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crocodile shark
The crocodile shark (''Pseudocarcharias kamoharai'') is a species of mackerel shark and the only member of the family Pseudocarchariidae. A specialized inhabitant of the mesopelagic zone, the crocodile shark can be found worldwide in tropical waters from the surface to a depth of . It performs a diel vertical migration, staying below a depth of during the day and ascending into shallower water at night to feed. Typically measuring only in length, the crocodile shark is the smallest living mackerel shark. It can be distinguished by its elongated cigar-shaped body, extremely large eyes, and relatively small fins.
An active-swimming predator of pelagic bony fishes, squid and shrimp, the crocodile shark has a sizable oily liver that allows it to maintain its position in the water column with minimal effort. The size and structure of its eyes suggests that it is adapted for hunting at night. The crocodile shark is aplacental viviparous, with females typically giving birth to litters of four. The fetuses are oophagous, meaning that they feed on undeveloped eggs ovulated for this purpose by their mother. Due to its small size, the crocodile shark poses little danger to humans and is of little commercial importance. However, substantial numbers are caught as bycatch, leading it to be assessed as Near Threatened by the International Union for Conservation of Nature (IUCN). This species was responsible for damaging deep sea fiberoptic cables when the technology was first deployed in 1985.
==Taxonomy and phylogeny==
The English common name "crocodile shark" is derived from its Japanese name ''mizuwani'' (水鰐, literally "water crocodile"), which refers to its sharp teeth and habit of snapping vigorously when taken out of the water.〔 Other common names for this species include Japanese ragged-tooth shark, Kamohara's sand-shark, and water crocodile.〔 The crocodile shark was first described as ''Carcharias kamoharai'' in a 1936 issue of ''Zoological Magazine (Tokyo)'' by ichthyologist Kiyomatsu Matsubara, based on a long specimen found at the Koti Fish Market in Japan. The type specimen is a long adult male found at a fish market in Su-ao, Taiwan.〔
After being shuffled between the genera ''Carcharias'' and ''Odontaspis'' in the family Odontaspididae by various authors, in 1973 Leonard Compagno resurrected Jean Cadenat's 1963 subgenus ''Pseudocarcharias'' from synonymy for this species and placed it within its own family.〔Martin, R.A. (Pseudocarchariidae ). ''ReefQuest Centre for Shark Research''. Retrieved on December 24, 2008.〕〔 The morphology of the crocodile shark suggests affinity with the megamouth shark (Megachasmidae), basking shark (Cetorhinidae), thresher sharks (Alopiidae), and mackerel sharks (Lamnidae). More recent phylogenetic analyses, based on mitochondrial DNA, have suggested that the crocodile shark is closely related to either the megamouth shark or the sand sharks (Odontaspididae). Alternately, analysis based on dentition suggests that the closest relatives of the crocodile shark are the thresher sharks, followed by the mackerel sharks.〔 Fossil ''Pseudocarcharias'' teeth dating to the Serravallian age (13.6–11.6 Ma) of the Miocene epoch have been found in Italy, and are identical to those of the modern-day crocodile 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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