Features and characteristics of the external structure of fish

The anatomy of underwater animals speaks of a high level of organization.


Fish are one of the most amazing aquatic animals. What features have allowed them to adapt to life in these conditions? From our article you will learn about the external structure of fish and their diversity.

The appearance of bony fish differs depending on the species. Common features of fish are the presence of an ossified skeleton, skin bones, and a special operculum. The structure of typical representatives of the bone world is generally reflected in the table of the structure of fish. It follows from the table that each organ, each feature in the form or external color of the skeleton carries a certain functional load.


No wonder they say about confident people: "Feels like a fish in water." These animals are unable to absorb oxygen from the air. Therefore, it is this environment that is comfortable for them. The only exception is a small group of lung-breathing fish. They have both gills and lungs. The latter allow them to experience unfavorable periods of drying up of water bodies and lack of oxygen.

Fish live in fresh and salt water. It depends on their type. So, gobies feel great even with an increase in salt concentration of 60%, and carp dies at the same time.

Fish are also adapted to different temperatures. This indicator is also individual. Californian Lucania prefers to live in water with a temperature of + 50. And dahlia, which lives in small streams in Chukotka, freezes and thaws along with the water.

Features of the external structure of fish

What conditions of the aquatic habitat should its inhabitants adapt to? First of all, this is friction and fluid pressure. Since these animals are constantly in motion, the body of the fish is streamlined. It consists of three sections: head, torso and tail. It is protected from mechanical damage by scales.

Fins provide movement and active movement in the water column. Paired are abdominal and thoracic, unpaired - dorsal, caudal and anal. Each of them performs its own functions. So, pectorals serve for slow movement forward and backward, and also allow you to change its trajectory. The dorsal and anal provide a constant position of the fish's body in the water column.

Cartilaginous fish

According to the structural features of fish, they are combined into two classes: Cartilaginous and Bone. The first is represented by sharks and rays. The external structure of this class of fish has significant differences. These are mainly marine inhabitants.

Cartilaginous fish lack gill covers and swim bladder. The respiratory organs open outward with independent openings. The skeleton of cartilaginous fish does not ossify. The ducts of the reproductive, digestive and excretory systems open into one opening - the cloaca.


At the mere mention of these fish, it becomes scary. Indeed, most sharks lead a predatory lifestyle. Although the whale and giant sharks, which are the largest representatives of the class, are quite harmless. Planktonic organisms form the basis of their diet.

The shark's body is streamlined. The tail fin is of particular importance for movement. In most species, it has different lobes. It is also called heterocercal. Moreover, the upper blade is much larger than the lower one.

The mouth is located on the elongated head in the form of a crescent. It is surrounded by a large number of teeth arranged in several rows. As some of them wear off, others grow on the inside.

Is it true that sharks are fish without scales? It's not like that at all. Although at first glance, her skin seems completely bare. Shark scales are called placoid. She is the oldest in origin. Placoid scales resemble teeth in composition, shape and chemical structure. This is a plate with a spike protruding outward. Shark scales have a wide base and a flattened shape. The plates are so tightly adjacent to each other that the skin looks bare. In fact, it protects the shark's body like an iron chain mail.

Plakoid scales also perform additional functions. It significantly reduces water resistance, which allows sharks to reach speeds of up to 80 km / h. And it also allows for almost silent movements. This is very important when hunting and attacking.


These fish have both a tail and scales. But they look very unusual. Their body is flattened in the dorsal-abdominal direction. The pectoral fins of the fish are fused with the head, resembling wings. We are talking about stingrays.

Most of them live in the seas, but inhabitants of fresh water bodies are also known. Depending on the habitat, the color of stingrays varies from yellowish to black. The eyes are located on the upper side of the body. Here is also a sprinkler. They represent the first pair of gill slits, which open the arches of the respiratory system.

The characteristic body shape is interrelated with their benthic lifestyle. Stingrays swim due to the undulating movements of the wide pectoral fins. But they spend most of their time at the bottom. Here they bury themselves in the sand or wait for prey. The diet of these fish consists of small invertebrates, fish or plankton.

Bony fish

This class is much more numerous. Its representatives are more than 20 thousand species. They live in all types of water bodies: from small streams to oceanic expanses.

These fish have more progressive structural features. These include the presence of a completely ossified skeleton and a swim bladder, which holds the body in the water column. Respiratory organs in bony fish are protected by gill covers. The latter not only protect them, but also take part in the implementation of respiratory movements.

Fish without scales: is it possible

There are numerous glands in the skin of fish. They have a protective function. The substances released by them prevent the penetration of pathogens into the interior, and reduce the friction of water during swimming. In some species, mucus contains toxic substances.

The body of bony fish is also covered with scales, which are derived from the skin. It looks like translucent flat plates. Individual scales overlap each other like shingles. With its front edge, each plate goes deep into the skin, and the back one covers the scales of the next row. The growth of these structures is similar to the formation of tree rings in trees. The growth of the plates occurs in the spring, and stops in the winter.

Do all fish have scales? Absolutely. But in some it completely covers the body, while in others it is located on the body in separate rows. The latter traditionally includes Cartilaginous fish and some Bone fish. For example, in beluga, sterlet, sturgeon and stellate sturgeon, sharp scales are located along the body in several strands.

Features of the covers

All the features of the external structure of fish allow them to better adapt to life in the aquatic environment. Not only the speed of movement allows them to hide from predators, but also the color of the covers. In many fish, it is protective. For example, the dorsal side of a perch is darker than the ventral side. This makes the fish less visible against the bottom. The belly of the perch is silvery. This makes it invisible against the background of the water surface for prey, which is located below. The transverse stripes provide excellent camouflage for the perch among the thickets of algae.

In other species, the color is variegated and bright. It is called warning because its owners are almost always poisonous. Flounder has the ability to change its color depending on environmental conditions.

What is the lateral line in fish

On both sides of the body, a thin strip is clearly visible to the naked eye. It stretches from the gill slits to the base of the tail. This structure is called the lateral line. It consists of receptors called neuromasts. The latter are formed by a group of hair cells.

The lateral line in fish is an organ for the perception of vibrations and movements in the environment. With its help, fish determine the direction and speed of the current. A similar structure was found in all larvae and in some adult species of amphibians, cephalopods, and crustaceans. Fish use it as a reference point in space, which is especially important when hunting.

Unusual views

Despite the large number of common features of the structure, there are a number of aquatic inhabitants that are in no way similar to representatives of this class. One of them is the drop fish. For most of her life, she has a normal appearance: tail, scales, fins ... However, when she emerges to the surface of the water, her body begins to swell and turn into a gelatinous creature that looks like a monster with a huge nose.

In coral ocean reefs you can find boxfish. It is shaped like a cube. You can add a bright yellow color with black dots to this unusual look. So far, scientists cannot explain why, in the process of evolutionary changes, the classic flat body shape changed to cubic.

So, the features of the external structure of fish that help them adapt to life in water include:

  • Streamlined body shape. It consists of a head, torso and tail. In benthic species that lead a sedentary lifestyle, the body is flattened in the dorsal-abdominal direction.
  • A large number of mucus-secreting glands.
  • Scales that completely cover the body of the fish or form longitudinal stripes.
  • In cartilaginous fish, the respiratory organs open outward with gill slits. In bone, they are closed with lids that protect the respiratory organs and participate in the implementation of respiratory movements.
  • The presence of several types of fins: paired and unpaired. The first group includes abdominal and thoracic. The dorsal, caudal, and anal are unpaired. They provide all types of movement, maneuverability and a stable position in the water column.