Sicklefin lemon shark

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 meaning "sharp", and '''' meaning "teeth". The sicklefin lemon shark may also be referred to as broadfin shark, Indian lemon shark, Indo-Pacific lemon shark, or in its native range simply "lemon shark", where it does not overlap with its Atlantic relative. 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. 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). ==Distribution and habitat==

The range of the sicklefin lemon shark extends from South Africa to the Red Sea (including Mauritius, the Seychelles, and Madagascar), continuing eastward along the coast of the Indian subcontinent to Southeast Asia, extending as far north as Taiwan and the Philippines, and as far south as New Guinea and northern Australia. This species is also found around numerous Pacific islands, including New Caledonia, Palau, the Marshall Islands, the Solomon Islands, Fiji, Vanuatu, and French Polynesia. This species favors still, murky waters and is most common in bays, estuaries, and lagoons, and over sandy flats and outer reefs. Sometimes, an individual may venture into open water; one was filmed in the 1971 documentary Blue Water, White Death, in the vicinity of a sperm whale (Physeter macrocephalus) carcass. Juvenile sicklefin lemon sharks are often found on reef flats or around mangroves, in water so shallow that their dorsal fins are exposed. In Herald Bight off Western Australia, a known nursery, juvenile sicklefin lemon sharks frequent open areas and mangroves in water under deep, but not areas covered by the seagrass Posidonia australis. ==Description==

The sicklefin lemon shark has a robust, stocky body and a short, broad head. The snout is rounded or almost wedge-shaped, with small nostrils bearing triangular flaps of skin in front. The eyes are small, with no spiracles. Short furrows are present at the corners of the mouth. This species attains a maximum known length of , though it typically does not exceed . ==Biology and ecology==

A sluggish species, the sicklefin lemon shark is usually seen cruising sedately just above the sea bottom or lying still on it, as unlike most requiem sharks, it is capable of actively pumping water over its gills. However, it will approach the surface in pursuit of food. Another study at Moorea in French Polynesia found that some of the local sharks were year-round residents of the island, while others were more transitory and visited only occasionally. Over 90% of the sicklefin lemon shark's diet consists of bottom- and shore-dwelling teleosts, including herring, mullets, mackerel, silversides, needlefish, smelt-whitings, porgies, sea catfish, triggerfish, parrotfish, and porcupinefish. Occasionally, they may also take cephalopods and crustaceans, and older individuals have been known to consume stingrays and guitarfish. This species may be preyed upon by larger sharks. Pseudogrillotia spratti, Phoreiobothrium perilocrocodilus, and Platybothrium jondoeorum. Sicklefin lemon sharks have been documented resting on the bottom and eliciting cleanings by bluestreak cleaner wrasses (Labroides dimidiatus), during which they may open their mouths and stop respiring for as long as 150 seconds to give the wrasses access to their mouths and gills. As with other members of its family, the sicklefin lemon shark is viviparous; the developing embryos are sustained by a placental connection formed from their depleted yolk sacs. Females give birth to one to 13 (usually six to 12) pups every other year in shallow nursery areas, following a gestation period of 10–11 months. Unlike in the American lemon shark, scant evidence indicates this species is philopatric (returns to the site of their birth to reproduce). Parturition occurs in October or November at Madagascar and Aldabra, and in January at French Polynesia; ovulation and mating for nonpregnant females takes place at around the same time. The embryos develop a placenta after around four months of gestation, when they still possess vestiges of external gills. The young are born at a length of . Juvenile sharks grow slowly, at a rate of per year. Both sexes attain sexual maturity at a length of . ==Human interactions==

Several apparently unprovoked attacks on humans have been attributed to the sicklefin lemon shark. Due to its formidable size and teeth, it is regarded as potentially dangerous, and certainly this species has been known to defend itself quickly and vigorously if touched, speared, or otherwise provoked or alarmed. Once agitated, this shark can be a persistent adversary; in one case, a shark forced a swimmer to seek refuge atop a coral head, and circled for hours before giving up. However, observations by divers show that normally this shark is shy and hesitant to approach, even when presented with bait. Often, the shark will move away when a diver enters visual range. Young sharks are reportedly more aggressive and inquisitive than the adults. At Mo'orea, sicklefin lemon sharks are the main attraction of daily shark-feeding ecotourism dives. This shark adapts well to captivity and is displayed in public aquariums. The IUCN has assessed the sicklefin lemon shark globally as Endangered; it is harvested using anchored and floating gillnets, beach nets, and longlines. The meat is sold fresh or dried and salted for human consumption, the fins used for shark fin soup, and the liver oil is processed for vitamins. The sicklefin lemon shark is highly susceptible to local overfishing, due to its slow reproductive rate and limited movements. Off Southeast Asia, this shark has become scarce under the pressure of expanding, unmonitored, and unregulated fisheries, and has been assessed there as Endangered by the IUCN. Widespread habitat degradation, including pollution and blast fishing on coral reefs and deforestation in mangroves, poses an additional threat to the survival of this species in the region. Already, the sicklefin lemon shark has been extirpated in parts of India and Thailand, and has not been reported from Indonesian markets in recent years despite being historically abundant there. Off Australia, the sicklefin lemon shark is taken in only small numbers intentionally and as bycatch, and has been listed under Least Concern. ==References==