The body of fish is quite complex and multifunctional. The possibility of staying under water with the performance of swimming manipulations and maintaining a stable position is determined by the special structure of the body. In addition to organs customary even for humans, critical parts are provided in the body of many underwater inhabitants to ensure buoyancy and stabilization. Of significant importance in this context is the swim bladder, which is an extension of the intestine. According to many scientists, this organ can be considered as a precursor to human lungs. But in fish, he performs his primary tasks, which are not limited only to the function of a kind of balancer.
Bladder formation
Bladder development begins in the larva, from the anterior intestine. Most freshwater fish retain this organ throughout their lives. At the time of release from the larva in the bubbles of the fry, there is still no gaseous composition. To fill it with air, the fishes have to rise to the surface and grab the necessary mixture on their own. At the stage of embryonic development, the swim bladder forms as a spinal outgrowth and is located under the spine. In the future, the channel that connects this part with the esophagus disappears. But this does not occur in all individuals. Based on the presence and absence of this channel, the fish are divided into closed and open bubble. In the first case, air duct overgrowth occurs, and gases are discharged through blood capillaries on the inner walls of the bladder. In open-bubble fish, this organ is connected to the intestine through the air duct, through which gases are removed.
Bubble gas filling
Gas glands stabilize the pressure of the bubble. In particular, they contribute to its increase, and if necessary, lowering involves the red body formed by a dense capillary network. Since pressure equalization in open-bubble fish is slower than in closed-bubble fish, they can quickly rise from the water depths. When catching individuals of the second type, fishermen sometimes observe how the swimming bladder protrudes from the mouth. This is due to the fact that the container swells under conditions of rapid rise to the surface from depth. Such fish, in particular, include zander, perch and stickleback. Some predators that live at the very bottom have a strongly reduced bubble.
Hydrostatic function
The fish bubble is a multifunctional organ, but its main task is to stabilize the situation in different conditions under water. This is a hydrostatic function, which, by the way, can be replaced by other parts of the body, as confirmed by examples of fish that do not have such a bubble. One way or another, the main function helps the fish to stay at certain depths, where the weight of the water displaced by the body corresponds to the mass of the individual. In practice, a hydrostatic function can manifest itself as follows: at the time of active immersion, the body contracts with the bubble, and when it ascends, on the contrary, it straightens. During the dive, the mass of the displaced volume is reduced and becomes less than the weight of the fish. Therefore, the fish can go down without much difficulty. The lower the immersion, the higher the pressure becomes and the more the body contracts. The reverse processes occur at the moments of ascent - the gas expands, as a result of which the mass is facilitated and the fish rises easily.
The functions of the senses
Along with hydrostatic function, this organ also acts in some way as a hearing aid. With it, fish can perceive noise and vibration waves. But not all species have this ability - in the category with this ability include carps and catfish. But sound perception is provided not by the swim bladder itself, but by a whole group of organs into which it enters. Special muscles, for example, can provoke vibrations of the walls of the bubble, which causes a sensation of vibration. It is noteworthy that in some species that have such a bubble, hydrostatics is completely absent, but the possibility of sound perception is preserved. This applies mainly to benthic fish, which spend most of their life on the same level under water.
Protective functions
At times of danger, minnows, for example, can release gas from the bubble and produce specific sounds, distinguishable by their relatives. At the same time, one should not think that sound generation is primitive and cannot be perceived by other inhabitants of the underwater world. Humpbacks are well known to fishermen with rumbling and grunting sounds. Moreover, the swim bladder, the fish had a trigger, literally terrified the teams of American submarines during the war - the sounds made were so expressive. Typically, such manifestations occur at times of nervous strain of fish. If, in the case of a hydrostatic function, the bubble operates under the influence of external pressure, then sound generation occurs as a special protective signal generated exclusively by fish.
Which fish do not have a swim bladder?
Sailing fish are deprived of this body, as well as varieties that lead a benthic lifestyle. Almost all deep-sea individuals also do without a swimming bladder. This is precisely the case when buoyancy can be provided by alternative methods - in particular, due to fat accumulations and their ability to not compress. A low body density in some fish also contributes to maintaining stability. But there is another principle for maintaining hydrostatic function. For example, a swimming bladder for a shark is not provided, therefore, it is forced to maintain a sufficient depth of immersion due to active manipulations of the body and fins.
Conclusion
It is no accident that many scientists draw parallels between the human respiratory system and the fish bubble. The evolutionary relationship unites these parts of the body, in the context of which it is worth considering the modern structure of fish. The fact that not all fish species have a swimming bladder causes its inconsistency. This does not mean that this body is unnecessary, but the processes of its atrophy and reduction indicate the possibility of doing without this part. In some cases, fish use internal fat and lower body density for the same hydrostatic function, and in others, fins.