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Sicklefin lemon shark
The sicklefin lemon shark, or sharptooth lemon shark (''Negaprion acutidens''), is a species of requiem shark belonging to the family Carcharhinidae, widely distributed in the tropical waters of the Indo-Pacific. It is closely related to the better-known lemon shark (''N. brevirostris'') of the Americas; the two species are almost identical in appearance, both being stout-bodied sharks with broad heads, two dorsal fins of nearly equal size, and a plain yellow-tinged coloration. As its common name suggests, the sicklefin lemon shark differs from its American counterpart in having more falcate (sickle-shaped) fins. This large species grows up to long. It generally inhabits water less than deep in a variety of habitats, from mangrove estuaries to coral reefs.
A slow-moving predator feeding mainly on bony fishes, the sicklefin lemon shark seldom travels long distances and many individuals can be found year-round at certain locations. Like other members of its family, this species is viviparous, with females giving birth to no more than 13 pups every other year, following a gestation period of 10–11 months. Although they are potentially dangerous to humans and known to respond vigorously to any provocation, under normal circumstances, sicklefin lemon sharks are cautious and tend to retreat if approached. The IUCN has assessed this species as Vulnerable; its low reproductive productivity and rate of movement limits the capacity of depleted stocks to recover. Off India and Southeast Asia, this species has been severely depleted or extirpated by unregulated exploitation for its meat, fins, and liver oil.
==Taxonomy and phylogeny==
The sicklefin lemon shark was first described as ''Carcharias acutidens'' by German naturalist Eduard Rüppell, in the 1837 ''Fische des Rothen Meeres'' (''Fishes of the Red Sea''). In 1940, Australian ichthyologist Gilbert Percy Whitley moved this species to his newly formed genus ''Negaprion''. The type specimen, designated in 1960, is a 68-cm-long individual caught in the Red Sea off Jeddah, Saudi Arabia. The specific epithet ''acutidens'' is derived from the Latin ''acutus'' meaning "sharp", and ''dens'' meaning "teeth". The sicklefin lemon shark may also be referred to as broadfin shark, Indian lemon shark, Indo-Pacific lemon shark, or simply lemon shark.
Based on microsatellite DNA evidence, the sicklefin lemon shark is thought to have diverged from its sister species ''N. brevirostris'' 10–14 million years ago, when the closure of the Tethys Sea separated lemon sharks in the Indian Ocean from those in the Atlantic.〔 The ancestral lemon shark species may have been ''N. eurybathrodon'', the fossilized teeth of which occur in both the United States and Pakistan. Morphological and molecular phylogenetic analyses suggest that ''Negaprion'' is grouped with the whitetip reef shark (''Triaenodon'') and the sliteye shark (''Loxodon'') in occupying an intermediate position on the carcharhinid evolutionary tree, between the most basal genera (''Galeocerdo'', ''Rhizoprionodon'', and ''Scoliodon'') and the most derived (''Carcharhinus'' and ''Sphyrna'').
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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