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The smooth hammerhead (''Sphyrna zygaena'') is a species of hammerhead shark, and part of the family Sphyrnidae. This species is named "smooth hammerhead" because of the distinctive shape of the head, which is flattened and laterally extended into a hammer shape (called the "cephalofoil"), without an indentation in the middle of the front margin (hence "smooth"). Unlike other hammerheads, this species prefers temperate waters and occurs worldwide at medium latitudes. In the summer, these sharks migrate towards the poles following cool water masses, sometimes forming schools numbering in the hundreds to thousands. The second-largest hammerhead shark after the great hammerhead shark, the smooth hammerhead can measure up to long. It is an active predator that takes a wide variety of bony fishes and invertebrates, with larger individuals also feeding on sharks and rays. As in the rest of its family, this shark is viviparous and gives birth to litters of 20–40 pups. A relatively common shark, it is captured, intentionally or otherwise, by many commercial fisheries throughout its range; its fins are extremely valuable for use in shark fin soup. This shark is potentially dangerous and has likely been responsible for a few attacks on humans, though it is less likely to encounter swimmers than other large hammerhead species due to its temperate habitat. ==Taxonomy and phylogeny== The Swedish natural historian Carl Linnaeus, known as the "father of taxonomy", originally described the smooth hammerhead as ''Squalus zygaena'' in the 1758 tenth edition of ''Systema Naturae'', without designating a type specimen. The name was later changed to ''Sphyrna zygaena''.〔 The specific epithet ''zygaena'' originates from the Greek word ''zygòn'', meaning "yoke", referring to the shape of its head. The Greek name ''zýgaina'' had already been used for the hammerhead shark by Aristotle in the second book of his ''History of Animals''. Other common names for this species include common hammerhead, common smooth hammerhead, round-headed hammerhead, or simply hammerhead. }} }} }} }} }} Studies based on morphology have generally regarded the smooth hammerhead as one of the more derived members of its family, grouped together with the scalloped hammerhead (''S. lewini'') and the great hammerhead (''S. mokarran''). Phylogenetic analyses based on nuclear and mitochondrial DNA have concluded differently: while the smooth and great hammerheads are closely related, they are not as closely related to the scalloped hammerhead as the other ''Sphyrna'' species. Furthermore, the smooth hammerhead is among the more basal hammerhead species, indicating that the first hammerheads to evolve had large cephalofoils. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Smooth hammerhead」の詳細全文を読む スポンサード リンク
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