Is the swim bladder really the floating and sinking organ of fish?How is it controlled?
Is the swim bladder really a fish’s floating and sinking organ?Inside the abdomen, near the back, there is a white or yellowish air sac called the swim bladder, which most people think is the tool for fish to float and dive freely in the water.But the swim bladder, strictly speaking, is an auxiliary part of buoyancy, and some fish rarely use it at all or have no swim bladder at all.Why is swim bladder only an auxiliary organ for fish?Many people believe that fish ascend and sink by compressing and expanding their swim bladders to reduce and increase their buoyancy.But there is a problem, that is, when we gut fish, there is no muscle tissue around the swim bladder. Since there is no muscle tissue, the compression and expansion of the swim bladder lose power. Therefore, it is a cognitive misunderstanding that fish float on the swim bladder.We learned about buoyancy in junior high school. When the density of an object is less than that of water, the object will float on the water; when the density of an object is equal to that of water, the object will float in the water; when the density of an object is greater than that of water, the object will sink to the bottom.So how do fish change density?The density of fish in the water actually almost will not change myself, it was sink or rise, mainly because the fins and reaction to the fish, simply is the fish surfaced, head, tail swinging, fin swaying back and forth, will give a fish with thrust, helping it to quickly reach the surface of the water, sinking is the same.Of course, the swim bladder is not completely ineffective in this process. When floating up, the swim bladder will increase gradually due to the decreasing pressure, which will accelerate the fish’s floating speed. When sinking down, with the increasing pressure, the swim bladder will decrease gradually, and its specific gravity will increase, and the fish can reach the bottom more quickly.Therefore, the fish’s swim bladder increases and decreases with the rise and fall of the fish, but this is not active regulation, but passive regulation, which is caused by the change of water pressure caused by the driving force from the fin and tail.However, it was the Germans who bionic the earliest submarines based on this principle.Which fish don’t have a swim bladder?While most fish have swim bladders, there are two groups of fish that are somewhat different.One has no swim bladder at all;Let’s start with fish with severely degraded swim bladders. The swim bladders of these fish are badly out of proportion to their size for one reason only – they are almost useless.Many fish are benthic or shallow-water active and rarely float or sink except when they die, so the swim bladder, an organ that helps float or sink, is severely degraded.In addition, for fish like tuna, which normally swim very fast (60-80 km/h), a large swim bladder can be a hindrancy-and thus, over time, their swim bladder has deteriorated considerably.Then there are fish without swim bladders.Do fish without swim bladders exist?Almost all cartilaginous fish (sharks, rays, stingrays, sturgeon, etc.) do not have bladders. These cartilaginous fish do not have prominent gill covers like bony fish, but instead have prominent gill slits.And the difference is that cartilaginous fish mostly rely on constant swimming to get water into the gill slits to breathe, like sharks they have to swim their whole lives, even when they sleep, otherwise they suffocate.Cartilaginous fish is different from bony fish in terms of evolution, so the presence or absence of swim bladder is also an evolutionary difference between them.Cartilaginous fish use swimming to control their position and depth in the water, and cartilaginous fish like sharks lack a swim bladder, but their livers are more developed than those of bony fish of the same size, and contain diacylglycerol ether, which also helps in floating and sinking.Is air in and out of the fish’s swim bladder as it floats?For fish that need to float and sink via a swim bladder, the swim bladder gradually becomes smaller as they dive, and the gas is discharged in two ways: the first is the laryngeal swim bladder of these fish, which is connected to a swim bladder tube, through which the gas from the swim bladder can enter the gills of the fish and be discharged out of the body.The second is the throatless swim bladder fish, whose bladders have been degenerated and which rely on special glands to produce lactic acid as a medium for excretion.Similarly, when a fish dives to a certain depth and tries to come up again, its swim bladder enlarges as well as its fins and tail. This enlargement requires gas, the source of which is the opposite of the gas emitted by the fish when it dives.For fish with swim bladder tubes, the gas source in the swim bladder is filtered into the body through the gill filaments. Most of the gas is supplied to various organs and tissues of the body, while a small part enters the swim bladder through the swim bladder tubes.For fish without a swim bladder, it dissolves some of the oxygen through lactic acid and then into the swim bladder.Fish without swim bladders are inefficient because they rely on lactic acid to dissolve oxygen. As a result, they move slowly from deep to shallow water, but if they reach the surface too quickly, their swim bladders rupture.The swim bladder is not the common organ of all fishes, among which cartilaginous fish do not have this organ. They float up and down by constantly swimming all their life. Moreover, like some fishes living in the benthic or shallow water, even if they have swim bladder, they are not developed.Moreover, the swim bladder is only an auxiliary organ for most fishes to float and sink, and the driving force for them to float and sink is mainly from the body swing. However, with the swim bladder, the efficiency can be increased by the contraction of the swim bladder when they float and sink.As for the source of gas in the swim bladder, oxygen filtered by the fish’s gills enters through the swim bladder tubes or lactic acid.