You fish they are the animalsvertebrates that first appeared in our planet. They are aquatic and have a series of adaptations that ensured the group's success in this environment. They can be found in different places, both in fresh water and in salt water.
They have great economic importance, being used as food, raised in aquariums and used in the practice of sport fishing. Fish can be divided into two large groups: bone and cartilaginous.
Read too: Shark - animal of the cartilaginous fish group
General characteristics of fish
Fish are aquatic animals that have different shapes and colors and a wide variety of habits. Some fish are voracious hunters, like the dreaded white shark, while others are harmless, like the whale shark, which feeds on the plankton. In terms of size, there is also great variation, with fish from a few millimeters to fish that reach many meters in length. Here we can mention again the whale shark, which can reach up to 12 meters in length.
Fish are animals that have breathing gill type, presenting structures (gills) specialized to perform gas exchange with the Water. The water passes through the gills, and the oxygen available in the water passes to the blood, and carbon dioxide from the blood passes into the water. It is noteworthy that some fish, such as the piramboia, have structures that function as lungs, being known, due to this characteristic, as lung fish.
O digestive system of fish is complete, the intestine being finalized in cloaca, in cartilaginous fish, and anus, in bony fish. O circulatory system it is of the closed type, with blood flowing inside the vessels. O heart of fish has only two cavities: an atrium and a ventricle. To ensure a complete circuit throughout the body, blood passes through the heart only once, being a circulation simple.
O excretory system of these animals is formed by a pair of kidneys. fish are ectothermic animals, that is, they are not able to keep their temperature constant, as the birds and mammals.
The fish have some sensory structures important, which ensure the location of the prey as well as the perception of the presence of predators. One of the sensory structures found in fish is the sideline, which allows them to capture movement in water.
Another important sensory structure is the Lorenzini ampoules, capable of sensing electrical fields emitted by animals and thus detecting prey. Lorenzini ampoules are found in cartilaginous fish. Fish also have senses such as sight, smell and hearing.
With regard to the reproduction, it is also possible to notice the difference between the groups. Some fish have external fertilization, while others have internal fertilization. There are oviparous, viviparous fish and there are also ovoviviparous species. In some species, there is a larval stage (indirect development) called fingerling.
Know more: Whale shark – the biggest fish in the world
Adaptations of fish to the aquatic environment
To survive in the aquatic environment, fish have a series of important adaptations. See, below, the main ones:
- hydrodynamic body,usually fusiform (cylindrical cup and tapered at the ends), which helps in swimming.
- body covered by scales and mucus, which reduces friction with water and also protects the animal against the action of microorganisms.
- gill breathing which guarantees the capture of oxygen from the water.
- presence of fins which act, for example, in propulsion and change of direction.
- presence of swim bladder (a kind of air pocket), in bony fish, and a big liver rich in lipids, in cartilaginous fish. These two structures ensure that fish can stay at a certain depth without using additional energy. In the case of the swim bladder, the fish is able to inflate or deflate the organ at will.
Read too: Seahorses - bony fish normally found in shallow water
Bone fish and cartilaginous fish
It is customary to divide fish into two large groups: bony and cartilaginous. As the names suggest, the bone fish have a bone skeleton, while the cartilaginous have a skeleton formed by cartilage. As an example of bony fish, we can mention the lambari and the painted. As examples of cartilaginous fish, we can mention the stingrays and the sharks.
Observing bony and cartilaginous fish, it is possible to see some basic differences between the two groups. In bone, the gills are covered by a kind of flap called operculum, absent in cartilaginous ones. THE swim bladder it is also a structure present only in bony fish, buoyancy being achieved in cartilaginous fish thanks to the presence of a liver rich in lipids.
THE mouth in bony fish it is located at the anterior end of the body, while in cartilaginous fish it has a ventral position. With regard to the digestive system, the intestine of the bones ends in the anus, while, in the cartilaginous ones, it ends in the cloaca. Another striking feature is the presence of a structure called clasper, present in cartilaginous fish, which ensures internal fertilization.
