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Speartooth shark
The speartooth shark (''Glyphis glyphis'') is an extremely rare species of requiem shark, belonging to the family Carcharhinidae. Only immature specimens, which inhabit the tidal reaches of large tropical rivers in northern Australia and New Guinea, are known. It is exclusively found in fast-moving, highly turbid waters over a wide range of salinities. This robustly built, gray-colored shark is characterized by a short and broad snout, tiny eyes, a relatively large second dorsal fin, and a black blotch beneath each pectoral fin near the tip. Another identifying trait are its teeth, which are large, triangular, and serrated in the upper jaw and narrow, spear-like, and serrated only near the tips in the lower jaw. Based on available specimens, adults probably grow to long.
Preying on demersal bony fishes and crustaceans, the speartooth shark is adapted for hunting in near-complete darkness. It is not as active as other requiem sharks, moving upstream and downstream with tidal currents so as to save energy. Reproduction is viviparous with females forming a placental connection to their young, though details are unknown. The speartooth shark is threatened by incidental capture in commercial and recreational fisheries, as well as by habitat degradation. Given its small population, restricted range, and stringent habitat requirements, this species is highly susceptible to these pressures and has been listed as Endangered by the International Union for Conservation of Nature (IUCN).
==Taxonomy==
German biologists Johannes Müller and Jakob Henle originally described the speartooth shark as ''Carcharias (Prionodon) glyphis'', in their 1839–41 ''Systematische Beschreibung der Plagiostomen''. They based their account on a stuffed juvenile female long, of unknown origin (possibly the Indian Ocean or the South China Sea).〔〔 In 1843, Swiss-American zoologist Louis Agassiz proposed the new genus ''Glyphis'' for this species and a fossil relative from Britain, ''G. hastalis''. However, the use of ''Glyphis'' to encompass the river sharks did not gain wide acceptance until Jack Garrick's 1982 revision of ''Carcharhinus''.〔
Müller and Henle's type specimen remained the sole known record of the speartooth shark until specimens of the "Bizant river shark" (''Glyphis'' sp. A) were caught in Australia in 1982. Subsequently work by Leonard Compagno, William White, and Peter Last confirmed initial suspicions that "sp. A" was the same species as ''G. glyphis''.〔 Thus, in Australia this shark may also be referred to as the Bizant river shark or the Queensland river shark.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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