Fish are the largest group of vertebrates (about 30 thousand species). Fish are permanent inhabitants of the aquatic environment. They inhabit almost all water bodies of the Earth: from streams to oceans. Many features of their structure are associated with life in water.
Most fish have a streamlined body shape (head, body and tail smoothly merge into each other).
The fish skin is covered with scales and has mucus secreting glands.
The main type of movement of fish is lateral bends of the whole body.
The fins of fish perform a steering function and keep the body in balance.
Respiratory organs – gills.
Many fish have a swim bladder that can change body density.
Fish are divided into two classes – Cartilaginous fish (sharks, rays) and Bony fish (sturgeon, salmon, herring, crucian carp, perch, pike, etc.). The main criterion for such a division is the substance that makes up the internal skeleton of fish (cartilage or bones).
Cartilaginous fish, unlike bony ones, do not have a swim bladder; the gills open outward with gill slits; no gill covers; the skeleton is made up of cartilage; most species live in salty waters, although there are freshwater ones as well. Internal fertilization is characteristic of cartilaginous fish.
The variety of living conditions in water influenced the appearance of fish and led to the emergence of a wide variety of forms: the emergence of many adaptations to specific living conditions. In a diverse underwater landscape, each fish species lives in the places to which it is best adapted, where it can find food, shelter and breeding grounds.
Depending on the habitat, ecological groups of fish are distinguished: freshwater, anadromous, brackish and marine.
Most fish live in the seas – sea fish. Some of them (sharks, tuna, cod) live in the water column, others (stingrays, flounders) – in the bottom layers or at the bottom of water bodies.
Freshwater fish, inhabiting rivers, lakes and ponds, constantly live in fresh water. Some of them have adapted to life in reservoirs with stagnant water (crucian carp, tench), others can live only in a fast flow of water (trout, grayling, chub, asp), others inhabit both stagnant and flowing water bodies (pike, pike perch, perch ).
Some fish live in the seas for most of their life, and breed in rivers (sturgeon, salmon, chum salmon, pink salmon), or vice versa (river eel). Such fish are called anadromous.
Brackish fish inhabit desalinated areas of the seas, the mouths of large rivers: such are many whitefish, roach, goby, river flounder.
In sea and fresh water bodies there are fish living in the water column and near the bottom. The water column is inhabited, for example, by such marine fish as anchovy, mackerel, tuna. Stingray and flounder live at the bottom.