Usually all aquatic vertebrates are called fish, although this classification is wrong, since other aquatic vertebrates such as whales are mammals. But the curious thing is that both fish and terrestrial vertebrates share the same ancestor. Fish are a group that, despite being very primitive, have had great evolutionary success, since the aquatic environment has allowed them to survive in a large number of habitats. Their adaptations have given them the ability to colonize from s altwater areas to freshwater regions in rivers and lakes, passing through species capable of living in both environments and going up rivers (such as salmon, for example).
If you want to continue learning about the characteristics of fish, a very diverse group that inhabits the planet's waters, continue reading this article on our site and we'll tell you all about them.
Main characteristics of the fish
Despite being a group with highly variable shapes, we could define fish by the following characteristics:
- Aquatic vertebrates: make up the most diverse vertebrate taxon today. Their adaptations for aquatic life have allowed them to colonize all kinds of aquatic environments. Its origin dates back to the end of the Silurian, more than 400 million years ago.
- Osseous skeleton: they have a bony skeleton with very few cartilaginous areas, this is their main difference with chondrichthyan fish.
- Ectotherms: that is, they depend on the environmental temperature to regulate their body temperature, unlike endotherms.
- Brachial respiration: they have a respiratory system where the main breathing organs are the gills, and they are covered by a structure called the operculum, which also serves to delimit the head and the rest of the body. Some species breathe through lungs derived from the swim bladder, which also helps them to float.
- Terminal mouth: they have a terminal mouth (not ventral as in the case of chondrichthyans) and their skull is made up of various bones articulated skins. In turn, these bones are what support the teeth, which cannot be replaced once they break or fall out.
- Pectoral and pelvic fins: they have smaller anterior pectoral fins and smaller posterior pelvic fins, both pairs. They also have one or two dorsal fins and a ventral anal fin.
- Homoproximal caudal fin odd: that is, the upper and lower lobes are the same. Some species also have a different caudal fin, divided into three lobes, it is present in coelacanths (sarcopterygian fish) and lungfish where the vertebrae extend to the end of the tail. It forms the main organ to generate the thrust by which most fish species move.
- Dermal scales: they have skin that is generally covered by dermal scales, with the presence of dentin, enamel and bone layers, which they vary according to their shape and can be cosmoid, ganoid and elasmoid scales, which in turn are divided into cycloid and ctenoid, which are divided by their smooth edges or with comb-like incisions, respectively.
Here we leave you more information about bony fish: Bony fish - Examples and characteristics.
Other characteristics of the fish
Within the characteristics of the fish, it is also worth mentioning the following:
How do fish swim?
Fish are capable of moving in a very dense medium such as water. This is mainly due to its hydrodynamic shape, which together with its powerful musculature in the trunk and tail area, propels its body forward by a lateral movement, often using the fins as a rudder for balance.
How do fish float?
Fish face the difficulty of staying afloat, as their bodies are denser than water. Some fish, such as sharks (which are chondrichthyan fish, that is, they are cartilaginous fish) do not have a swim bladder, so they need some systems to stay at a certain height in the water column, such as keeping in continuous movement.
However, other fish have an organ dedicated to buoyancy, the swim bladder, in which they hold 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.
How do fish breathe?
Traditionally we say that all fish breathe through gills, a membrane structure that allows the direct passage of oxygen from the water to the blood. However, this characteristic is not generalized, as there is a group of fish closely related to terrestrial vertebrates, and this is the case of the lungfish or Dipnoos, which are capable of both gill and lung respiration.
For more information, you can consult this other article on How do fish breathe?
Osmosis in fish
Freshwater fish live in an environment with few s alts, while in their blood the concentration of these is much higher, this is produced by a process called osmosis, the massive entry of water into your body, and the massive exit of s alts to the outside.
That is why they need various adaptations to regulate this process, which is why they absorb s alts in their gills (which are in contact directly with the water, contrary to its hermetic and scale-covered skin) or by releasing highly filtered and diluted urine.
Meanwhile, s altwater fish face the opposite problem, they live in very s alty environments, so they are at risk to get dehydrated. To get rid of excess s alt, they are able to release it through the gills or through highly concentrated, almost unfiltered urine.
Trophic behavior of fish
The diet of the fish is very varied, from a diet based on the remains of animals on the bottom, vegetable matter, to the predation of other fish or molluscs. This last characteristic has allowed them to evolve their visual capacity, agility and balance to obtain food. Migration
There are examples of fish migrating from freshwater to s altwater, or vice versa. The best known case is that of Salmonids, an example of anadromous fish that spend their adult life in the sea, but return to freshwater to spawn (i.e. that lay eggs), being able to use certain environmental information to find the river in which it was born and lay its eggs there. While other species, such as eels, are catadromous, since they live in fresh water but migrate to s alt water to reproduce.
Fish reproduction and growth
Most fish are dioecious (have both sexes), and oviparous (with external fertilization and external development), being capable of releasing their eggs into the environment, burying them, or even carrying them in their mouths, sometimes also engaging in egg-guarding behavior. However, there are some examples of ovoviviparous tropical fish (eggs are stored in the ovarian cavity until hatching). On the other hand, some sharks have a placenta through which the young are nourished, this being a viviparous gestation.
The subsequent development of the fish is generally associated with the environmental conditions, mainly to the temperature, being the fish from more tropical those that have a faster development. Unlike other groups of animals, fish continue to grow in their adult stage without limits, reaching enormous sizes in some cases.
For more information, we encourage you to read this other article on How do fish reproduce?
Characteristics of the fish according to their group
Nor can we forget the characteristics of the fish according to their group:
Agnathus fish
They are jawless fish, a very primitive group and includes hagfish and lampreys. Despite not having vertebrae, they are considered vertebrates, due to characteristics observed in their skull or in their embryonic development. They have the following characteristics:
- Eel-shaped body.
- They are usually scavengers or parasites, living attached to other fish.
- They don't have vertebrae.
- They do not undergo internal ossification.
- They have bare skin, as they lack scales.
- They lack paired fins.
Gnathostome fish
This group includes all other fishThis also includes most of the current vertebrates, such as the rest of the fish, amphibians, reptiles, birds and mammals. They are also called jawed fish and have the following characteristics:
- They have jaws.
- Even and odd fins (pectoral, dorsal, anal, ventral or pelvic and caudal).
This group includes:
- Chondrichthyans: cartilaginous fish such as sharks, rays and chimaeras. Its skeleton is made up of cartilage.
- Osteichthyos: ie bony fish. This includes all the fish that we can find today (divided into ray-finned fish and lobe-finned fish, or actinopterygians and sarcopterygians, respectively).
