A kype is a hook-like secondary sex characteristic which develops at the distal tip of the lower jaw in some male salmonids prior to the spawning season. [1] [2] The structure usually develops in the weeks prior to, and during, migration to the spawning grounds. In addition to the development of the kype, a large depression forms in the two halves of the premaxilla in the upper jaw, allowing the kype to fit into the premaxilla when the mouth is closed. [3]
The kype functions as a secondary sexual characteristic and influences the formation of dominance hierarchies at the spawning grounds. The size of the kype is believed to determine male spawning frequency.
The kype grows rapidly from bony needles proliferating from the tip of the dentary (the anterior and largest of the bones making up the lower jaw). The needles form a mesh, but do not interfere with the connective tissues used by bone marrow. At the snout, the needles strengthen into Sharpey's fibres. The speed at which the kype skeleton develops results in many osteoblasts and proteoglycans appearing along the growth zone. The dentary itself is made of compact bone, but the kype tissue contains chondrocytes and cartilage. [3] [4] The kype formation process has been described as "making bone as fast as possible and with as little material as possible". [4]
Some species of salmon are semelparous (they have a single reproductive bout before death) whereas others are iteroparous (they spawn multiple times after maturation). In iteroparous cases, at least in Atlantic salmon, the kype is not fully resorbed after the breeding season, although basal parts of the kype skeleton are re-modelled into regular dentary bone. [3] Some fish never lose their kype. Rather, as they re-enter subsequent spawning seasons, their kypes continue to grow. This fast growing skeletal tissue fuses with the dense dentary, becoming a permanent, growing kype. [5]
Many male trout (e.g. Brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss)) and salmon develop a kype prior to spawning periods. [5] In pre-spawning Salmo and Salvelinus males, the lower jaw elongates and the hook develops; female salmon do not develop a kype. [3] Bull trout (Salvelinus confluentus) are adfluvial (adults spawn in streams but subadults and adults migrate to lakes for feeding) and sometimes develop a kype, however, although this may occur in some populations, it remains absent in others. [6]
Among American species of charr, the kype reaches its maximum size in the large anadromous males, Dolly Varden trout (Salvelinus malma) and brook trout (Salvelinus fontinalis), whereas it is reportedly absent or hardly visible in large nonanadromous males, Arctic char (Salvelinus alpinus) and lake trout (Salvelinus namaycush). [7]
In salmonids of the genus Oncorhynchus (meaning "hooked snout"), the upper jaw becomes more elongated than the lower thereby forming a "snout". [3] In some species, the development of the "kype" (in this study defined as the distance from the middle of eye to the tip of the snout) is used as an indicator of a difference in behavioural mating strategies. Chinook salmon (Oncorhynchus tshawytscha) express one of two fixed alternative reproductive tactics. Individuals expressing these are referred to as "hooknose" or "jack". Hooknose males leave their natal rivers at the end of their first year of life, but then return after maturing for 3 to 5 years on average. Once returned, they fight for position in a dominance hierarchy to gain closer access to spawning females. Alternatively, jacks are presumably resident in their natal rivers their entire lives, reach sexual maturity precociously (after 2 years), and use a sneaking tactic, by darting from nearby refuges to steal fertilisations from hooknose males. [8]
Development of the kype often occurs in association with other seasonal changes. In the Atlantic salmon (Salmo salar), kype development is accompanied by a morphogenesis of bones and cartilages in the ethmoidal zone (the anterior region of the skull) changing the appearance of both jaws, the appearance of "breeding teeth" and resorption of scales (more so in males than females). [9] Some salmonids may develop a predominant hump under their dorsal fin.
Charles Darwin considered the kype to be a product of sexual selection and as a tool for fighting among males. For example, male salmon have been seen in the wild using their kype to firmly grasp an opponent's tail. [10] Others have suggested it has no function, and observed the kype seems to prevent the use of the breeding teeth which sometimes develop alongside the kype. One suggestion was that the kype is merely the result of a surplus of chemicals, not used for the production of sex products. [3]
Today, the kype is regarded as a secondary sex characteristic displayed by males at the spawning grounds. Therefore, its function is considered to be helping the fish establish a hierarchy among other males where those with larger kypes are dominant over animals with smaller kypes, [2] [3] and/or characteristics that could be of importance in inter- and intra-sexual evaluations of individual quality. The size of the kype is believed to determine male spawning frequency. [11]
The extinct sabertooth salmon, Oncorhynchus rastrosus , was first named for its prominent premaxillary dentition. It possessed an enormous conical tooth on each premaxilla. There is no visible kype on the dentary, implying a different strategy for forming mate dominance. [12]
Salmon is the common name for several commercially important species of euryhaline ray-finned fish from the genera Salmo and Oncorhynchus of the family Salmonidae, native to tributaries of the North Atlantic (Salmo) and North Pacific (Oncorhynchus) basins. Other closely related fish in the same family include trout, char, grayling, whitefish, lenok and taimen, all coldwater fish of the subarctic and cooler temperate regions with some sporadic endorheic populations in Central Asia.
Trout is a generic common name for numerous species of carnivorous freshwater ray-finned fishes belonging to the genera Oncorhynchus, Salmo and Salvelinus, all of which are members of the subfamily Salmoninae in the family Salmonidae. The word trout is also used for some similar-shaped but non-salmonid fish, such as the spotted seatrout/speckled trout.
Salmonidae is a family of ray-finned fish that constitutes the only currently extant family in the order Salmoniformes, consisting of 11 extant genera and over 200 species collectively known as "salmonids" or "salmonoids". The family includes salmon, trout, char, graylings, freshwater whitefishes, taimens and lenoks, all coldwater mid-level predatory fish that inhabit the subarctic and cool temperate waters of the Northern Hemisphere. The Atlantic salmon, whose Latin name became that of its genus Salmo, is also the eponym of the family and order names.
The masu salmon, also known as masu or cherry trout in Japan, is a species of salmonid belonging to the genus Oncorhynchus, found in the North Pacific along Northeast/East Asian coasts from the Russian Far East to south through Korea, Japan and Taiwan. Although generally accepted as a salmon in the West, the fish is actually regarded as a trout in Japan as it is the most commonly seen freshwater salmonid in the Japanese archipelago.
A salmon run is an annual fish migration event where many salmonid species, which are typically hatched in fresh water and live most of the adult life downstream in the ocean, swim back against the stream to the upper reaches of rivers to spawn on the gravel beds of small creeks. After spawning, all species of Pacific salmon and most Atlantic salmon die, and the salmon life cycle starts over again with the new generation of hatchlings.
The rainbow trout is a species of trout native to cold-water tributaries of the Pacific Ocean in Asia and North America. The steelhead is an anadromous (sea-run) form of the coastal rainbow trout(O. m. irideus) or Columbia River redband trout (O. m. gairdneri) that usually returns to freshwater to spawn after living two to three years in the ocean. Freshwater forms that have been introduced into the Great Lakes and migrate into tributaries to spawn are also called steelhead.
The Atlantic salmon is a species of ray-finned fish in the family Salmonidae. It is the third largest of the Salmonidae, behind Siberian taimen and Pacific Chinook salmon, growing up to a meter in length. Atlantic salmon are found in the northern Atlantic Ocean and in rivers that flow into it. Most populations are anadromous, hatching in streams and rivers but moving out to sea as they grow where they mature, after which the adults seasonally move upstream again to spawn.
The Arctic char or Arctic charr is a cold-water fish in the family Salmonidae, native to alpine lakes, as well as Arctic and subarctic coastal waters in the Holarctic.
Sea trout is the common name usually applied to anadromous (sea-run) forms of brown trout, and is often referred to as Salmo trutta morpha trutta. Other names for anadromous brown trout are bull trout, sewin (Wales), peel or peal, mort, finnock (Scotland), white trout (Ireland), Dollaghan and salmon trout (culinary).
The coho salmon is a species of anadromous fish in the salmon family and one of the five Pacific salmon species. Coho salmon are also known as silver salmon or "silvers". The scientific species name is based on the Russian common name kizhuch (кижуч).
Oncorhynchus is a genus of ray-finned fish in the subfamily Salmoninae of the family Salmonidae, native to coldwater tributaries of the North Pacific basin. The genus contains twelve extant species, namely six species of Pacific salmon and six species of Pacific trout, all of which are migratory mid-level predatory fish that display natal homing and semelparity.
Sea lice are copepods of the family Caligidae within the order Siphonostomatoida. They are marine ectoparasites that feed on the mucus, epidermal tissue, and blood of host fish. The roughly 559 species in 37 genera include around 162 Lepeophtheirus and 268 Caligus species.
The Naugatuck River is a 40.2-mile-long (64.7 km) river in the U.S. state of Connecticut. Its waters carve out the Naugatuck River Valley in the western reaches of the state, flowing generally due south and eventually emptying into the Housatonic River at Derby, Connecticut and thence 11 miles (18 km) to Long Island Sound. The Plume and Atwood Dam in Thomaston, completed in 1960 following the Great Flood of 1955, creates a reservoir on the river and is the last barrier to salmon and trout migrating up from the sea.
The westslope cutthroat trout, also known as the black-spotted trout, common cutthroat trout and red-throated trout is a subspecies of the cutthroat trout and is a freshwater fish in the salmon family of order Salmoniformes. The cutthroat is the Montana state fish. This subspecies is a species of concern in its Montana and British Columbia ranges and is considered threatened in its native range in Alberta.
Oncorhynchus rastrosus also known as the saber-toothed salmon, or spike-toothed salmon is an extinct species of salmon that lived along the Pacific coast of North America and Japan. They first appeared in the late Miocene in California, then died out some time during the Early Pliocene.
The coastal cutthroat trout, also known as the sea-run cutthroat trout, blue-back trout or harvest trout, is one of the several subspecies of cutthroat trout found in Western North America. The coastal cutthroat trout occurs in four distinct forms. A semi-anadromous or sea-run form is the most well known. Freshwater forms occur in both large and small rivers and streams and lake environments. The native range of the coastal cutthroat trout extends south from the southern coastline of the Kenai Peninsula in Alaska to the Eel River in Northern California. Coastal cutthroat trout are resident in tributary streams and rivers of the Pacific basin and are rarely found more than 100 miles (160 km) from the ocean.
The kokanee salmon, also known as the kokanee trout, little redfish, silver trout, kikanning, Kennerly's salmon, Kennerly's trout, or Walla, is the non-anadromous form of the sockeye salmon. There is some debate as to whether the kokanee and its sea-going relative are separate species; geographic isolation, failure to interbreed, and genetic distinction point toward a recent divergence in the history of the two groups. The divergence most likely occurred around 15,000 years ago when a large ice melt created a series of freshwater lakes and rivers across the northern part of North America. While some members of the salmon and trout family (salmonids) went out to sea (anadromous), others stayed behind in fresh water (non-anadromous). The separation of the sockeye and the kokanee created a unique example of sympatric speciation that is relatively new in evolutionary terms. While they occupy the same areas and habitats during the breeding season, when ocean-going sockeye salmon return to freshwater to spawn, the two populations do not mate with each other in some regions, suggesting speciation.