The Borneo shark (Carcharhinus borneensis) is a very rare and little-known species of requiem shark, family Carcharhinidae, found in the shallow coastal waters of northwestern Borneo and possibly elsewhere in Southeast Asia. A small shark reaching 70 cm (28 in) in length, this species is the only member of its genus with enlarged pores above the corners of its mouth. It has a slender body, colored gray above and white below, and a long, pointed snout. With only a handful of confirmed sightings and heavy fishing occurring within its range, the Borneo shark has been assessed as Endangered by the International Union for Conservation of Nature (IUCN).
Taxonomy and phylogeny
The Borneo shark was originally described as Carcharias (Prionodon) borneensis by Dutch ichthyologist Pieter Bleeker, in an 1858 issue of the scientific journal Acta Societatis Regiae Scientiarum Indo-Neêrlandicae. He based his account on a 24 cm (9.4 in) long immature male caught off Singkawang, Borneo. Subsequent authors have recognized this shark as belonging to the genus Carcharhinus.
The evolutionary relationships of the Borneo shark are uncertain. In 1988, Leonard Compagno placed it in an informal phenetic group also containing the smalltail shark (C. porosus), blackspot shark (C. sealei), spottail shark (C. sorrah), creek whaler (C. fitzroyensis), whitecheek shark (C. dussumieri), hardnose shark (C. macloti), and Pondicherry shark (C. hemiodon). It also bears morphological similarities to Rhizoprionodon species, though it clearly belongs in Carcharhinus.
Distribution and habitat
All recent specimens of the Borneo shark have come solely from the shallow, inshore waters off Mukah in northwestern Borneo. Historically its range may have been wider, with two specimens known from Kalimantan in the 1800s, one known from southern China in 1936, and unconfirmed records from the Philippines and Java.
The Borneo shark is slim-bodied, with a long, pointed snout and oblique, slit-like nostrils preceded by narrow, nipple-shaped flaps of skin. The eyes are rather large and circular, and equipped with nictitating membranes (protective third eyelids). The corners of the sizable mouth bear short, indistinct furrows, and immediately above are a series of enlarged pores that are unique within the genus. There are 25–26 upper and 23–25 lower tooth rows. The upper teeth have a single narrow, oblique cusp with strongly serrated edges, and large cusplets on the trailing side. The lower teeth are similar, but tend to be more slender and finely serrated. The five pairs of gill slits are short.
The pectoral fins are short, pointed, and falcate (sickle-shaped), while the pelvic fins are small and triangular with a nearly straight trailing margin. The first dorsal fin is fairly large and triangular, with a blunt apex sloping down to a sinuous trailing margin; its origin lies over the free rear tips of the pectoral fins. The second dorsal fin is small and low, and originates over the middle of the anal fin base. There is no ridge between the dorsal fins. The caudal fin is narrow, with a well-developed lower lobe and a strong ventral notch near the tip of the upper lobe. The dermal denticles are small and overlapping, each with three horizontal ridges leading to marginal teeth. The Borneo shark is slate-gray above, darkening towards the tips of the dorsal fins and upper caudal fin lobe, and white below. There are faint, lighter edges on the pectoral, pelvic, and anal fins. The maximum known size is 70 cm (28 in).
Biology and ecology
Virtually nothing is known of the natural history of the Borneo shark. Like other members of its family, it is viviparous with the fetuses sustained by a placental connection. One recorded female contained six pups. Newborns measure around 24–28 cm (9.4–11 in) long; males mature at under 59 cm (23 in) long, and females at under 61 cm (24 in).
Until recently, only five confirmed specimens of the Borneo shark were known, all of them immature and collected before 1937. Several extensive surveys in the region since have failed to find it and the species was feared extinct, but in 2007 researchers from Universiti Malaysia Sabah rediscovered it during a survey of the fishery resources of Sabah and Sarawak. The Borneo shark is subject to heavy fishing pressure within its range, largely from the East Asian demand for fins, and it remains at high risk of extinction. The International Union for Conservation of Nature (IUCN) has listed it as Endangered.
- ^ a b c d e Compagno, L.J.V. (2005). Carcharhinus borneensis. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.1. Downloaded on May 1, 2010.
- ^ Bleeker, P. (1858). "Twaalfde bijdrage tot de kennis der vischfauna van Borneo. Visschen van Sinkawang". Acta Societatis Regiae Scientiarum Indo-Neêrlandicae 5 (7): 1–10.
- ^ a b c Compagno, L.J.V. (1984). Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date. Food and Agricultural Organization. pp. 458–459. ISBN 9251013845.
- ^ Compagno, L.J.V. (1988). Sharks of the Order Carcharhiniformes. Princeton University Press. pp. 319–320. ISBN 069108453X.
- ^ a b c d e White, W.T., P.R. Last, and A.P.K. Lim (2010). "Rediscovery of the rare and endangered Borneo Shark Carcharhinus borneensis (Bleeker, 1858) (Carcharhiniformes: Carcharhinidae)". In Last, P.R., W.T. White, and J.J. Pogonoski. Descriptions of New Sharks and Rays from Borneo. CSIRO Marine and Atmospheric Research. pp. 17–28. ISBN 9781921605574.
- ^ a b Last Seen in 1859, Rare Borneo Shark Spotted Again in Malaysia; New Species of Ray, Crab ID'd. UnderwaterTimes.com. March 24, 2007. Retrieved on May 1, 2010.