Swim bladder

The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish[1]) to control their buoyancy, and thus to stay at their current water depth without having to waste energy in swimming.[2] Also, the dorsal position of the swim bladder means the center of mass is below the center of volume, allowing it to act as a stabilizing agent. Additionally, the swim bladder functions as a resonating chamber, to produce or receive sound.

The swim bladder is evolutionarily homologous to the lungs. Charles Darwin remarked upon this in On the Origin of Species.[3] Darwin reasoned that the lung in air-breathing vertebrates had derived from a more primitive swim bladder.

In the embryonic stages, some species, such as redlip blenny,[4] have lost the swim bladder again, mostly bottom dwellers like the weather fish. Other fish like the Opah and the Pomfret use their pectoral fins to swim and balance the weight of the head to keep a horizontal position. The normally bottom dwelling sea robin can use their pectoral fins to produce lift while swimming.

The gas/tissue interface at the swim bladder produces a strong reflection of sound, which is used in sonar equipment to find fish.

Cartilaginous fish, such as sharks and rays, do not have swim bladders. Some of them can control their depth only by swimming (using dynamic lift); others store fats or oils with density less than that of seawater to produce a neutral or near neutral buoyancy, which does not change with depth.

Swim bladder
The swim bladder of a rudd
Air bladder in a bleak
Internal positioning of the swim bladder of a bleak
S: anterior, S': posterior portion of the air bladder
œ: œsophagus; l: air passage of the air bladder

Structure and function

Oste023c labelled
Swim bladder from a bony (teleost) fish
How gas is pumped into the swim bladder using counter-current exchange.

The swim bladder normally consists of two gas-filled sacs located in the dorsal 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. The walls of the bladder contain very few blood vessels and are lined with guanine crystals, which make them impermeable to gases. By adjusting the gas pressurising organ 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 swim bladders, a connection is retained between the swim bladder and the gut, the pneumatic duct, allowing the fish to fill up the swim bladder by "gulping" air. Excess gas can be removed in a similar manner.

In more derived varieties of fish (the physoclisti) the connection to the digestive tract is lost. In early life stages, these fish must rise to the surface to fill up their swim bladders; in later stages, the pneumatic duct disappears, and the gas gland has to introduce gas (usually oxygen) to the bladder to increase its volume and thus increase buoyancy. In order to introduce gas into the bladder, the gas gland excretes lactic acid and produces carbon dioxide. The resulting acidity causes the hemoglobin of the blood to lose its oxygen (Root effect) which then diffuses partly into the swim bladder. The blood flowing back to the body first enters a rete mirabile 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, requiring a pressure of hundreds of bars.[5] Elsewhere, at a similar structure known as the oval window, the bladder is in contact with blood and the oxygen can diffuse back out again. Together with oxygen, other gases are salted out in the swim bladder which accounts for the high pressures of other gases as well.[6]

The combination of gases in the bladder varies. In shallow water fish, the ratios closely approximate that of the atmosphere, while deep sea fish tend to have higher percentages of oxygen. For instance, the eel Synaphobranchus has been observed to have 75.1% oxygen, 20.5% nitrogen, 3.1% carbon dioxide, and 0.4% argon 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.

The swim bladder in some species, mainly fresh water fishes (common carp, catfish, bowfin) is interconnected with the inner ear of the fish. They are connected by four bones called the Weberian ossicles from the Weberian apparatus. These bones can carry the vibrations to the saccule and the lagena (anatomy). They are suited for detecting sound and vibrations due to its low density in comparison to the density of the fish's body tissues. This increases the ability of sound detection.[7] The swim bladder can radiate the pressure of sound which help increase its sensitivity and expand its hearing. In some deep sea fishes like the Antimora, the swim bladder maybe also connected to the macula of saccule in order for the inner ear to receive a sensation from the sound pressure.[8] In red-bellied piranha, the swimbladder may play an important role in sound production as a resonator. The sounds created by piranhas are generated through rapid contractions of the sonic muscles and is associated with the swimbladder.[9]

Teleosts are thought to lack a sense of absolute hydrostatic pressure, which could be used to determine absolute depth.[10] However, it has been suggested that teleosts may be able to determine their depth by sensing the rate of change of swim-bladder volume.[11]


PSM V20 D769 Lepidosiren annectens using the air bladder as a lung
The West African lungfish possesses a lung homologous to swim bladders

Swim bladders are evolutionarily closely related (i.e., homologous) to lungs. Traditional wisdom has long held 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 vertebrates and some fish (e.g., lungfish, gar, and bichir) and into the swim bladders of the ray-finned fish. In 1997, Farmer proposed that lungs evolved to supply the heart with oxygen. In fish, blood circulates from the gills to the skeletal muscle, and only then to the heart. During intense exercise, the oxygen in the blood gets used by the skeletal muscle before the blood reaches the heart. Primitive lungs gave an advantage by supplying the heart with oxygenated blood via the cardiac shunt. This theory is robustly supported by the fossil record, the ecology of extant air-breathing fishes, and the physiology of extant fishes.[12] In embryonal 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 (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.[12] Correspondingly, these fish also have both heterocercal and stiff, wing-like pectoral fins which provide the necessary lift needed due to the lack of swim bladders. Teleost fish with swim bladders have neutral buoyancy, and have no need for this lift.[13]

Deep scattering layer

California headlightfish
Most mesopelagic fishes are small filter feeders which ascend at night using their swimbladders to feed in the nutrient rich waters of the epipelagic zone. During the day, they return to the dark, cold, oxygen deficient waters of the mesopelagic where they are relatively safe from predators. Lanternfish account for as much as 65 percent of all deep sea fish biomass and are largely responsible for the deep scattering layer of the world's oceans.

Sonar operators, using the newly developed sonar technology during World War II, were puzzled by what appeared to be a false sea floor 300–500 metres deep at day, and less deep at night. This turned out to be due to millions of marine organisms, most particularly small mesopelagic fish, with swimbladders that reflected the sonar. These organisms migrate up into shallower water at dusk to feed on plankton. The layer is deeper when the moon is out, and can become shallower when clouds obscure the moon.[14]

Most mesopelagic fish make daily vertical migrations, moving at night into the epipelagic zone, often following similar migrations of zooplankton, and returning to the depths for safety during the day.[15][16] These vertical migrations often occur over large vertical distances, and are undertaken with the assistance of a swim bladder. The swim bladder is inflated when the fish wants to move up, and, given the high pressures in the mesoplegic zone, this requires significant energy. As the fish ascends, the pressure in the swimbladder must adjust to prevent it from bursting. When the fish wants to return to the depths, the swimbladder is deflated.[17] Some mesopelagic fishes make daily migrations through the thermocline, where the temperature changes between 10 and 20 °C, thus displaying considerable tolerance for temperature change.

Sampling via deep trawling indicates that lanternfish account for as much as 65% of all deep sea fish biomass.[18] Indeed, lanternfish are among the most widely distributed, populous, and diverse of all vertebrates, playing an important ecological role as prey for larger organisms. The estimated global biomass of lanternfish is 550–660 million metric tonnes, several times the entire world fisheries catch. Lanternfish also account for much of the biomass responsible for the deep scattering layer of the world's oceans. Sonar reflects off the millions of lanternfish swim bladders, giving the appearance of a false bottom.[19]

Human uses

In some Asian cultures, the swim bladders of certain large fishes are considered a food delicacy. In China they are known as fish maw, 花膠/鱼鳔,[20] and are served in soups or stews.

The vanity price of a vanishing kind of maw is behind the imminent extinction of the vaquita, the world's smallest dolphin breed. Only found in Mexico's Gulf of California, the once numerous vaquita now number less than 60 in total. Vaquita die in gillnets[21] set to catch totoaba (the world's largest drum fish). Totoaba are being hunted to extinction for its maw, which can sell for as much $10,000 per kilogram.

Swim bladders are also used in the food industry as a source of collagen. They can be made into a strong, water-resistant glue, or used to make isinglass for the clarification of beer.[22] In earlier times they were used to make condoms.[23]

Swim bladder disease

Swim bladder disease is a common ailment in aquarium fish. A fish with swim bladder disorder can float nose down tail up, or can float to the top or sink to the bottom of the aquarium.[24]

Risk of injury

Many anthropogenic activities, like pile driving or even Seismic wave, that could result from climate change or natural causes, can create high-intensity sound waves that cause a certain amount of damage to fish that possess a gas bladder. Physostomes can release air in order to decrease the tension in the gas bladder that may cause internal injuries to other vital organs, while physoclisti can't expel air fast enough, making it more difficult to avoid any major injuries.[25] Some of the commonly seen injuries included ruptured gas bladder and renal Haemorrhage. These mostly affect the overall health of the fish and didn't affect their mortality rate.[25] Investigators used the High-Intensity-Controlled Impedance Fluid Filled (HICI-FT), a stainless-steel wave tube with an electromagnetic shaker. It simulates high-energy sound waves in aquatic far-field, plane-wave acoustic conditions.[26][27]

Similar structures in other organisms

Siphonophores have a special swim 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.[28]



Swim bladder display in a Malacca shopping mall

Fish maw soup

Fish maw soup

Goldfish with swim bladder disease

Swim bladder disease has resulted in this female ryukin goldfish floating upside down


  1. ^ "More on Morphology". www.ucmp.berkeley.edu.
  2. ^ "Fish". Microsoft Encarta Encyclopedia Deluxe 1999. Microsoft. 1999.
  3. ^ a b Darwin, Charles (1859) Origin of Species Page 190, reprinted 1872 by D. Appleton.
  4. ^ Nursall, J. R. (1989). "Buoyancy is provided by lipids of larval redlip blennies, Ophioblennius atlanticus". Copeia. 3 (3): 614–621. doi:10.2307/1445488. JSTOR 1445488.
  5. ^ Pelster B (December 2001). "The generation of hyperbaric oxygen tensions in fish". News Physiol. Sci. 16 (6): 287–91. PMID 11719607.
  6. ^ "Secretion Of Nitrogen Into The Swimbladder Of Fish. Ii. Molecular Mechanism. Secretion Of Noble Gases". Biolbull.org. 1981-12-01. Retrieved 2013-06-24.
  7. ^ Kardong, Kenneth. Vertebrates: Comparative Anatomy, Function, Evolution. New York: McGraw-Hill Education. p. 701. ISBN 9780073524238.
  8. ^ Deng, Xiaohong; Wagner, Hans-Joachim; Popper, Arthur N. (2011-01-01). "The inner ear and its coupling to the swim bladder in the deep-sea fish Antimora rostrata (Teleostei: Moridae)". Deep Sea Research Part I: Oceanographic Research Papers. 58 (1): 27–37. doi:10.1016/j.dsr.2010.11.001. PMC 3082141. PMID 21532967.
  9. ^ Onuki, A; Ohmori Y.; Somiya H. (January 2006). "Spinal Nerve Innervation to the Sonic Muscle and Sonic Motor Nucleus in Red Piranha, Pygocentrus nattereri (Characiformes, Ostariophysi)". Brain, Behavior and Evolution. 67 (2): 11–122. doi:10.1159/000089185. PMID 16254416.
  10. ^ Bone, Q.; Moore, Richard H. Biology of fishes (3rd., Thoroughly updated and rev ed.). Taylor & Francis. ISBN 9780415375627.
  11. ^ Taylor, Graham K.; Holbrook, Robert Iain; de Perera, Theresa Burt (6 September 2010). "Fractional rate of change of swim-bladder volume is reliably related to absolute depth during vertical displacements in teleost fish". Journal of the Royal Society Interface. 7 (50): 1379–1382. doi:10.1098/rsif.2009.0522. PMC 2894882. PMID 20190038.
  12. ^ a b Farmer, Colleen (1997). "Did lungs and the intracardiac shunt evolve to oxygenate the heart in vertebrates" (PDF). Paleobiology. 23 (3): 358–372. doi:10.1017/S0094837300019734.
  13. ^ Kardong, KV (1998) Vertebrates: Comparative Anatomy, Function, Evolution2nd edition, illustrated, revised. Published by WCB/McGraw-Hill, p. 12 ISBN 0-697-28654-1
  14. ^ Ryan P "Deep-sea creatures: The mesopelagic zone" Te Ara - the Encyclopedia of New Zealand. Updated 21 September 2007.
  15. ^ Moyle, Peter B.; Cech, Joseph J. (2004). Fishes : an introduction to ichthyology (5th ed.). Upper Saddle River, N.J.: Pearson/Prentice Hall. p. 585. ISBN 9780131008472.
  16. ^ Bone, Quentin; Moore, Richard H. (2008). "Chapter 2.3. Marine habitats. Mesopelagic fishes". Biology of fishes (3rd ed.). New York: Taylor & Francis. p. 38. ISBN 9780203885222.
  17. ^ Douglas EL, Friedl WA and Pickwell GV (1976) "Fishes in oxygen-minimum zones: blood oxygenation characteristics" Science, 191 (4230) 957–959.
  18. ^ Hulley, P. Alexander (1998). Paxton, J.R.; Eschmeyer, W.N., eds. Encyclopedia of Fishes. San Diego: Academic Press. pp. 127–128. ISBN 978-0-12-547665-2.
  19. ^ R. Cornejo; R. Koppelmann & T. Sutton. "Deep-sea fish diversity and ecology in the benthic boundary layer".
  20. ^ Teresa M. (2009) A Tradition of Soup: Flavors from China's Pearl River Delta Page 70, North Atlantic Books. ISBN 9781556437656.
  21. ^ "'Extinction Is Imminent': New report from Vaquita Recovery Team (CIRVA) is released". IUCN SSC - Cetacean Specialist Group. 2016-06-06. Retrieved 2017-01-25.
  22. ^ Bridge, T. W. (1905) [1] "The Natural History of Isinglass"
  23. ^ Huxley, Julian (1957) "Material of early contraceptive sheaths." British Medical Journal, 1 (5018): 581–582.
  24. ^ Johnson, Erik L. and Richard E. Hess (2006) Fancy Goldfish: A Complete Guide to Care and Collecting, Weatherhill, Shambhala Publications, Inc. ISBN 0-8348-0448-4
  25. ^ a b Halvorsen, Michele B.; Casper, Brandon M.; Matthews, Frazer; Carlson, Thomas J.; Popper, Arthur N. (2012-12-07). "Effects of exposure to pile-driving sounds on the lake sturgeon, Nile tilapia and hogchoker". Proceedings of the Royal Society B: Biological Sciences. 279 (1748): 4705–4714. doi:10.1098/rspb.2012.1544. ISSN 0962-8452. PMC 3497083. PMID 23055066.
  26. ^ Halvorsen, Michele B.; Casper, Brandon M.; Woodley, Christa M.; Carlson, Thomas J.; Popper, Arthur N. (2012-06-20). "Threshold for Onset of Injury in Chinook Salmon from Exposure to Impulsive Pile Driving Sounds". PLOS ONE. 7 (6): e38968. doi:10.1371/journal.pone.0038968. ISSN 1932-6203. PMC 3380060. PMID 22745695.
  27. ^ Popper, Arthur N.; Hawkins, Anthony (2012-01-26). The Effects of Noise on Aquatic Life. Springer Science & Business Media. ISBN 9781441973115.
  28. ^ Clark, F. E.; C. E. Lane (1961). "Composition of float gases of Physalia physalis". Fed. Proc. 107 (3): 673–674. doi:10.3181/00379727-107-26724.

Further references

  • Bond, Carl E. (1996) Biology of Fishes, 2nd ed., Saunders, pp. 283–290.
  • Pelster, Bernd (1997) "Buoyancy at depth" In: WS Hoar, DJ Randall and AP Farrell (Eds) Deep-Sea Fishes, pages 195–237, Academic Press. ISBN 9780080585406.

The Amiidae are a family of basal ray-finned fishes. The bowfin is the only species to survive today, although additional species in all four subfamilies of Amiidae are known from Jurassic, Cretaceous, and Eocene fossils.Bowfins are now found throughout eastern North America, typically in slow-moving backwaters, canals, and ox-bow lakes. When the oxygen level is low (as often happens in still waters), the bowfin can rise to the surface and gulp air into its swim bladder, which is lined with blood vessels and can serve as a primitive lung.

Channel catfish

The channel catfish (Ictalurus punctatus) is North America's most numerous catfish species. It is the official fish of Kansas, Missouri, Iowa, Nebraska, and Tennessee, and is informally referred to as a "channel cat". In the United States, they are the most fished catfish species with around 8 million anglers targeting them per year. The popularity of channel catfish for food has contributed to the rapid expansion of aquaculture of this species in the United States.


The flying gurnards are a family, Dactylopteridae, of marine fish notable for their greatly enlarged pectoral fins. As they cannot literally fly or glide in the air (like flying fish), an alternative name preferred by some authors is helmet gurnards. They have been regarded as the only family in the suborder Dactylopteroidei of the Scorpaeniformes but more recent molecular classifications put them in the order Syngnathiformes, in the superfamily Centriscoidea.They have been observed to "walk" along sandy sea floors while looking for crustaceans, other small invertebrates and small fish by using their pelvic fins. Like the true gurnards (sea robins), to which they may be related, they possess a swim bladder with two lobes and a "drumming muscle" that can beat against the swim bladder to produce sounds. They have heavy, protective scales and the undersides of their huge pectoral fins are brightly coloured, perhaps to startle predators.Most species are in the Indo-Pacific genus Dactyloptena, but the single member of Dactylopterus is from warmer parts of the Atlantic. The adults live on the sea bottom, but many species have an extended larval stage, which floats freely in the oceans.

Fish anatomy

Fish anatomy is the study of the form or morphology of fishes. It can be contrasted with fish physiology, which is the study of how the component parts of fish function together in the living fish. In practice, fish anatomy and fish physiology complement each other, the former dealing with the structure of a fish, its organs or component parts and how they are put together, such as might be observed on the dissecting table or under the microscope, and the latter dealing with how those components function together in living fish.

The anatomy of fish is often shaped by the physical characteristics of water, the medium in which fish live. Water is much denser than air, holds a relatively small amount of dissolved oxygen, and absorbs more light than air does. The body of a fish is divided into a head, trunk and tail, although the divisions between the three are not always externally visible. The skeleton, which forms the support structure inside the fish, is either made of cartilage, in cartilaginous fish, or bone in bony fish. The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong. The ribs attach to the spine and there are no limbs or limb girdles. The main external features of the fish, the fins, are composed of either bony or soft spines called rays which, with the exception of the caudal fins, have no direct connection with the spine. They are supported by the muscles which compose the main part of the trunk. The heart has two chambers and pumps the blood through the respiratory surfaces of the gills and on round the body in a single circulatory loop. The eyes are adapted for seeing underwater and have only local vision. There is an inner ear but no external or middle ear. Low frequency vibrations are detected by the lateral line system of sense organs that run along the length of the sides of fish, and these respond to nearby movements and to changes in water pressure.Sharks and rays are basal fish with numerous primitive anatomical features similar to those of ancient fish, including skeletons composed of cartilage. Their bodies tend to be dorso-ventrally flattened, they usually have five pairs of gill slits and a large mouth set on the underside of the head. The dermis is covered with separate dermal placoid scales. They have a cloaca into which the urinary and genital passages open, but not a swim bladder. Cartilaginous fish produce a small number of large, yolky eggs. Some species are ovoviviparous and the young develop internally but others are oviparous and the larvae develop externally in egg cases.The bony fish lineage shows more derived anatomical traits, often with major evolutionary changes from the features of ancient fish. They have a bony skeleton, are generally laterally flattened, have five pairs of gills protected by an operculum, and a mouth at or near the tip of the snout. The dermis is covered with overlapping scales. Bony fish have a swim bladder which helps them maintain a constant depth in the water column, but not a cloaca. They mostly spawn a large number of small eggs with little yolk which they broadcast into the water column.


Gars (or garpike) are members of the Lepisosteiformes (or Semionotiformes), an ancient holosteian order of ray-finned fish; fossils from this order are known from the Late Jurassic onwards. The family Lepisosteidae includes 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. Gars have elongated bodies that are heavily armored with ganoid scales, and fronted by similarly elongated jaws filled with long, sharp teeth. All of the gars are relatively large fish, but the alligator gar (Atractosteus spatula) is the largest, as specimens have been reported to be 3 m (9.8 ft) in length; however, they typically grow to 2 m (6.5 ft) and weigh over 45 kg (100 lb). Unusually, their vascularised swim bladders can function as lungs, and most gars surface periodically to take a gulp of air. Gar flesh is edible and the hard skin and scales of gars are used by humans; however their eggs are highly toxic.


The Gobiiformes are an order of fish that includes the gobies. The order, which was previously considered a suborder of Perciformes, is made up of 2,211 species that are divided into seven different families. Phylogenetic relationships of the Gobiiformes have been elucidated using molecular data. Gobiiforms are generally small fish and are mostly marine (saltwater) fishes, but roughly 10% of the population inhabit fresh waters. This order is made up of mainly benthic or sand-burrowing fish. Benthic fish live on the bottom of a body of water. Like in most benthic organisms, gobiiforms do not have a gas bladder or swim bladder which keeps them from suspending in the water column, so they must stay on the bottom.


The Moridae are a family of cod-like fishes, known as codlings, hakelings, and moras.

Morids are marine fishes found throughout the world, and may be found at depths to 2,500 m (8,200 ft), although most prefer shallower waters. In appearance, they greatly resemble the typical cods, from which can only be distinguished by their skeletal features and the structure of the swim bladder.They grow up to 90 cm (35 in) long (red codling, Pseudophycis bachus).

Neutral buoyancy

Neutral buoyancy occurs when a object's average density is equal to the density of the fluid in which it is immersed, resulting in the buoyant force balancing the force of gravity that would otherwise cause the object to sink (if the body's density is greater than the density of the fluid in which it is immersed) or rise (if it's less). An object that has neutral buoyancy will neither sink nor rise.

In scuba diving, the ability to maintain neutral buoyancy through controlled breathing, accurate weighting, and management of the buoyancy compensator is an important skill. A scuba diver maintains neutral buoyancy by continuous correction, usually by controlled breathing, as neutral buoyancy is an unstable condition for a compressible object in a liquid.

Notothenia microlepidota

Notothenia microlepidota, the black cod or small-scaled cod, is a species of notothen native to the Pacific waters around New Zealand and Macquarie Island. This species can reach a total length of 70 cm (28 in). It is a commercially important species.The juveniles are silvery in appearance with a pronounced tail fork. The adult has a less pronounced fork in the tail, with body colors of silver, yellow, and reddish brown. The scales are very small, and the two lateral lines have a considerable overlap.

Being a Subantarctic species, the black cod has special adaptations such as antifreeze proteins in its blood, as well as adipose tissues to offset its lack of a swim bladder, giving it neutral buoyancy. Sablefish, Anoplopoma fimbria, is occasionally called black cod, as well, but it is not a true cod. In New Zealand, Maori cod is also known as "black cod".


Ostariophysi is the second-largest superorder of fish. Members of this superorder are called ostariophysians. This diverse group contains almost 8,000 species, about 28% of known fish species in the world and 68% of freshwater species, and are present on all continents except Antarctica. They have a number of common characteristics such as an alarm substance and a Weberian apparatus. Members of this group include fish important to people for food, sport, the aquarium industry, and research.

Poacher (fish)

The poachers are a family (Agonidae) of small, bottom-dwelling, cold-water marine fish. They are also known as alligatorfishes, starsnouts, hooknoses, and rockheads. Poachers are notable for having elongated bodies covered by scales modified into bony plates, and for using their large pectoral fins to move in short bursts. The family includes about 47 species in some 20 genera, some of which are quite widespread.

The pelvic fins are nearly vestigial, typically consisting of one small spine and a few rays. The swim bladder is not present.

At 42 centimetres (17 in) in length, the dragon poacher Percis japonica is the largest member of the family, while Bothragonus occidentalis is 7 cm (2.8 in) long as an adult; most are in the 20–30 cm range.

Poachers generally feed on small crustaceans and marine worms found on the bottom. Some species camouflage themselves with hydras, sponges, or seaweed. They live at 1,280 m (4,200 ft) deep, with only a few species preferring shallower, coastal waters. All but one species are restricted to the Northern Hemisphere.


The roosterfish, Nematistius pectoralis, is a game fish found in the warmer waters of the East Pacific from Baja California to Peru. It is the only species in the genus Nematistius and the family Nematistiidae. It is distinguished by its "rooster comb", seven very long spines of the dorsal fin.

The roosterfish has an unusual arrangement of its ears: the swim bladder penetrates the brain through the large foramina and makes contact with the inner ear. It uses its swim bladder to amplify sounds.

Roosterfish can reach over 1.6 m (5 ft 3 in) in length and over 50 kg (110 lb) in weight. The weight of the average fish hooked is about 20 lb (9.1 kg). The fish is popular as a game fish, but it is not considered a good eating fish. Catch and release is strongly recommended.

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 (lower pH) lowers hemoglobin's affinity and carrying capacity for oxygen. The Root effect is to be distinguished from the Bohr effect where only the affinity to oxygen is reduced. Hemoglobins showing the Root effect show a loss of cooperativity at low pH. This results in the Hb-O2 dissociation curve being shifted downward and not just to the right. At low pH, hemoglobins showing the Root effect don't become fully oxygenated even at oxygen tensions up to 20kPa. This effect allows hemoglobin in fish with swim bladders to unload oxygen into the swim bladder against a high oxygen gradient. The effect is also noted in the choroid rete, the network of blood vessels which carries oxygen to the retina. In the absence of the Root effect, retia will result in the diffusion of some oxygen directly from the arterial blood to the venous blood, making such systems less effective for the concentration of oxygen. It has also been hypothesized that the loss of affinity is used to provide more oxygen to red muscle during acidotic stress.


The Sciaenidae are a family of fish commonly called drums or croakers in reference to the repetitive throbbing or drumming sounds they make. The family includes the weakfish, and consists of about 275 species in about 70 genera; it belongs to the order Perciformes.

Spring viraemia of carp

Spring viraemia of carp, also known as swim bladder inflammation, is caused by Carp sprivivirus, also called Rhabdovirus carpio. It is listed as a notifiable disease under the World Organisation for Animal Health.

Swim bladder disease

Swim bladder disease, also called swim bladder disorder or flipover, is a common ailment in aquarium fish. The swim bladder is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming. A fish with swim bladder disorder can float nose down tail up, or can float to the top or sink to the bottom of the aquarium.


Tarpons are large air-breathing fish of the genus Megalops; one species is native to the Atlantic, and the other to the Indo-Pacific Seas. They are the only members of the family Megalopidae.


Tinnsjå (English: Lake Tinn), also called Tinnsjø and Tinnsjøen, is one of the largest lakes in Norway, and one of the deepest in Europe. It is located between the municipalities of Tinn and Notodden in Telemark county. At its source in the west, the Måna river flows out of Møsvatn and past Rjukan into Tinnsjå. From the north, the river Mår flows from the Mår, Gøystavatn, and Kalhovdfjorden lakes into Tinnsjå. Tinnsjå is part of the Skiensvassdrag, and drains via the Tinnelva river in the south, down to Heddalsvatn.

In 1944, during the German occupation of Norway, the ferry SF Hydro was sunk in Tinnsjå by the Norwegian resistance. The Germans were using the ferry to transport a large quantity of heavy water to Germany, where it was to be used for nuclear weapon research. The heavy water had been produced at Vemork, a factory located in Rjukan. The wreck of the ferry was discovered in 1993. In 2004, it was investigated and filmed for an episode of NOVA; heavy water samples were recovered and deuterium isotopic enrichment was confirmed.

In 2004 a film crew shooting footage for a new documentary on the heavy water sabotage became aware of an unusual fish, swimming near the lake bottom at a depth of 430 m. Two specimens of the previously unknown fish were captured in April 2005. Analysis revealed the fish to be closely related to Arctic char. The light-colored, translucent fish is up to 15 cm long and lacks a swim bladder.

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. The presence of the structure is one of the most important and phylogenetically significant distinguishing characteristics of the Ostariophysi. The structure itself consists of a set of minute bones that originate from the first few vertebrae to develop in an embryonic ostariophysan. These bones grow to physically connect the auditory system, specifically the inner ear, to the swim bladder. The structure acts as an amplifier of sound waves that would otherwise be only slightly perceivable by the inner ear structure alone.

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