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Fish Anatomy 101

Head, shoulders, fins and tail. fish anatomy 101

Can fish see color? Do they drink water? Are you a 'fish out of water' when friends and family ask questions about your new pets? Understanding what healthy, happy fish look like will help you be a better pet parent and spot health problems before it's too late.

Similarities and Differences

What we have in common:

Eyes
While scientists don’t know everything about fish anatomy yet, it is believed that fish have eyes that work like ours. Scientists believe fish can see colors, and some even think that fish may even be able to see more colors than humans. It’s also possible that fish can see ultraviolet light, which means they can see not only what humans see, but also things we can't. Depending on what species of fish you have, their eyes can be located close or further away from their snout. Both are normal, and can actually give you an indication of what some fish like to eat.

Mouth
Unlike humans, fish use their mouths for both taste and smell. To aid with hunting meals, fish also possess chemoreceptors on their heads and snouts, which helps them track down their food via smell.

Ears
Fish do in fact have ears and can hear, though they mostly hear low-pitched sounds. Their ears have otoliths, or tiny bones on top of hairs in the inner ear, which help interpret vibrations as sound. Just like your ears, the inner ear also helps your fish balance and remain upright.

Skin
Like you, fish have skin, but you can probably already tell it’s a bit different from yours. It’s scaly and slimy, but it’s supposed to be that way. Like humans, fish have three main layers of skin: the epidermis (outside layer), dermis (middle layer) and hypodermis (innermost layer).

A fish’s scales are firmly attached to the dermis or middle layer of skin. The dermis is also home to pigment scales that give your fish their bright colors. Fish have a color-controlling nerve that, if pinched, will make a fish’s color become very bright in that area. The outer layer, or epidermis, covers the scales and contains the cells that produce the natural slimy layer, which protects the fish.

Fish don’t turn green when they’re seasick, but their skin is a good indicator of illness. Slime is good, but too much slime can mean something is wrong. Fungi, bacterial infections and parasitic infections can all affect fish skin. Healthy skin is smooth and free of bald patches, red or white spots and missing scales.

Did you know fish have skin that—just like you—can get sunburned? Unlike humans, fish don’t turn red when their skin is burned. Instead, they begin to lose their coloring and become pale when exposed to too much light. If you notice your little swimmers are getting pale, no need to lather on the SPF; just make sure your fish tank isn’t in direct sunlight or exposed to any ultra-violet rays.

Vent and Droppings
Everybody poops, even fish. While it isn’t pretty, waste can also be a clue to your fish’s health. Be sure to check your fish’s vent area to make sure it is not swollen or red and that no feces are hanging onto the vent, as this could be the sign of a problem. Waste should never be pale or bloody. Also, check the vent area for any worms, which are dark in color, tube-shaped and hang from the vent.

What we don’t have in common:

Gills
As you probably know, fish breathe through their gills, which filter oxygen out of the water. Saltwater fish drink water through their mouths and excrete the salt through their gills. Healthy gills are extremely important and should be red with no white spots. Gills can be susceptible to a number of parasites, so check for symptoms regularly.

Fins
Fish fins come in a variety of interesting shapes, sizes and colors and are part of what makes your fish look so cool. Different fins on the top, bottom and sides of your fish work together to move the fish through the water and change directions. The best way to recognize healthy fins is to check for any bloody streaks or raggedness. Make sure fins are intact and that your fish can move, using them to swim and not holding them close to the body.

Air Bladder
You might wonder what makes fish float and control their depth in the tank. How do they come to the surface to eat or sink to the bottom to swim laps around sunken treasure? Fish have an internal organ called an air bladder, or swim bladder, that they can fill with gasses to control their depth. Some experts even believe that vibrations in the fish's air bladder may enhance their hearing.

Now that you know what your fish should look like when they’re at their best, you’ll be able to spot a sick fish before any problems become serious.