The
gas bladder (also
fish maw, less accurately
swim bladder or
air bladder) is an internal gas-filled
organIn biology and anatomy, an organ is a collection of tissues joined in structural unit to serve a common function ....
that contributes to the ability of a
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
to control its
buoyancyIn physics, buoyancy is the upward force that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least seem lighter....
, and thus to stay at the current water depth without having to waste energy in swimming.
Gas bladders are only found in ray-finned fish. In the embryonic stages some species have lost the swim bladder again, mostly bottom dwellers like the weather fish.
The
gas bladder (also
fish maw, less accurately
swim bladder or
air bladder) is an internal gas-filled
organIn biology and anatomy, an organ is a collection of tissues joined in structural unit to serve a common function ....
that contributes to the ability of a
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
to control its
buoyancyIn physics, buoyancy is the upward force that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least seem lighter....
, and thus to stay at the current water depth without having to waste energy in swimming.
Species
Gas bladders are only found in ray-finned fish. In the embryonic stages some species have lost the swim bladder again, mostly bottom dwellers like the weather fish. Other fishes like the
OpahOpah are large, colorful, deep-bodied pelagic Lampriform fish comprising the small family Lampridae . There are only two living species in a single genus: Lampris...
and the
PomfretPomfret are perciform fishes belonging to the family Bramidae.They are found in the Atlantic, Indian, and Pacific Oceans, and the largest species, the Atlantic pomfret, Brama brama, grows up to one metre long....
use their pectoral fins to swim and balance the weight of the head to keep a horizontal position. The normally bottom dwelling
sea robinSea robins are bottom-feeding scorpaeniform fishes in the family Triglidae. They get their name from their large pectoral fins, which, when swimming, open and close like a bird's wings in flight....
can use their pectoral fins to produce lift while swimming. The
cartilaginous fishChondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
(e.g. sharks and rays) and lobe-finned fish do not have gas bladders. They can control their depth only by swimming (using dynamic lift); others store fats or oils for the purpose.
Structure and function
The gas bladder normally consists of two gas-filled sacs located in the
dorsalIn anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...
portion of the fish, although in a few primitive species, there is only a single sac. It has flexible walls that contract or expand according to the ambient
pressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
. The walls of the bladder contain very few blood vessels and are lined with
guanineGuanine is one of the five main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, thymine, and uracil. In DNA, guanine is paired with cytosine. With the formula C
5H
5N
5O, guanine is a derivative of purine, consisting of a fused...
crystals, which make them impermeable to gases. By adjusting the gas pressure using the gas gland or oval window the fish can obtain neutral buoyancy and ascend and descend to a large range of depths. Due to the dorsal position it gives the fish lateral stability.
In physostomous gas bladders, a connection is retained between the gas bladder and the gut, the
pneumatic duct, allowing the fish to fill up the gas bladder by "gulping" air and filling the gas bladder. In more derived varieties of fish, the physoclisti, the bladder has a gas gland that can introduce gases (usually
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
) to the bladder to increase its
volumeThe volume of any solid, liquid, gas, plasma, theoretical object, or vacuum is how much three-dimensional space it occupies, often quantified numerically. One-dimensional figures and two-dimensional shapes are assigned zero volume in the three-dimensional space...
and thus increase
buoyancyIn physics, buoyancy is the upward force that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least seem lighter....
. To reduce buoyancy, gases are released from the bladder into the blood stream and then expelled into the water via the gills.
In order to introduce gas into the bladder, the gas gland excretes
lactic acidLactic acid , also known as milk acid, is a chemical compound that plays a role in several biochemical processes. It was first isolated in 1780 by a Swedish chemist, Carl Wilhelm Scheele, and is a carboxylic acid with a chemical formula of C
3H
6O
3...
and produces
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
the resulting acidity causes the
hemoglobinHemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates, and the tissues of some invertebrates....
of the blood to lose its oxygen (
Root effectHemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates, and the tissues of some invertebrates....
) which then
diffusesMolecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material...
partly into the gas bladder. The blood flowing back to the body first enters a
rete mirabileA rete mirabile is a complex of arteries and veins lying very close to each other, found in some vertebrates...
where virtually all the carbon dioxide and oxygen produced in the gas gland diffuse back to the arteries supplying the gas gland. Thus a very high gas pressure of oxygen can be obtained, which can even account for the presence of gas in the swim bladders of deep sea fish like the
eelTrue eels are an order of fish, which consists of four suborders, 19 families, 110 genera and approximately 600 species. Most eels are predators...
, requiring a pressure of hundreds of
barThe bar is a unit of pressure equal to 100 kilopascals, and rougly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the decibar , centibar , and millibar...
. Elsewhere, at a similar structure known as the
oval window, the bladder is in contact with blood and the oxygen can diffuse back. Together with oxygen other gasses are salted out in the gas bladder which accounts for the high pressures of other gasses as well.
The combination of gases in the bladder varies; in shallow water fish, the ratios closely approximate that of the
atmosphereThe Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
, while deep sea fish tend to have higher percentages of oxygen. For instance, the
eelTrue eels are an order of fish, which consists of four suborders, 19 families, 110 genera and approximately 600 species. Most eels are predators...
Synaphobranchus has been observed to have 75.1% oxygen, 20.5%
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
, 3.1%
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
, and 0.4%
argonArgon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is present in the Earth's atmosphere at 0.94%. Terrestrially, it is the most abundant and most frequently used of the noble gases...
in its gas bladder.
Physoclist gas bladders have one important disadvantage: they prohibit fast rising, as the bladder would burst. Physostomes can "burp" out gas, though this complicates the process of re-submergence.
In some fish, mainly freshwater species, the gas bladder is connected to the
labyrinthThe labyrinth is a system of fluid passages in the inner ear, including both the cochlea, which is part of the auditory system, and the vestibular system, which provides the sense of balance...
of the inner
earThe ear is the organ that detects sound. The vertebrate ear shows a common biology from fish to humans, with variations in structure according to order and species. It not only acts as a receiver for sound, but plays a major role in the sense of balance and body position...
by the
Weberian apparatusThe Weberian apparatus is an anatomical structure that connects the swim bladder to the auditory system in fishes belonging to the Superorder Ostariophysi. When it is fully developed in adult fish, the elements of the apparatus are sometimes collectively referred to as the Weberian ossicles...
, which provides a precise sense of water pressure (and thus depth), and improves hearing.
Evolution
Gas bladders are evolutionarily closely related (i.e.
homologousIn evolutionary biology, homology refers to any similarity between characteristics of organisms that is due to their shared ancestry. The word homologous derives from the ancient Greek ομολογειν, 'to agree'. There are examples in different branches of biology...
) to
lungThe lung or pulmonary system is the essential respiration organ in air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located in the chest on either side of the heart...
s. It is believed that the first lungs, simple sacs connected to the gut that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
s and some fish (e.g.
lungfishLungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best-known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...
,
garIn American English the name gar is strictly applied to members of the Lepisosteidae, a family including seven living species of fish in two genera that inhabit fresh, brackish, and occasionally marine, waters of eastern North America, Central America, and the Caribbean islands.-Etymology:In...
, and
bichirThe bichirs are a family, Polypteridae, of archaic-looking ray-finned fishes, the sole family in the order Polypteriformes.All species occur in freshwater habitats in tropical Africa and the Nile River system, mainly swampy, shallow floodplains and estuaries.-Anatomy and appearance:Bichirs are...
) and into the gas bladders of the ray-finned fish. In
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
nal development, both lung and gas bladder originate as an outpocketing from the gut; in the case of gas bladders, this connection to the gut continues to exist as the pneumatic duct in the more "primitive" ray-finned fish, and is lost in some of the more derived teleost orders. There are no animals which have both lungs and a gas bladder.
The
cartilaginous fishChondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
(e.g. sharks and rays) split from the other fishes about 420 million years ago and lack both lungs and gas bladders, suggesting that these structures evolved after that split. Correspondingly, these fish also have a heterocercal fin which provides the necessary lift needed due to the lack of swim bladders. On the other hand, teleost fish with swim bladders have neutral buoyancy and have no need for this lift.
Human uses
In some Asian cultures, fish maw of certain large sea fishes is considered a food delicacy. It is usually served braised or in stews. Fish maws are also used in the food industry as a source of collagen. Fish maw can also be made into a strong, water-resistant glue. Fish swim bladders are also used to make
isinglassIsinglass is a substance obtained from the swimbladders of fish . It is a form of collagen used mainly for the clarification of wine and beer.-Use in foods and drinks:...
for the clarification of
beerBeer is the world's oldest and most widely consumed alcoholic beverage and the third most popular drink overall after water and tea. It is produced by the brewing and fermentation of starches, mainly derived from cereal grains—most commonly malted barley, although wheat, maize , and rice are widely...
.
Similar structures in other organisms
Siphonophores have a special gas bladder that allows the jellyfish-like colonies to float along the surface of the water while their tentacles trail below. This organ is unrelated to the one in fish.