Gas bladder
Encyclopedia
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
that contributes to the ability of a fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
to control its buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
, and thus to stay at the current water depth without having to waste energy in swimming. The swim bladder is also of use as a stabilizing agent because in the upright position the center of mass is below the center of volume due to the dorsal
Dorsum (biology)
In 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...
position of the swim bladder. Another function of the swim bladder is the use as a resonating chamber
Resonator
A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical...
to produce or receive sound.
The swim bladder is evolutionarily homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
to the lungs, and Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
himself remarked upon this in On the Origin of Species.
Species
Swim 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 fish like the OpahOpah
Opah 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 Pomfret
Pomfret
Pomfret 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 long....
use their pectoral fins to swim and balance the weight of the head to keep a horizontal position. The normally bottom dwelling sea robin
Sea robin
Sea robins, also known as gurnard, 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 fish
Chondrichthyes
Chondrichthyes 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 swim bladders. They can control their depth only by swimming (using dynamic lift); others store fats or oils for the purpose.
Structure and function
The swim bladder normally consists of two gas-filled sacs located in the dorsalDorsum (biology)
In 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 pressure
Pressure
Pressure 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 guanine
Guanine
Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...
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
Physostome
Physostomes are fishes that have a pneumatic duct connecting the gas bladder to the alimentary canal. This allows the gas bladder to be filled or emptied via the mouth. In contrast, fishes without any connection to their gas bladder are called Physoclisti. The physostome fish encompass the...
swim bladders, a connection is retained between the swim bladder and the gut
Gut (zoology)
In zoology, the gut, also known as the alimentary canal or alimentary tract, is a tube by which bilaterian animals transfer food to the digestion organs. In large bilaterians the gut generally also has an exit, the anus, by which the animal disposes of solid wastes...
, the pneumatic duct, allowing the fish to fill up the swim bladder by "gulping" air and filling the swim bladder. Excess gas can be removed in a similar manner.
In more derived varieties of fish, the physoclisti
Physoclisti
Physoclisti are fishes that lack a connection between the gas bladder and the alimentary canal. Addition and removal of the gasses from the gas bladder in these fishes occurs through specialised structures called the gas gland and ovale respectively...
, the connection to the digestive tract is lost. In early life stages, fish have to rise to the surface to fill up their swim bladders, however, in later stages the connection disappears and the gas gland has to introduce gas (usually oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
) to the bladder to increase its volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
and thus increase buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
. In order to introduce gas into the bladder, the gas gland excretes lactic acid
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
and produces carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. The resulting acidity causes the hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
of the blood to lose its oxygen (Root effect
Root effect
The Root Effect is a physiological phenomenon that occurs in fish hemoglobin, named after its discoverer R. W. Root. It is the phenomenon where an increased proton or carbon dioxide concentration lowers hemoglobin's affinity and carrying capacity for oxygen . The Root effect is to be...
) which then diffuses
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
partly into the swim bladder. The blood flowing back to the body first enters a rete mirabile
Rete mirabile
A rete mirabile is a complex of arteries and veins lying very close to each other, found in some vertebrates. The rete mirabile utilizes countercurrent blood flow within the net...
where virtually all the excess carbon dioxide and oxygen produced in the gas gland diffuses 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 eel
Eel
Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...
, requiring a pressure of hundreds of bars
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , 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 gases are salted out in the swim bladder which accounts for the high pressures of other gases as well.
The combination of gases in the bladder varies. In shallow water fish, the ratios closely approximate that of the atmosphere
Earth's atmosphere
The atmosphere of Earth 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 eel
Eel
Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...
Synaphobranchus
Synaphobranchus
Synaphobranchus is a genus of eels in the cutthroat eel family Synaphobranchidae. It currently contains the following species:* Synaphobranchus affinis Günther, 1877...
has been observed to have 75.1% oxygen, 20.5% nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, 3.1% carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, and 0.4% argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
in its swim bladder.
Physoclist swim 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 (e.g. common carp
Common carp
The Common carp is a widespread freshwater fish of eutrophic waters in lakes and large rivers in Europe and Asia. The wild populations are considered vulnerable to extinction, but the species has also been domesticated and introduced into environments worldwide, and is often considered an invasive...
, wels catfish
Wels catfish
The wels catfish , also called sheatfish, is a large catfish found in wide areas of central, southern, and eastern Europe, and near the Baltic and Caspian Seas. It is a scaleless fresh and brackish water fish recognizable by its broad, flat head and wide mouth...
), the swim bladder is connected to the labyrinth of the inner ear
Ear
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....
by the Weberian apparatus
Weberian apparatus
The 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...
, a bony structure derived from the vertebrae, which provides a precise sense of water pressure (and thus depth), and improves hearing.
Evolution
Swim bladders are evolutionarily closely related (i.e. homologousHomology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
) to lung
Lung
The lung is the essential respiration organ in many 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 near the backbone on either side of the heart...
s. It is believed that the first lungs, simple sacs connected to the gut
Gut (zoology)
In zoology, the gut, also known as the alimentary canal or alimentary tract, is a tube by which bilaterian animals transfer food to the digestion organs. In large bilaterians the gut generally also has an exit, the anus, by which the animal disposes of solid wastes...
that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s and some fish (e.g. lungfish
Lungfish
Lungfish 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...
, gar
Gar
In 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 bichir
Bichir
The 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 swim bladders of the ray-finned fish. In embryo
Embryo
An 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 swim bladder originate as an outpocketing from the gut; in the case of swim 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 swim bladder.
The cartilaginous fish
Chondrichthyes
Chondrichthyes 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 swim 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
Isinglass
Isinglass is a substance obtained from the dried swim bladders of fish. It is a form of collagen used mainly for the clarification of wine and beer. It can also be cooked into a paste for specialized gluing purposes....
for the clarification of beer
Beer
Beer is the world's most widely consumed andprobably oldest alcoholic beverage; it is the third most popular drink overall, after water and tea. It is produced by the brewing and fermentation of sugars, mainly derived from malted cereal grains, most commonly malted barley and malted wheat...
.
The gas/tissue interface at the swim bladder produces a strong reflection of sound, which is used in sonar
Sonar
Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...
equipment to find fish.