Definition of Fish/Fish

In the traditional classification of animals, fish are a class of aquatic vertebrates, which have been adapted to live both in fresh water, such as rivers and lakes, and in salt water, and they breathe through gills.

Most fish have fins that allow them to swim. They have gills, which are respiratory organs adapted to extract dissolved oxygen from water, and are unable to breathe in air. This means that, strange as it may seem to us, fish die from suffocation out of water. Currently, there are more than 20,000 known species of fish, with different shapes, sizes and eating habits. In the traditional classification, three main groups of fish are recognized: bony fish, cartilaginous fish, and jawless fish.

fish evolution

The first fossil records of fish appear in the Cambrian, more than 500 million years ago. At that time, the mainland was almost uninhabited, since there were no terrestrial organisms. The seas teemed with life, and worm-like creatures swam freely through them.

These creatures have an unusual and novel structure among animals: the notochord, which is a dorsal "rope" that runs through the back of the entire body and helps maintain its shape. Later in the evolution of animals, the notochord would be the basis for the evolution of the vertebral column and complex nervous systems, so characteristic of modern animals.

The first fish were simple, their bodies were cylindrical, and they fed by filtering food particles from the water. From these primitive fish a great diversity of fish arose during later geological periods. Many of these groups of fish are already extinct, and we only know them from their fossils, but other groups have survived to this day.

Among these groups of extinct fish, there was one, the lobe-finned fish, which is of particular evolutionary importance. This group of fish developed primitive lungs and legs that allowed it to live temporarily out of water and begin to walk clumsily in the mud. Land vertebrates (amphibians, reptiles, birds, and mammals) descend from this group of fish.

Diversity of modern fish

Bony fish are the most numerous and diverse, and surely, it is the bony fish that we think of when we think of fish.

The skeleton of these fish is made up of bones (hence their name) and they have a unique flotation organ: the swim bladder. The bladder is like a balloon that allows them to regulate the depth at which they float: when the balloon fills with air, the fish rises, and when it contracts, the fish sinks. By controlling the amount of gas within the bladder, the fish can control the depth at which it remains suspended in the water. In other words, if the fish stops swimming, it remains "suspended" at a certain depth and does not sink to the bottom.

Bony fish are found in both fresh and salt water and are present almost everywhere in the world. They are one of the few groups of fish that left the sea to enter the rivers, so almost all freshwater fish are bony fish.

Their diet is varied, and there are carnivorous, herbivorous and detritivorous fish.

Some examples of commercially important marine bony fish are tuna, sardines, and clownfish.

Cartilaginous fish have an internal skeleton made up of cartilage, unlike bone, cartilage is a flexible and resistant tissue. Cartilaginous fish are exclusively marine

They do not have a swim bladder, so when they stop swimming they sink to the bottom. However, this does not mean that these fish swim restlessly: some rest on the bottom.

Cartilaginous fish include rays, sharks, and chimaeras. The latter live in the depths of the ocean and are therefore not as well known.

Most of them live in the sea, but there are a few species of freshwater sharks and rays. They are carnivorous species, which feed on fish, crustaceans or mollusks.

Sharks are excellent hunters and have highly specialized senses to detect their prey. They can smell blood in the water from miles away and detect the weak electrical field generated by all living organisms around them. This electroreception ability also allows sharks to detect Earth's magnetic field, which they use as a GPS to navigate and orient themselves in the ocean.

Jawless fish are the most primitive and least known of all fish. They have no jaw or scales, and feed by suction or filtering. Lampreys, a group of jawless fish, are shaped like eels (but true eels are bony fish) but unlike eels; they have an oral disc with teeth grouped concentrically around the mouth.

Lampreys are parasitic fish that feed on the blood of other fish, but do not kill the fish they feed on. The disk is like a suction cup that serves to attach itself to the skin of the fish, with its teeth it opens its skin and sucks its body fluids.

Lampreys are born in rivers and return to the sea, where they spend their entire adult lives. To reproduce and spawn, the adults return to the rivers.

The other group of jawless fish are the hagfish, exclusively marine and similar to lampreys, they are scavengers, that is, they feed on dead organisms they find in the sea.