Some fish have these various decorations and adaptations, many do not.

• Cirri. Hair-like projections that extend from the forehead area (called the nape).
• Fin-spine. A hard fin ray that provides not only support to the fin, but also a protective device.
• Barbel. A whisker-like projection or projections located under the mouth of a fish.
• Snout. The mouth-area of the fish.
• Nape. The forehead-area before the dorsal fin begins.
• Lateral line. A row of highly sensitive nerves aligned usually along the center of the body. These nerves help the fish detect water pressure changes and other environmental conditions.
• Dorsal fins. These can be single or in pairs. These fins are on the top and toward the front of the body when seen from the side. Sometimes, there is a second dorsal fin behind it – if so, the plain dorsal fin gets promoted to first dorsal fin.
• Pectoral fins. In some fish these fins, found on opposite sides of the fish behind the gill slit or close to them, have adapted to something more like arms.
• Ventral fins. One or more fins located on the bottom of the fish when seen in profile.
• Anal fin. A fin that is positioned very close to where the tail begins and opposite the second dorsal fin. Not all fish have this fin.
• Finlets. Some fish, like tuna, have a small series of fins that run along behind a bigger dorsal fin.
• Gill slits. The obvious slits that are the opening to the gills. They commonly denote where the head ends and the body begins.
• Gill-plate cover. The “flap” of skin that covers the gills.
• Scales. Fingernail-like plates that cover the skin of many fish. Some scales are bigger, and more obvious, than others.

The tail fin, also called the caudal fin, can be either a single fin or forked. If it is forked, the upper and lower parts of the fork are called the upper and lower lobes, respectively.

• Border of tail. The lower and upper parts of a tail fin that make up part of the “fan” of the tail.
• Margin of tail. The edge of the tail fin that lies on the end or outside of the tail fin “fan.”
• Base of tail. The boundary where the body of the fish ends and the tail fin begins.