Have you ever wondered how fish manage to be buoyant in water all day, and still have the energy to take care of their needs?
A buoyant, or a well balanced body, invests minimum energy to remain deep in the water, because a buoyant body requires less motion energy under the water than a weighted body.
For this reason, many fish have developed numerous techniques to reduce their body weight, or reduce their specific gravity (making it similar to the specific gravity of water). Fish that have a specific gravity that is close or equal to the specific gravity of water will be buoyant, neither sinking nor floating to the surface. Fat is less dense than water. One way to reduce the flotation of a body in water is by increasing its percentage of body fat. Approximately one third of its body weight needs to be fat for the fish to remain buoyant in the water. This is the situation with many cartilage bodied marine animals (sharks and stingrays) – the cartilage helps them remain buoyant in the water. Deep water sharks use a substance called SQUALENE – a fatty material with a density that is significantly lower than that of sea water.
This is not the only way to optimize buoyancy; another way of reducing density is by adding air pockets inside the fish’s body. Many fish have gas-filled bladders (floating or swimming bladders) which are used for this purpose. The amount of gas required in the swimming bladder to achieve buoyancy depends mostly on the type of water – fresh or salt water. Fresh water provides less buoyancy since it is less dense than sea water (this is why it easier to float in sea water than in a fresh water swimming pool). Therefore, fresh water fish need to have larger flotation bladders. According to research calculations, fresh water fish need to have a flotation bladder with a volume that is equal to 7% of its total body volume. On the other hand, salt water fish only need a flotation bladder with a volume equal to 5% of its total body weight. According to measurements performed on fish, the bladders of fresh water fish are 7-11% the volume of their body, compared to salt water fish which have bladders that are 4-6% the volume of their entire body.
Fish that have a flotation bladder with a fixed amount of gas are only able to maintain their buoyancy at a certain depth. The reason for this is that pressure increases with depth. The pressure compresses the gas in the bladder, this decreases the bladder volume, and increases the fish’s density, causing the fish to become heavier. This type of fish will have to invest more energy to prevent it from sinking. The same applies in the opposite situation, if the pressure increases; the volume of the bladder increases, the fish becomes lighter than the water, making it necessary for the fish to invest a large amount of energy to prevent it from over floating upwards (in extreme situations, overexpansion of the bladder, especially in deep water fish, can cause the bladder to burst). In order for fish to be able to ascend, descend, and maintain buoyancy in water, they have to be able to regulate the amount of gas in the bladder. Some of these fish have a connection between the bladder, and the mouth, this enables them to inhale and exhale through the mouth, and regulate the amount of air in their bladder.
This type of connection between the mouth and the bladder does not exist in most fish. Hence, these types of fish need to have another type of special mechanism to regulate the amount of gas in the flotation bladder. They do this activity using a two-way connection to transfer gas from the flotation bladder to the blood vessels bordering the bladder.
Emptying the bladder is a passive action that occurs when pressure in the bladder increases. The increase in pressure “pushes” the surplus gas into the blood cells, which pass them onwards.
Filling the bladder is an active action that is assisted by a special gland called the gas gland.
In most cases the gas glad can be found in the center of the body, under the dorsal fin. The bladder walls are flexible, and they can expand or contract according to the environmental pressure. The walls of the swimming bladder are coated with a special crystal (Quanine) to prevent gas from penetrating through the walls and into the body.
Not all fish require flotation bladders. Fish that dwell close to the seabed do not usually live in the water column; therefore, they have no need for buoyancy or for a flotation bladder.
Gas bladders are evolutionary relatives of the lungs. Researchers believe that the first lungs were those of terrestrial vertebrates, from which they developed into the flotation bladders of fish today.
Other marine animals discovered additional underwater flotation, and movement techniques.
Buoyancy is a major issue for octopuses, cuttlefish, and nautilus, which all belong to the cephalopod group (in other words, they are marine mollusks), since cephalopods are actually mollusks that used to live on the seabed in their “past”. Here too, the ability to rise from the seabed, and swim in the water column was made possible by a change in the cephalopod’s specific gravity, making it equivalent to that of sea water.
Cuttlefish discovered an original method of changing their weight by using their “skeleton framework”. Their “skeleton framework” is similar to that of the oval shape that you often see in birdcages or on the beach after a storm.
This skeleton framework is very light, and is completely perforated. The perforations are either filled with liquid, or air (ascent / descent). A nautilus is a living fossil with a spiral exterior shell, similar to a snail’s shell, and 90 short hunting tentacles. The shell has air pockets which serve as a flotation mechanism, enabling the nautilus to regulate its buoyancy by emptying and filling the air pockets.
Octopuses use an exchange of body fluids containing either a high or low density of fluoride or sulfuric ions, to control their buoyancy.
Floating bladders are not only used to help the buoyancy process, but also to produce and receive sounds under water. These sounds are produced with vibrations, probably for both courting and for warning purposes. The flotation organ also enhances hearing, since it is also used to amplify marine sounds.
When we enter the water to scuba dive, we usually use diving balances, in exactly the same manner in which fish use their flotation. During a dive, neutral buoyancy helps us to prevent damage the reef, keep in balance, and also not waste unnecessary energy in trying to remain buoyant – exactly like fish.
One of the pleasures of diving, in addition to observing life underwater, is undoubtedly the sensation of floating.
Floating in the sea is exactly the same as floating in outer space, and it is no coincidence that astronauts practice buoyancy or working in space by diving.
Author: Aviv Levi, Scientific Manager, Underwater Marine Observatory Park