Fish represent the largest class of known species among vertebrates. Fish are believed to have emerged around 45 million years ago.
But how do they spread across the planet?
Fish occupy the salty waters of the seas and oceans and the fresh waters of rivers, lakes and weirs. In this group there are about 24,000 species, of which more than half live in salt water. The average size of the fish can vary from one centimeter to about 18 meters.
They were probably the first vertebrates to emerge on Earth, and they were small, jawless, had cartilaginous spines and a shell lining their bodies. In evolution, there have been a number of adaptations that have given fish better survival conditions in their habitat - no heavy armor, fast swimmers, jaws and biting.
Once limited to the filtration of nutrient particles suspended in water or deposited at the bottom and prey on some types of invertebrates, fish have also become efficient predators.
Since their inception, they have successfully occupied the salty and sweet aquatic environments, and continue to this day.
Life-enhancing features in water
Fish have several characteristics that favor the performance of their activities in the environment in which they live. Among them, the following stand out:
- body with a generally hydrodynamic shape, ie flattened laterally and elongated, which favors its displacement in water;
- presence of fins, locomotion structures that, in terms of location, may be pectoral, ventral, dorsal, caudal and anal;
- body usually covered by smooth scales, whose organization decreases the friction with the water while the animal moves; In addition, the skin is endowed with mucus producing glands, which also helps to reduce friction with water;
- segmented trunk musculature, which allows the performance of wave movements.
Fish are animals pecyrothermic. This means your body temperature varies with that of the environment. The body temperature of fish generally remains more or less close to ambient temperature.
Breathing and blood circulation
Most fish breathe through gills, also known as gills. Water continually enters the fish's mouth, bathes the gills and exits through the openings on either side of the head.
In the gills, where there are many blood vessels, oxygen gas dissolved in water passes into the blood. At the same time, the carbon dioxide that forms in the animal's body and which is in the blood passes into the water and is eliminated from the body.
The heart of the fish has two cavities one atrium and one ventricle - and through it only unoxygenated blood circulates. After passing through the heart, unoxygenated blood goes into an artery and into the gills where it receives oxygen gas. This now oxygenated blood is distributed to all organs of the animal's body.
Food and digestion
Some fish are herbivores, feeding mainly on algae. Others are carnivores, and feed on other fish and various animals such as mollusks and crustaceans.
In the abyssal zones - the great, lightless ocean chasms - where photosynthesizing beings do not survive, there are many fish detritivores, which feed on organic remains from the illuminated surface, as well as carnivorous fish.
The digestive system of fish consists of the mouth, pharynx, esophagus, stomach and intestine, as well as attached glands, such as the liver and pancreas.
Fish have many sense organs
- Olfactory pouch - They are formed by cells located in the nostrils and associated with the perception of smells of substances dissolved in water. The sense of smell of fish is usually very sharp. The shark, for example, can "sniff" fresh blood dozens of feet away.
- Eyes - They allow you to form sharp images at close range. At greater distances, they only notice moving objects on the water surface. Some fish have color perception and some do not. Sharks and rays (also known as stingrays), for example, do not distinguish colors. The eyes are usually large and have no eyelids or tear glands.
- Sideline - It is formed by a row of pores on either side of the body, with ramifications in the head. The pores communicate with a channel located under the scales, in which there are sensory cells. Through the sensory cells, the fish perceives the differences in water pressure, which gradually increases with depth. It also senses currents and vibrations in the water, detecting the presence of a prey, a predator or the movements of other fish that are swimming next to it, which is very important for trips in schools. It also perceives the direction of water movement, which facilitates its movement in darkness or murky waters.