Normally, all aquatic vertebrates are called fish, although this classification is incorrect because other aquatic vertebrates, such as whales, are mammals. But the funny thing is that fish and terrestrial vertebrates share the same ancestor. Fish are a group that, although very primitive, has been very successful in evolution, since the aquatic environment has allowed them to survive a large number of habitats. Their adaptations have given them the ability to colonize saltwater areas, freshwater areas of rivers and lakes, species capable of living in both environments and overrun rivers (such as salmon).
If you want to continue discovering the general characteristics of fish, a very diverse group that inhabits the waters of the planet, continue reading this article of Animal Expert and we tell you everything.
Although it is a group with very variable shapes, we could define fish by the following characteristics:
Among the characteristics of fish, it is also worth mentioning the following:
Fish can move in environments as dense as water. This is mainly due to its hydrodynamic shape which, with its powerful musculature in the trunk and tail area, propels your body forward through a lateral movement, usually using the fins as rudder to maintain balance.
Fish have difficulty staying afloat because their bodies are denser than water. Some fish, such as sharks (which are condritic fish, i.e. cartilaginous fish) do not have a swimming bladder, so they need certain systems to maintain a height in the water column, such as maintaining continuous movement.
However, other fish have an organ dedicated to buoyancy, the swim bladder, in which they maintain a specific amount of air to float. Some fish remain at the same depth throughout their lives, while others have the ability to fill and empty their swim bladder to regulate their depth.
Traditionally, we say that all fish breathe through the gills, a membrane structure that allows oxygen to pass directly from the water into the bloodstream. However, this characteristic is not widespread, as there is a group of fish closely related to terrestrial vertebrates, and this is the case of fish lunged or Dipnoos, able to breathe in both the gills and the lungs.
For more information, read this other article on How Do Fish Breathe?
Freshwater fish live in an environment with few salts, while in their blood their concentration is much higher, this occurs through a process called osmosis, the massive ingress of water into their bodies and the massive outlet of salts outwards.
This is why they need various adaptations to regulate this process, so that they absorb salts in their gills (which are in direct contact with water, as opposed to their hermetic and flaky skin) or release highly filtered and diluted urine.
Meanwhile, saltwater fish face the opposite problem, they live in very salty environments, so they run the risk of dehydration. To get rid of excess salt, they are able to be released by the gills or by highly concentrated urine, almost unfiltered.
The diet of fish is very varied, from feeding animal remains from the bottom, plant matter, to predation of other fish or molluscs. This last feature allowed them to evolve their visual ability, agility and balance to obtain food.
There are examples of fish migrating from fresh to salt water, or vice versa. The best known case is salmonids, an example of an anadrome fish that spend their adult life at sea but return to fresh water to spawn (i.e. lay eggs), who can use some environmental information to find the river in which their eggs are located. they were born and laid their eggs there. While other species, such as eels, are catadromes because they live in fresh water, but migrate to salt water to breed.
Most fish are dioe (both sexes) and oviparous (with external fertilization and external development), which can release their eggs in the middle, bury them or even take them to the mouth, sometimes paying attention also to eggs. However, there are some examples of ovoviviparous tropical fish (eggs are stored in the ovarian cavity until hatched). On the other hand, sharks have a placenta through which the young feed, being a viviparous pregnancy.
Subsequent fish development is generally associated with environmental conditions, particularly temperature, and fish from more tropical areas have faster development. Unlike other groups of animals, fish continue to grow to adulthood without limits, reaching huge sizes in some cases.
For more information, also read this other article on How do fish breed?
Nor can we forget those of your group
They are fish without jaws, it is a very primitive group and includes mixinids and lampreys. Although they do not have vertebrae, they are considered vertebrates, because of the characteristics seen in their skulls or by their embryonic development. They have the following features:
This group includes all the rest of the fish. Most current vertebrates are also included here, as are other fish, amphibians, reptiles, birds and mammals. They are also called jaw fish and have the following characteristics:
This group includes
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