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Milk shark
The milk shark (''Rhizoprionodon acutus'') is a species of requiem shark, and part of the family Carcharhinidae, whose common name comes from an Indian belief that consumption of its meat promotes lactation. The largest and most widely distributed member of its genus, the milk shark typically measures long, and can be found in coastal tropical waters throughout the eastern Atlantic and the Indo-Pacific regions. Occurring from the surface to a depth of , this species is common near beaches and in estuaries, and has been recorded swimming up rivers in Cambodia. Juveniles are known to inhabit tidal pools and seagrass meadows. The milk shark has a slender body with a long, pointed snout and large eyes, and is a nondescript gray above and white below. This shark can be distinguished from similar species in its range by the long furrows at the corners of its mouth, and seven to 15 enlarged pores just above them.
Among the most abundant sharks within its range, the milk shark feeds primarily on small bony fishes, but also takes cephalopods and crustaceans. In turn, it often falls prey to larger sharks and possibly marine mammals. In common with other members of its family, this species is viviparous, with the developing embryos sustained by a placental connection. Females give birth to one to eight young either during a defined breeding season or throughout the year, depending on location. The reproductive cycle is usually annual, but may be biennial or triennial. Large numbers of milk sharks are caught by artisanal and commercial fisheries in many countries for meat, fins, and fishmeal. Despite this, the International Union for Conservation of Nature has assessed this species as being of Least Concern, because its wide distribution and relatively high productivity seemingly allow present levels of exploitation to be sustained.
==Taxonomy and phylogeny==
The German naturalist Eduard Rüppell published the first scientific description of the milk shark, as ''Carcharias acutus'' (the specific epithet means "sharp" in Latin), in his 1837 ''Fische des Rothen Meeres'' (''Fishes of the Red Sea''). It has since been listed under several different genera, including ''Carcharhinus'' and ''Scoliodon'', before finally being placed in the genus ''Rhizoprionodon'' via synonymization with the type species, ''R. crenidens''.〔 As Rüppell did not mention a type specimen, in 1960, Wolfgang Klausewitz designated a -long male caught off Jeddah, Saudi Arabia as the lectotype for this species.〔
The common name "milk shark" comes from a belief held in India that eating this shark's meat enhances lactation. Other names for this species include fish shark, grey dog shark, little blue shark, Longmans dogshark, milk dog shark, sharp-nosed (milk) shark, Walbeehm's sharp-nosed shark, and white-eye shark. A 1992 phylogenetic analysis by Gavin Naylor, based on allozymes, found that the milk shark is the most basal of the four ''Rhizoprionodon'' species examined. The extinct ''R. fischeuri'', known from Middle Miocene (16–12 Ma) deposits in southern France and Portugal, may be the same as ''R. acutus''.〔
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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