Swimming is the oldest way to move living things. For they simply do not have other chances for movement in the aquatic environment.
To purposefully move in the aquatic environment, during the evolution of fish, they acquired a whole list of devices, ranging from body shape to organs that only they have. Now we will take a look at what Nature endowed with one of the most ancient and numerous Chord groups.
Swimming bubble
The swimming bladder is the main “device” through which fish swim. But! It is present only in bony fish. Therefore, we first consider how the bony fishes exploit this organ, and then we are interested in how the cartilages move in the water.
So, the swim bladder is two different-sized, hollow sausages, separated by a jumper. They are the outgrowth of the esophagus. In the process of evolution, they were transformed into the lungs, characteristic of the more developed - amniotes, classes of terrestrial animals.
How does the swim bladder work?
Due to the presence of a swimming bladder, the fish is held at the desired depth. The mechanism of the organ is very simple. Remember the law of Archimedes. The swimming bladder is filled with air. Falling below the level at which the mass of fish coincides with the volume of water displaced by it, the body of the animal undergoes compression. Naturally, at this moment, the swimming bladder is also compressed, from which air is forced out.Due to this, the volume of water displaced by the fish is reduced. The balance between the weight of the fish and the volume of the displaced fluid is disrupted, which allows the animal to go even lower.
If the fish emerges, then approaching the surface of the water increases the amount of gas absorbed by the animal. Some of them enter the swim bladder, expanding it. The bubble “bursts” the body of the animal, increasing the volume of displaced water. As a result of this action, the specific gravity of the fish decreases, and it literally pushes it to the surface.
In total, the swim bladder provides the fish with immersion, ascent and zero buoyancy in the mode of minimal energy consumption.
How do cartilaginous fish swim?
A typical representative of the class of cartilaginous fish is sharks. They appeared on Earth much earlier than bony fish. They have no swimming bladder. Therefore, they are forced to constantly move in order to adjust their position in the water column. Even in a dream, these animals must move their tail, otherwise they will simply drown, as it does not sound paradoxical in relation to fish.
Body shape, external integument
The shape of the body of the fish is another adaptation to movements in dense, in comparison with air, water masses. The bodies of animals, except for near-bottom and deep-sea species, are spindle-shaped, streamlined, which create minimal resistance to the environment. In addition, do not forget about the scales, which increase gliding, reducing the energy consumption of the animal during swimming.
Musculoskeletal system
So that the fish could swim, they formed a radically new - in comparison with the more ancient mixins and lampreys, the musculoskeletal system. First, fins appeared in fish. A pair of thoracic, abdominal. And one abdominal, dorsal and caudal fin. They are “tied” to the muscles, the contractions of which cause the fins to change their position, generating movement. As a result of this, the animal can move in a horizontal, vertical plane, turn around.
In addition to the fins, movement is supported by the work of the muscles of the body. Red muscle fibers are involved in the process of long, monotonous swimming. White muscle fibers “turn on” when you need a jerk, speed, energetic, but short-lived movement.
Other components for movement in the aquatic environment
In fact, the whole organism of fish is adapted for movement and life in the thickness of water masses. For example, the saturation of the body with oxygen using the gills, especially the placement of the senses, the functional of the digestive, excretory systems.
Yes, and keep in mind when discussing the ability of fish to swim, it should be borne in mind that it was these animals in the aquatic environment that reached the maximum level of adaptation in comparison with more primitive forms. The next evolutionary step was the creation of organisms that "learned" to crawl, walk, fly. One of the first “migrants” to land was the Selicant Fish Detachment, represented today by relic coelacanth.
