| Pigeye shark | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Division: | Selachii |
| Order: | Carcharhiniformes |
| Family: | Carcharhinidae |
| Genus: | Carcharhinus |
| Species: | C. amboinensis |
| Binomial name | |
| Carcharhinus amboinensis | |
 | |
| Range of the pigeye shark[2] | |
| Synonyms | |
Carcharias amboinensisMüller & Henle, 1839 | |
Thepigeye shark orJava shark (Carcharhinus amboinensis) is an uncommonspecies ofrequiem shark, in thefamily Carcharhinidae, found in the warm coastal waters of the eastern Atlantic and westernIndo-Pacific. It prefers shallow, murky environments with soft bottoms, and tends to roam within a fairly localised area. With its bulky grey body, small eyes, and short, blunt snout, the pigeye shark looks almost identical to (and is often confused with) the better-knownbull shark (C. leucas). The two species differ invertebral count, the relative sizes of thedorsal fins, and other subtle traits. This shark typically reaches lengths of 1.9–2.5 m (6.2–8.2 ft).
The pigeye shark is anapex predator that mostly hunts low in thewater column. It has a varied diet, consisting mainly ofbony andcartilaginous fishes and also includingcrustaceans,molluscs,sea snakes, andcetaceans. This speciesgives birth to live young, with the developingembryos sustained to term via aplacental connection to their mother. Litters of three to thirteen pups are born after agestation period of nine or twelve months. Young sharks spend their first few years of life in shelteredinshore habitats such asbays, where their movements follow tidal and seasonal patterns. The pigeye shark's size anddentition make it potentially dangerous, though it has not been known to attack humans. The shark is infrequently caught inshark nets protecting beaches and byfisheries, which use it formeat andfins. TheIUCN presently assesses this species as vulnerable.
German biologistsJohannes Müller andJakob Henle described the pigeye shark and named itCarcharias (Prionodon) amboinensis in their 1839Systematische Beschreibung der Plagiostomen. Later authors reassigned it to thegenusCarcharhinus. Thetype specimen is a stuffed female 74 cm (29 in) long, originally caught offAmbon Island in Indonesia, from which thespecific epithet is derived.[3][4] Severaljunior synonyms are known for this species, among themTriaenodon obtusus, which was based on a near-birth pigeye sharkfoetus.[4]
Fossil teeth of the pigeye shark are known from theLate Miocene ofBrunei, representing the oldest known record of the species.[5][6]
Since the pigeye shark so strongly resembles the bull shark,morphology-basedphylogenetic studies have considered the two species to be closely related.[7][8] Neither this nor any other arrangement is strongly supported bymolecular phylogenetic research, which to date has been inconclusive regarding this shark's evolutionary relationship to otherCarcharhinus species.[9][10]
Genetic analysis of pigeye sharks across northern Australia suggest that the evolutionary history of this species was affected by coastline changes during thePleistoceneepoch (2.6 million to 12,000 years ago). The patterns of diversity found in itsmitochondrial DNA are consistent with the repeated splitting and merging of its populations as geographical barriers were alternately formed and inundated. The most recent of these barriers was aland bridge across theTorres Strait that reopened only some 6,000 years ago; as a result, significant genetic separation exists between the sharks found offWestern Australia and theNorthern Territory and those found offQueensland.[11]
The pigeye shark is a very robust-bodied species with a short, broad, and rounded snout. The small and circular eyes are equipped withnictitating membranes. The anterior rims of the nostrils bear medium-sized flaps of skin. The mouth forms a wide arch and has barely noticeable furrows at the corners. There are 11–13 (usually 12) upper and 10–12 (usually 11) lowertooth rows on each side; in addition, there are single rows of tiny teeth at the upper and lower symphyses (jaw midpoints). The teeth are broad and triangular with serrated edges; those in the lower jaw are slightly narrower, more upright, and more finely serrated than those in the upper. The five pairs ofgill slits are of moderate length.[2][4][12]
The firstdorsal fin is large and triangular, with a pointed apex and a concave trailing margin; it originates roughly over the posterior insertions of thepectoral fins. The second dorsal fin is less than a third as high as the first, and originates ahead of theanal fin. There is no midline ridge between the dorsal fins. The long pectoral fins are broad and slightly falcate (sickle-shaped), becoming narrow and pointed at the tips. The anal fin has a sharply notched trailing margin. Thecaudal peduncle has a deep notch on its upper surface at thecaudal fin origin. The caudal fin is asymmetrical, with a well-developed lower lobe and a longer upper lobe with a notch in the trailing margin near its tip.[2][4][12]
The skin is covered by rather largedermal denticles, which become more tightly packed and overlapping with age; each denticle bears three to five horizontal ridges and five posterior teeth.[2] This species is grey above and white below, with a faint pale band on the flanks. The second dorsal fin and lower caudal fin lobe darken at the tips, particularly in juveniles.[4] Analbino individual was caught off Queensland in 1987, which was the first known example of albinism in arequiem shark.[13] An adult pigeye shark typically measures 1.9–2.5 m (6.2–8.2 ft) long, while the largest individuals reach 2.8 m (9.2 ft) long.[2]
The pigeye shark can be most reliably distinguished from the bull shark by the number of precaudal (before the caudal fin)vertebrae (89–95 inC. amboinensis versus 101–123 inC. leucas). Externally, it has a greater size difference between its dorsal fins (first-to-second height ratio >3.1:1 versus ≤3.1:1 inC. leucas) and the notch in its anal fin margin forms an acute angle (versus a right angle inC. leucas). This species also usually has fewer tooth rows in the lower jaw (10–12 on each side versus 12–13 inC. leucas).[2][4]
Though widely distributed in thetropical andsubtropical marine waters ofEurasia, Africa, andOceania, the pigeye shark does not appear to be common anywhere. Existing records are patchy, and the full extent of its range may be obscured by confusion with the bull shark.[1] In the eastern Atlantic, it is found offCape Verde and Senegal, and from Nigeria to Namibia;[2] there is a singleMediterranean record from offCrotone, Italy.[14] It occurs all along the continental periphery of the Indian Ocean, from eastern South Africa to theArabian Peninsula (includingMadagascar, theSeychelles, andMauritius), toSoutheast Asia and northern Australia. Its range extends into the Pacific, northward to the Philippines and southern China, and eastward toNew Guinea and someMicronesian islands.[2] Tagging and genetic data indicate that pigeye sharks, particularly juveniles, are not stronglymigratory and tend to remain in a local area. The longest recorded distance covered by an adult is 1,080 km (670 mi).[11][12]
The pigeye shark inhabits coastal waters down to a depth of 150 m (490 ft), favouring environments with finesediment and murky water. It sometimes entersestuaries, but unlike the bull shark, it does not ascend rivers and avoidsbrackish water.[2][15] The movements andhabitat usage of juvenile pigeye sharks have been extensively studied inCleveland Bay in northeastern Queensland. Young sharks live in the bay year-round, staying mostly in the eastern side where the input from three rivers produces strong currents and highturbidity. Individualhome ranges are relatively small, averaging 30 km2 (12 sq mi), and increase in size with age. The juveniles generally stay in water less than 40 m (130 ft) deep, with the youngest sharks spending the most time in the shallowest parts of the bay. They swim into theintertidal zone with the rising tide and depart as the tide recedes; this movement may relate to exploiting foraging opportunities on the submerged mud flats, or to avoiding predation or competition by staying out of the deeper waters occupied by larger sharks. There is also an annual movement cycle where the juveniles move closer to the river mouths during thedry season and farther from them during thewet season; since the rainy season brings a higher flow of fresh water into the bay, the sharks may be responding directly or indirectly to the resultant decrease insalinity anddissolved oxygen levels.[16][17]
The pigeye shark is a largely solitary animal, though occasionally several individuals may be found at the same location.[15] In theMozambique Channel, it outnumbers the bull shark on the east side while the opposite is true on the west side, suggesting there may becompetitive exclusion between these similar species.[4]Parasites documented from the pigeye shark include themyxosporeanKudoa carcharhini,[18] thecopepodsPandarus smithii andP. cranchii,[19] and thetapewormsCallitetrarhynchus gracilis,[20]Cathetocephalus sp.,[21]Floriceps minacanthus,[22]Heteronybelinia australis,[23]Otobothrium australe,O. crenacolle,[24] andProtogrillotia sp.[20] Young pigeye sharks are potentially vulnerable to predation by larger sharks. The naturalmortality for juveniles inCleveland Bay has been measured at no more than 5% per year; this rate is comparable to that in juvenile bull sharks, and is much lower than in juvenileblacktip sharks (C. limbatus) orlemon sharks (Negaprion brevirostris).[25]
Though the pigeye shark will take prey from anywhere in thewater column, it tends to hunt close to thesea floor.[15] Anapex predator, it feeds mainly onteleost fishes such ascroakers,flatfishes, andcutlassfishes, and to a lesser extent oncartilaginous fishes,cephalopods, anddecapod crustaceans. It has also been recorded eatinggastropods,sea snakes,dolphins, andwhalecarrion.[4][26] Other sharks andrays figure much more prominently in the diets of South African pigeye sharks than those from other regions; the types consumed include requiem sharks,catsharks,angel sharks,guitarfishes,stingrays, andeagle rays.[15]
The pigeye shark isviviparous; like in other requiem sharks, after the developingembryo depletes its supply ofyolk, it is sustained to term by its mother through aplacental connection formed from the emptyyolk sac.[4] Mature females have a single functionalovary and two functionaluteruses. Reproductive details vary among regions: off South Africa, thegestation period lasts about 12 months, with mating and birthing both occurring in late summer. The litters range from three to seven pups (average five) and the newborns are around 75–79 cm (30–31 in) long.[1][15] Off northern Australia, the gestation period lasts 9 months, with birthing taking place in November and December. The litters range from six to 13 pups (average 9) and the newborns are around 59–66 cm (23–26 in) long.[27]
Young sharks can be found in shallowinshore environments such as bays until at least three years of age, suggesting this species uses these sheltered habitats as nurseries.[28] As the sharks grow older, they venture farther from land into deeper water, more and more often, until they eventually disperse.[16][29] This is a long-lived, slow-growing species; males grow faster and reach a smaller ultimate size than females.Sexual maturity is attained at around 2.1 m (6.9 ft) long and 12 years of age for males, and 2.2 m (7.2 ft) long and 13 years of age for females. Themaximum lifespan is at least 26 years for males and 30 years for females.[27][30]
Large and formidably toothed, the pigeye shark is regarded as potentially dangerous to humans, and has been implicated in at least one attack on a human. In July 2020, at Fitzroy Island off the east coast of Queensland, Australia, free diver Anika Craney was bitten on her lower leg by what was later identified as a pigeye shark.[31][32] Craney survived the encounter despite sustaining a serious injury, including having her artery severed. This species is caught infrequently onlonglines and ingillnets, and is used formeat andfins.[12] As a predator, though, the shark canaccumulateciguatera toxins produced bydinoflagellates within its tissues. In November 1993, some 500 people inManakara, Madagascar, were poisoned, 98 of them fatally, after eating meat from a pigeye shark. This was the first recorded mass ciguatera outbreak caused by a shark, as well as the first with a significant death toll.[33] TheIUCN has listed the pigeye shark overall as vulnerable, while noting that its rarity may render it susceptible tooverfishing.[1] InKwaZulu-Natal, South Africa, small numbers of pigeye sharks are caught inshark nets set up to protect beaches. The catch rate and the average size of sharks caught both decreased between 1978 and 1998, leading to concerns that the local population may be depleted. Thus, the IUCN has given this species a regional assessment ofNear Threatened in the southwestern Indian Ocean.[1]
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