As aquarists, it is essential to know fish anatomy or the morphology of fishes. Each part of the fish’s body plays an important role.

Knowing the name, location, and functions of the various features will enable you to provide better care for the marvelous creatures that add so much beauty to your life.

Fishes come in a wide range of forms. Many fish have special modifications shaped by the environment they live in. The morphology of a fish can tell us a great deal about their habitat and how they live.

Let us begin with the anatomical positions to understand the location of each feature.

Anatomical term Location
Anterior The head end or close to the head
Posterior The tail end; closer to the tail
Dorsal The back or the top
Ventral The belly or the front
Lateral The flank or the side of the body

Over time, like all other animals, fish have adapted for survival in their environment. While fish species may look different from one another, their outer body parts serve the same purpose.

Furthermore, the external anatomy features of a fish species can tell us a great deal about it. For instance, its habitat, the food it eats, and how it protects itself from predators.

The Body of the Fish

Fish are shaped by the habitat they live in. The body of a fish has three parts: the head, the trunk, and the tail. However, the division between the three may not always be apparent.

The body of a fast-moving fish is streamlined or fusiform. Some fish may be eel-shaped or filiform. Other fish are worm-shaped or vermiform. Fish are generally either compressed, i.e., laterally thin, or depressed, i.e., dorso-ventrally flat.

– Parts of a fish

The typical external anatomical features of fish include:

  • dorsal fin
  • anal fin
  • caudal fin
  • pectoral fins
  • ventral fins
  • gills
  • lateral line
  • nares
  • mouth
  • scales
  • body shape

The Fish Fins

The most distinctive features of a fish are its fins. Apart from the tail or caudal fin, fins are not directly connected with the spine. Fins are supported only by muscles, and their primary function is to help the fish move through the water.

Fins serve the same purpose as human limbs. They allow fish to balance themselves, turn, or protect themselves. Some fish, for instance, the flying fish and frogfish, use their fins for gliding or crawling.

Fins are either placed in a single line along the centerline of the fish, for instance, the anal fin, the dorsal fin, and the tail fin, or in pairs like the pectoral fins and ventral fins.

Pectoral fins help fish balance themselves. The top or dorsal fin is also used for balance, but its primary function is protection. The anal fin and ventral fin are located on the underside or belly of fish and help them maneuver and balance themselves.

– Median Fins

Median fins are single fins that run horizontally down the center of the fish’s body. The dorsal fin is a median fin positioned on the back of the fish. The caudal fin and the anal fin are also median fins situated on the side of the body.

The dorsal fin helps the fish retain its balance. This fin prevents the fish from rolling over if they turn sharply or stop suddenly. Dorsal fins can also fulfill other purposes. The lionfish use their dorsal fins to protect themselves.

Although most fish have one dorsal fin, some fish have two or even three dorsal fins.

The anal fin is the posterior bottom fin located behind the anus. Fish use this fin to steady themselves while swimming. This fish also helps fish to steer themselves, for instance, through crevices. Not all fish possess anal fins, and in some species, they are tiny.

– Caudal or Tail Fin

The caudal fin is commonly known as the tail fin. The primary purpose of this fin is to help propel the fish forward.

The caudal peduncle is located at the base of the caudal fin, where the strong swimming muscles of the tail are. The caudal fin and the caudal peduncle together push the fish through the water.

Different breeds have different caudal fins, enabling them to swim fast, slow, or greater distances.

– Paired Fins

Fish possess two sets of paired fins: the pectoral and the pelvic fins. Paired fins are aligned in pairs, similar to arms and legs in humans. Both the pelvic and the pectoral fins are paired fins.

The pectoral fins, which are perpendicular, are on the sides of the fish, generally just beyond the operculum. Pectoral fins serve the same purpose for fish as arms do for humans. These fins help when the fish changes direction and hovers above the seafloor.

The pelvic fins are located horizontally on the belly of the fish, behind the pectoral fins. Pelvic fins correspond to legs; they are connected with the pelvis of the fish. They help the fish to go up or down through the water. Fish use the pelvic fins when they need to turn sharply or to stop quickly.

Fish Heart

The heart of a fish is different from the heart of mammals. They have a two-chambered heart. The heart consists of one atrium to receive the blood and one ventricle to pump it out.

However, the fish heart has an entry and an exit compartment that can be considered chambers. As a result, the fish heart is also said to be four-chambered.

Unlike a mammal, blood moves through all four chambers sequentially in a fish heart. Venous blood enters a thin-walled sac known as the sinus venosus, then into the atrium, and finally through a thick-walled pump or the ventricle. The blood then flows out into the conus arteriosus or elasmobranchs or the bulbous arteriosus or teleosts, which sends the blood to the gills and the rest of the body.

The circulatory system in fish moves in a single circuit. Blood circulates from the heart to the gills and from the gills to the rest of the body.

The fish heart is located slightly behind and below the gills. The heart of slow-moving species is relatively small, while active, strong swimmers such as the Blue Mackerel have a larger heart.

Fish Intestine

The intestine is the part of the digestive tract where much of the food is digested. This organ is also where the nutrients are passed along so that the body can absorb them. Similar to other vertebrates, the intestines of fish are made up of two sections: the small intestine and the large intestine.

In bony fish, the intestine is shorter, approximately one and a half times the length of the fish’s body. It generally has small pouch-like structures along its length that increase the intestine’s overall surface area and improve the digestive process.

Piscivorous fish such as bass have relatively short intestines because their food is easy for the intestines to break down and digest. Herbivorous fish like tilapia need longer intestines because plant matter is generally fibrous and tough and fibrous. It is harder to break down and provide nutrients to the fish.

Some breeds, such as lungfish, sturgeons, and sharks, don’t have a small intestine as such. Hence, the digestive part of the stomach forms a spiral intestine that connects to the rectum.

Here the intestine itself is relatively straight, but there’s a long, spiral fold along the inner surface. This fold creates a valve-like structure that increases the length of the intestine along with its surface area and enables the intestine to work more efficiently.

In some fish, for instance, lampreys, the spiral valve in the intestine is tiny. The reason for this may be their diet, which doesn’t require much digestion. Hagfish, for example, don’t have a spiral valve. Digestion takes place along almost the entire length of the intestine, which is not subdivided into different sections.

Fish Spine

All fish have a skeleton made of cartilage and is soft or bone, which is hard. The central part of the fish skeleton is the spine which compares to the vertebral column of other vertebrates, including humans.

Cartilaginous fishes, such as rays, skates, sharks, and chimeras, have a soft skeleton. The rest are bony fish and have a stiff frame.

The primary function of the backbone and skeleton is to support and safeguard the vital inner organs, muscles, and other parts of the fish. The vertebral column gives the body of the fish its basic shape and supports all other bones.

The backbone, or the vertebral column, is hollow. It is a series of small bones called vertebrae that have a small hole. These tiny holes form the canal through which the spinal cord passes. The vertebrae hold the spinal cord and protect it from injury.

The spaces between the vertebrae make it flexible so that the backbone can bend and the nerves can reach all the organs and tissues in the body. The rib bones surround the body cavity. The rest of the bones support the rays and the spines.

Hagfish are the only breed of fish that we know of, thus far, that doesn’t have a backbone, even though they are classified as fish.

Fish Nares

All fish can smell. Although it may seem odd that fish have what appear to be nostrils, they do have nostrils. In fact, fish often have four nostrils. The function of the nostrils is not to help the fish breathe, and they do not open into the mouth or the gills.

The nostrils of a fish, actually organs of smell, are pretty sensitive. They have paired holes or nares situated on their snout, right over their mouths. Just below the nare openings are many tiny sacs or chambers under the skin that contain smell receptors. These are connected to the brain by nerves, allowing the fish to smell.

Water, carrying scents, moves through the nares as the fish swims or while they face into a current. Once the receptors detect a smell, the nerves send signals to the fish’s brain that interprets the scent. These smells may enable a fish to find food or warn them of danger.

Some fish, like sharks, catfish, and eels, have a well-developed sense of smell.

Internal Organs

Fish possess most of the usual internal organs except for a chest cavity and lungs. They have a stomach, a liver, a spleen, intestines, kidneys, and reproductive organs. The internal organs generally take up a minimal area towards the front of the body, so most of the fish’s trunk is actually its tail.

Most bony fish breeds have a unique internal organ called the swim bladder or air bladder. It is a hollow, gas-filled organ usually found in the abdomen, and it assists fish move up or down in the water.

When fish regulate the amount of air in the bladder, they can adjust the depth at which they float. They don’t need to swim continually, allowing fish to conserve energy by remaining suspended in the water. Certain fish species use the swim bladder to produce sounds.

When anglers want to release fish caught from very deep water, they sometimes need to remove the air from their swim bladder before returning to deep waters.

It is interesting to note that elasmobranchs, i.e., the shark and ray family members, do not have a swim bladder.


Fish anatomy is a fascinating subject. We hope we have helped you unlock some of the mysteries of those species we call freshwater aquarium fish. Here is a glance at some interesting facts.

  • The body of a fast-moving fish is fusiform.
  • Some fish may be filiform.
  • Other fish are vermiform.
  • In a fish’s heart, blood moves through all four chambers sequentially.
  • The fish’s swim bladder helps them stay afloat.
  • The nostrils of a fish, called nares, help them smell.

We have yet to learn much about aquarium fish since they are our pets and deserve our respect and the best possible care.

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