Fish are the oldest vertebrates

Fish are the oldest vertebrates. They inhabit almost all aquatic environments on Earth - from freshwater lakes and rivers to saltwater seas and oceans.

The earliest fish was a round-mouthed, jawless fish that appeared on the earth during the Cambrian period 450 million years ago. Fishes are easily distinguished by their appearance, and they form the largest group of vertebrates: there are more than 22,000 species of fish out of a total of 50,000 species of vertebrates.

School of fish on the bottom of the sea

Not all animals that live in water are fish. For example: Whales are mammals. However, all fish species are well adapted to life in the water. They use their fins for locomotion. Fish have two pairs of fins - pectoral fins and pelvic fins, located on both sides of the body; and a caudal fin, which grows from the tail; and depending on the species, there are one or two dorsal fins on the back and one on the buttocks. anal fin. They have a gas-filled sac called a swim bladder that allows the fish to sink, float and maintain position in the water. Only rays and sharks do not have this organ. Fish also have gills for breathing, and in most species the gills are covered by opercular bones. The gills are located on both sides of the head and behind the mouth. They filter the water swallowed from the mouth, obtain oxygen from the water, and then discharge the water through openings called gill slits. Different species of fish vary greatly in size. Their body consists of 3 parts: head, trunk and tail. The skin is covered with scales, which vary in size and number. There is a distinct line on each side called the lateral line, which is a sensory organ used to determine direction. Some bony fish have muscles separated by tiny bones.

lamprey

The first fish that appeared on the earth had a round, jawless mouth. Now only 70 species of such fish exist, forming jawless fish. Among these fish, lampreys are the most famous because they have no scales and have an elongated, round body, much like an eel. Lampreys attach themselves to other fish through their mouths with suckers and feed on the host's blood. Other fish are divided into two main categories: bony fish and cartilaginous fish.

A bony fish has a skeleton. Among these fish are native bony fish, in which only part of the skeleton is bony. Examples include lobe fishes (including coelacanths), lungfishes, and sturgeons (such as sturgeons). The difference between these fish and more evolved bony fishes is that the skeletons of bony fishes are entirely composed of hard bones. Moray eels, soles and crappie, as representatives of bony fish, have different shapes, but they all have extremely symmetrical tail fins and are covered with tiny scales (with a few exceptions, including eels and some carps). There are several categories of bony fish. Eels are fish whose larvae look very different from their adults. Herrings are fish that live in groups. Carp include almost all freshwater fish. Perch and tuna are fish whose tail fins are supported by strong spokes. Known as "spinyfins," they form the largest group of bony fishes.

Cartilage fish have a skeleton composed entirely of cartilage and reinforced with calcium. These fish are mainly sharks and rays.

When an eel is born, it is a small fish with a flat body, called a "baby eel". When mature, they have a long, smooth body, usually without scales, and a continuous fin along the back. Eels live in rivers and lakes in Europe and the Americas. They migrate to the algae sea northeast of the West Indies in the North Atlantic, where they reproduce. They die after the young are born. The baby eels take on their adult form on their way back across the Atlantic, and on their way back, they also begin to give birth in the Sargasso Sea.

Herring live in the North Sea, English Channel and Baltic Sea. Adult fish have a pale belly and a dark blue or nearly black back. Like sardines and sprats, they live in colonies, sometimes thousands of fish living together. This is an effective method of self-defense for each fish, since a predator can only choose one target among many fish. When attacked, schools of fish disappear almost immediately.

There are thousands of species of fish belonging to the carp family, almost all over the world. These freshwater fish have large scales and their teeth are anchored not to their jaws but to their throats. Their mouths can move forward to suck in food. Many species of the genus Cyprinus live mainly in calm rivers, small ponds and lakes in Asia and Europe, and the shapes and colors of each species vary greatly. Some species have only a few large scales (mirror carp) or almost no scales (grass carp). These fish are easy to breed and breeders have created many variations. Carp mainly feed on plants and invertebrates. During the spawning season, it depends on the temperature of the water, which should not be too cold (at least 20°C). Females produce hundreds or thousands of eggs, but once most of the fry are born, they become food for other fish and even adult carp.

Koi

Spiny fins appeared about 60 million years ago. Bass is a typical representative of this type of fish. Their fins are supported by hard, sharp spokes, and their huge tail fins have spines. Bass live in lakes and rivers in Europe and North America, where they eat invertebrates and small fish, including their own young. Other species of these fish live in the ocean, such as tuna and swordfish, which are powerful swimmers and can swim up to 100 kilometers per hour. Tuna can weigh up to 500 kilograms and are carnivores. Among fish, they have the unique ability to maintain a body temperature higher than that of the water, and their species include albacore tuna in the Pacific and bluefin tuna in the Mediterranean and Atlantic oceans.

Sharks and rays are the main representatives of modern cartilaginous fishes, which may have appeared 410 million years ago. As their name suggests, they have a skeleton made of cartilage. Cartilage is a flexible material that hardens when filled with calcium and is a solid like bone. Cartilaginous fishes thrive in temperate and thermal oceans. They breathe through gills in the water. The gills communicate directly with the outside world through several gill slits on the back of the head. There are approximately 550 species of cartilaginous fish, 370 of which are sharks, and the rest essentially consist of flat-bodied rays and electric rays.

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