Pisces - Who are fish and how do they live

Pisces - Who are fish and how do they live

Fish belong to the large class of vertebrates and are heterothermic animals, in other words cold-blooded animals that is to say that the body temperature is regulated by the external temperature. They alone make up more than half of all the rest of the vertebrates put together (mammals, birds, reptiles and amphibians).


Fish are aquatic animals that live in both fresh and marine waters all over the world, from the warmest to the coldest regions, to the most ideal conditions and the most hostile to life.

The greatest concentration of marine fish species is found along the coral reefs thanks to the abundance of food, while those of fresh water are found in greater concentration in the great African lakes and in the water courses of the tropical rainforests, especially in the Amazon River.


On the approximately 22,000 species it is very difficult to give morphological indications of a general nature as they are too different from each other. In fact, it goes from fish that have a completely rounded shape as in puffer fish to those that resemble a snake as in eels or in completely flattened forms as in sole. In general, the shape is more frequently compressed, which tapers at the head and tail.

The dimensions are very varied, ranging from fish of a few millimeters (Pandaka pygmea) to fish measuring over 15 m (whale shark).

A peculiarity of the fish is the color as there is a great variety: in fact, it passes from fish of a thousand colors of tropical regions (probably a need for intraspecies recognition or as a warning for potential predators in the case of poisonous fish) to more sober and classic, dark colors in the back part and clear in the ventral part (probably a camouflage system as a predator looking down sees dark and vice versa looking towards the other sees clear confusing it with the sunlight).

The body of the fish is generally covered by scales (although in some species they are absent) and is equipped with fins that are nothing more than membranes that have a propulsive or directional function. Usually one is placed in the dorsal part of the body, in the middle part; one is placed at the extremity of the animal and is generally the propelling organ; one or more fins are then found in the ventral part along the midline and you can then find pectoral fins on the sides, behind the gills and others placed between the head and the anus at the level of the abdomen (also in this case there are numerous differentiations in fact of fins between species and species).

A peculiarity of the fish body are the scales (scales) arranged in a partially superimposed way (imbricate). In some species the scales are bone plates; in others, as in eels, they are so small as to give the effect of a smooth skin; or there are fish where there just aren't. In any case, where they are present they represent a sort of armor that protects and supports the body of the fish.


The fish as vertebrates have a skeleton (endoskeleton) formed by a skull, a vertebral column, ribs and bony structures that serve to support the fins.

Breathing occurs through a gill system that allows oxygen to be retained from the water and expelled carbon dioxide through the blood, but there are also fish that are able to breathe atmospheric air through well-developed lungs.

Fish have a digestive system that begins with a mouth with teeth, more or less developed depending on the species, continues with a pharynx, an esophagus, a stomach and an intestine that ends in the anal opening. All fish also have a liver and pancreas.

There is a circulatory system consisting of a heart and a main artery that runs under the spinal column. The heart sends blood first to the gills to be freed of carbon dioxide and enriched with oxygen, then to the head and then to the rest of the body through the artery.

In fish we find a fairly simple nervous system consisting of a brain and a spinal cord. The size and development of both vary from species to species.

Another peculiarity of the fish is the swim bladder which is a sort of sack that originates as a protuberance of the alimentary canal which has the function of allowing the fish to remain at the depth it wishes without problems, practically adapting the fish to the pressure variations of the different depths. .

Fish can have more or less developed eyesight depending on the species. In fish without ears, noises are perceived in the form of sound vibrations through the bones of the skull, thus reaching a sort of inner ear. The smells are perceived through the nostrils and in some fish they are particularly developed.


Most fish have separate sexes but there are also cases of fish hermaphrodites. They are for the most part oviparous, that is to say they produce eggs outside the female's body and generally there is no parental care so once the eggs have been laid, they are abandoned to themselves (there are very rare cases of defense of the nest and of the young). However, there are also forms of viviparity (e.g. sharks) that is to say that they have internal fertilization and give birth to young in an advanced stage of development and ovoviviparity, that is to say that the eggs hatch in the oviduct of the female which gives birth to perfectly formed individuals. .

Below are the monographic files of the main fish.

Individual bird species information sheets

Video: Osmoregulation in Fish