Fish Anatomy 101: Internal and External Parts of a Fish

You are probably aware that fish comes in different colors, sizes, and have parts like the gills, fins, and scales. But, do you know that there are unique parts of a fish that are only specific to some species? If not, here’s a lesson about fish anatomy and how these appendages work together to make fish a fish. 

What Are the External Parts of a Fish?

Illustration of the external parts of the fish.

The external anatomy of a fish includes the mouth, scales, fins, nares, eyes, and gills. However, you should know that some have more external parts to compensate for their living environment. For instance, the sea dragon’s body shape mimics its habitat. On the other hand, butterflyfishes have spots on their body that looks like an eye to confuse the enemies. The most common external parts include:

Lateral Line

This is a visible line running down the fish’s length, consisting of fluid-filled sacs and hair-like elements that open to the water through microscopic holes. The lateral line’s primary purpose is to detect pressure changes and motion in the water. It also assists the fish in determining water movement direction for oxygen absorption and proper movement. The lateral line helps the fish detect nearby prey, predators, and other stationary objects in the water.


Fish uses the mouth for feeding purposes. The mouth’s shape, size, and placement speak more about their feeding habits. Those with superior mouths usually ambush their prey. On the other hand, fish like catfish have inferior mouths are known as bottom-feeders. Fish that has a terminal mouth are mid-water feeders, usually omnivores.


Although fish don’t breathe through noses, they have a pair of nostrils called nares, which detects odors in the water. 

Fish use their sense of smell to locate a mate, detect chemicals in the water and predators. For instance, if a predator injures its prey, it releases a scent that alerts other fishes to flee. The fish’s living environment also has unique scents that salmon use to get back to the streams they were born in. Species living in muddy water, like catfish, have a stronger sense of smell than those in clear streams.

Speaking of catfish, did you know that their “whiskers “are called barbels? 


Did you know that deep-sea fish have eyesight similar to a middle-aged human? Most species have well-developed eyes that help them zero in and focus on prey. Some of them can even differentiate colors. 

The lenses move back and forth to focus on near or far objects. Fish living in different habitats have different eye sizes and shapes. For instance, predator fish have forward-facing eyes that offer better depth perception. On the other hand, the prey fish have large eyes on the sides, which gives them a more extensive view to spot predators.


Scales are a protective layer that shields the skin against damage. The scales contain a layer of slime that protects the fish from infection caused by bacteria and parasites in the water.

Some even have serrated scales for self-defense, others have smooth scales, and some, like eels, have no scales. Scales also reduce water-resistant resistance and friction, which aids in swimming.

Operculum/Gill Cover

The operculum is a bony plate whose central role is to cover and protect the sensitive gills. It also serves a role in respiration. The gill cover opens up and closes to facilitate water entry, allowing the fish to acquire the dissolved oxygen in the water. It also marks the division between the fish’s head and the body.


Wondering where fish’s waste goes out after the digestive system? It’s through the vent. This part is not only for removing waste, however. It is also the outlet for sperm in males and eggs in women. 


Paired fins are the most distinctive features of a fish. For instance, you’ll find sharp fins and an adipose fin in catfish and short and countershading dorsal in tunas. 


The tail fin’s primary role is to give the fish forward propulsion and speed. Species with crescent-shaped caudal fins swim continuously for long distances, while the forked caudal fin helps with rapid swimming. The slow swimming fish have rounded fins that also aid in maneuvering, while the truncated caudal allows the fish to turn quickly.


Pectoral fins are usually on either side, just past the operculum. Their primary purpose is to help propulsion, enable side-by-side movement, and offer support while swimming.  


The pelvic fins are usually at the lower front, and they aid instability. Fish also use the fins to slow down or stop. In some species like the clingfishes, pelvic fins help them hold onto stationary objects under the water.  


These are the fins located at the back. It also gives the fish stability as it takes a sharp turn or suddenly stops.


The anal fin lies just behind the anus. It supports the dorsal fin, making the fish stable while swimming, and controls rolling motion. In the bony fish, anal fins help in reproduction.

Fun fact: Tunas use their caudal fin most of the time, but it’s their pectoral fins that they rely on for maneuvering and steering. 

What Are the Internal Parts of a Fish?

A person using a knife to slice open a fish.

The internal fish organs include the spine, kidneys, heart, brain, stomach, and intestines. They perform essential body functions like respiration, circulation, reproduction, impulse transmission, and digestion. The major internal parts and their functions include:


As in other vertebrates, fish have a kidney whose primary function is to excrete nitrogenous waste. It filters out liquid waste from the blood, which later passes out of the body. Kidneys also regulate saltwater concentration to help fish survive in freshwater or high saline marine environments.


The fish liver produces fluids like albumin and bile that aid in digestion. It also removes toxins from the food consumed and cleans the bloodstream. The liver helps prevent blood coagulation. It also stores nutrients, fats, and carbohydrates and aids in xenobiotic metabolism.


Fish has a two-chambered heart that circulates blood throughout its body. The heart pumps the blood from the auricle to the ventricle, to the gills for oxygenation, and then to all other parts. The pumped blood transports nutrients and oxygen to the organs and carries waste products to the liver and kidneys for elimination.

Spinal Cord

The fish spinal cord connects the brain to the rest of the body. Its primary role is to coordinate sensory stimuli. This means it relays information to the brain and transports motor responses from the brain to the peripheral nerves.


The fish spine is the primary structural framework that connects the skull to the tail. It contains numerous hollow vertebrae that help protect the spinal cord. The spine prevents excessive strain while in motion and aids with tail movement.


Did you know that fish brains can process emotions and sensory information like humans? A fish can tell when a friend or enemy is around. The brain also controls muscle movement, vision, and hunger perception. Most fishes have a small brain-to-body ratio, but other species like sharks have massive brains with improved cognitive ability.

Swim Bladder

The swim bladder is a hollow, air-filled cavity located in the fish’s body cavity, which plays a vital role in neutral buoyancy.

When the bladder releases oxygen, the fish becomes less buoyant and can sink into deep water. By adjusting the amount of air in the bladder, the fish can suspend itself and maintain stability despite the changing external conditions.

Stomach and Intestine

The stomach is responsible for food digestion, while the absorption of nutrients takes place in the intestines. Fish that feed on plants have longer intestines since the plant matters are very fibrous and take longer to break down. On the other hand, the carnivores have relatively short intestines.


Gonads are the reproductive organ. They provide the sex hormones responsible for developing male or female characteristics. Male gonads are usually white or cream-colored, located in the abdominal cavity near the air bladder. Female gonads or ovaries look like a mass of eggs suspended from the dorsal wall of the abdominal cavity.


Gills are the gas exchange organs of the fish with a network of blood vessels that gives them a bright red color. Fish continuously take water through the mouth, which runs over the gills. The blood capillaries in the gills absorb dissolved oxygen from the water. Gills also take in oxygen from the air, absorb nutrients, and excrete waste into the water.

Pyloric Caeca

The pyloric caecum is an organ with finger-like projections located at the stomach and intestines intersection. It secretes hydrolytic enzymes that help indigestion. Pyloric caecum also serves as a fermentation chamber and increases the gut’s surface area. It plays a role in the absorption of the digested food too.


Fish contains skeletal, smooth, and heart muscles tissues. The skeletal muscles contract and relax to facilitate movement. On the other hand, the smooth muscles move the internal organs of the body and control blood and other fluids’ movement. The muscle is what humans eat, and it comprises the fillet.  

Summing Up

While we all know that have gills, fins, and scales, there are more parts than just the few known. The lateral line, swim bladder, operculum, heart, gonads, and spine all play a vital role in the survival and functioning of a fish. After reading the above guide, you should understand the behaviors and adaptations of different species based on their body parts.