| Sicklefin lemon shark | |
|---|---|
 | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Division: | Selachii |
| Order: | Carcharhiniformes |
| Family: | Carcharhinidae |
| Genus: | Negaprion |
| Species: | N. acutidens |
| Binomial name | |
| Negaprion acutidens (Rüppell, 1837) | |
 | |
| Range of the sicklefin lemon shark | |
| Synonyms | |
Aprionodon acutidens queenslandicusWhitley, 1939 * ambiguous synonym | |
Thesicklefin lemon shark (Negaprion acutidens) orsharptooth lemon shark is aspecies ofrequiem shark belonging to thefamily Carcharhinidae, widely distributed in the tropical waters of theIndo-Pacific. It is closely related to the better-knownlemon shark (N. brevirostris) of theAmericas; the two species are almost identical in appearance, both being stout-bodied sharks with broad heads, twodorsal fins of nearly equal size, and a plain yellow-tinged coloration. As itscommon name suggests, the sicklefin lemon shark differs from its American counterpart in having more falcate (sickle-shaped) fins. This large species grows up to 3.8 m (12 ft) long. It generally inhabits water less than 92 m (302 ft) deep in a variety ofhabitats, frommangroveestuaries tocoral reefs.
A slow-movingpredator feeding mainly onbony 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 isviviparous, with females giving birth to no more than 13 pups every other year, following agestation 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. TheIUCN has assessed this species as Endangered; its low reproductive productivity and rate of movement limits the capacity of depleted stocks to recover. OffIndia andSoutheast Asia, this species has been severely depleted orextirpated by unregulated exploitation for its meat, fins, andliver oil.
The sicklefin lemon shark was first described asCarcharias acutidens by German naturalistEduard Rüppell, in the 1837Fische des Rothen Meeres (Fishes of the Red Sea). In 1940, Australian ichthyologistGilbert Percy Whitley moved this species to his newly formed genusNegaprion. Thetype specimen, designated in 1960, is a 68-cm-long individual caught in theRed Sea offJeddah,Saudi Arabia.[3] Thespecific epithetacutidens is derived from theLatinacutus meaning "sharp", anddens 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.[2]
Based onmicrosatelliteDNA evidence, the sicklefin lemon shark is thought to havediverged from itssister speciesN. brevirostris 10–14 million years ago, when the closure of theTethys Sea separated lemon sharks in theIndian Ocean from those in theAtlantic.[4] The ancestral lemon shark species may have beenN. eurybathrodon, thefossilized teeth of which occur in both theUnited States andPakistan.[4]Morphological andmolecular phylogenetic analyses suggest thatNegaprion is grouped with thewhitetip reef shark (Triaenodon) and thesliteye shark (Loxodon) in occupying an intermediate position on the carcharhinidevolutionary tree, between the mostbasal genera (Galeocerdo,Rhizoprionodon, andScoliodon) and the mostderived (Carcharhinus andSphyrna).[5]
Therange of the sicklefin lemon shark extends fromSouth Africa to the Red Sea (includingMauritius, theSeychelles, andMadagascar), continuing eastward along the coast of theIndian subcontinent toSoutheast Asia, extending as far north asTaiwan and thePhilippines, and as far south asNew Guinea and northernAustralia. This species is also found around numerous Pacific islands, includingNew Caledonia,Palau, theMarshall Islands, theSolomon Islands,Fiji,Vanuatu, andFrench Polynesia.[6][3] This species likely colonized the central Pacific by infrequently "hopping" from one island to the next. Significant levels of genetic differentiation between sicklefin lemon sharks in Australia and French Polynesia, 7,500 km (4,700 mi) apart, suggest that little intermingling occurs between regional subpopulations.[4]
An inhabitant of coastalcontinental and insular shelves, the sicklefin lemon shark occurs from theintertidal zone to a depth of 92 m (302 ft).[7] This species favors still, murky waters and is most common inbays,estuaries, andlagoons, and over sandy flats and outerreefs. Sometimes, an individual may venture intoopen water; one was filmed in the 1971documentaryBlue Water, White Death, in the vicinity of asperm whale (Physeter macrocephalus) carcass. Juvenile sicklefin lemon sharks are often found on reef flats or aroundmangroves, in water so shallow that their dorsal fins are exposed.[3][8] In Herald Bight offWestern Australia, a known nursery, juvenile sicklefin lemon sharks frequent open areas and mangroves in water under 3 m (9.8 ft) deep, but not areas covered by theseagrassPosidonia australis.[9]
The sicklefin lemon shark has a robust, stocky body and a short, broad head. The snout is rounded or almost wedge-shaped, with smallnostrils bearing triangular flaps of skin in front. The eyes are small, with nospiracles. Short furrows are present at the corners of the mouth.[3] Thirteen to 16 (usually 14) tooth rows occur on either side of both jaws, not including the tiny teeth at the symphysis (the jaw midline). The upper teeth have a large cusp rising from a broad base, with a notch on each side; these teeth become increasingly angled towards the corners of the mouth. The lower teeth resemble the upper teeth, but are narrower and more erect.[8] The teeth of sharks over 1.4 m (4.6 ft) long are finely serrated.[3]
The fins (especially thedorsal,pectoral, andpelvics) of the sicklefin lemon shark are more falcate (sickle-shaped) than those of the otherwise very similar American lemon shark. The first dorsal fin is positioned closer to the pelvic than the pectoral fins. The second dorsal fin, nearly equal to the first in size, is located over or slightly forward of theanal fin. No ridge is seen between the dorsal fins. The pectoral fins are long and broad, originating below the space between the third and fourth gill slits. The anal fin has a strong notch in the rear margin. The precaudal pit (a cavity just forward of thecaudal fin) is oriented longitudinally.[3] The largedermal denticles are overlapping and bear three to five horizontal ridges each.[7] The coloration is a plain yellowish brown or gray above and lighter below, with more yellow on the fins.[10] This species attains a maximum known length of 3.8 m (12 ft),[2] though it typically does not exceed 3.1 m (10 ft).[8]
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.[3] This shark seldom undertakes long-distance movements.[4] A study atAldabra Atoll in the Seychelles found, of the tagged sharks that were later recaptured, over 90% were still within 2 km (1.2 mi) of their initial tagging location.[11] Another study atMoorea 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.[12]
Over 90% of the sicklefin lemon shark's diet consists of bottom- and shore-dwellingteleosts, includingherring,mullets,mackerel,silversides,needlefish,smelt-whitings,porgies,sea catfish,triggerfish,parrotfish, andporcupinefish. Occasionally, they may also takecephalopods andcrustaceans, and older individuals have been known to consumestingrays andguitarfish.[3][11][13][14] This species may be preyed upon by larger sharks.[7] Knownparasites include thetapewormsParaorygmatobothrium arnoldi,[15]Pseudogrillotia spratti,[16]Phoreiobothrium perilocrocodilus,[17] andPlatybothrium jondoeorum.[18] Sicklefin lemon sharks have been documented resting on the bottom and eliciting cleanings bybluestreak 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.[19]
As with other members of its family, the sicklefin lemon shark isviviparous; the developingembryos are sustained by aplacental connection formed from their depletedyolk sacs. Females give birth to one to 13 (usually six to 12) pups every other year in shallow nursery areas, following agestation period of 10–11 months.[3][11] Unlike in the American lemon shark, scant evidence indicates this species is philopatric (returns to the site of their birth to reproduce).[4] Parturition occurs in October or November at Madagascar and Aldabra, and in January at French Polynesia;ovulation andmating 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 45–80 cm (18–31 in).[3][11] Juvenile sharks grow slowly, at a rate of 12.5–15.5 cm (4.9–6.1 in) per year. Both sexes attainsexual maturity at a length of 2.2–2.4 m (7.2–7.9 ft).[7]
Several apparently unprovokedattacks 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.[3] At Moorea, sicklefin lemon sharks are the main attraction of daily shark-feedingecotourism dives.[12] This shark adapts well to captivity and is displayed inpublic aquariums.[3]
TheIUCN has assessed the sicklefin lemon shark globally as Endangered; it is harvested using anchored and floatinggillnets, beach nets, andlonglines.[6] The meat is sold fresh or dried and salted for human consumption, the fins used forshark fin soup, and theliver oil is processed forvitamins.[3] The sicklefin lemon shark is highly susceptible to localoverfishing, due to its slow reproductive rate and limited movements. Off Southeast Asia, this shark has become scarce under the pressure of expanding, unmonitored, and unregulatedfisheries, and has been assessed there asEndangered by the IUCN. Widespreadhabitat degradation, includingpollution andblast fishing on coral reefs anddeforestation in mangroves, poses an additional threat to the survival of this species in the region. Already, the sicklefin lemon shark has beenextirpated in parts ofIndia andThailand, and has not been reported fromIndonesian markets in recent years despite being historically abundant there. Off Australia, the sicklefin lemon shark is taken in only small numbers intentionally and asbycatch, and has been listed underLeast Concern.[6]
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