Borneo Shark

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. 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 peduncle bears a deep, crescent-shaped pit at the origin of the upper caudal fin lobe. The asymmetrical caudal fin has a well-developed lower lobe and a longer, narrow upper lobe with a strong ventral notch near the tip. …

All recent specimens of the Borneo shark have been collected solely from fishery landing sites at Mukah in Sarawak, despite thorough surveys across the rest of Borneo (including at the locality of the type specimen). Thus, its range may now be restricted to shallow, inshore waters in northwestern Borneo. Of the five earlier specimens, four came from Borneo and one from Zhoushan Island in China, hinting at a wider historical distribution. This species was also recorded from Borongan in the Philippines in 1895, and Java in 1933; these records cannot be substantiated and there have been no subsequent sightings from these areas.

Fossil record. Fossils of the Borneo shark are known from as early as the Late Miocene. At the site of Ambug Hill in Brunei, their remains have been found alongside those of blacktip sharks and members of the genera Paragaleus, Chiloscyllium, Rhinobatos, Scoliodon, and Lamiopsis.

Bony fishes are probably the main food of the Borneo shark. It is viviparous like other requiem sharks, with the developing embryos provisioned by the mother through a placental connection formed from the depleted yolk sac. The litter size is six, and the pups are born at close to 24–28 cm (9.4–11.0 in) long. From the available specimens, the length at sexual maturity can be surmised to be under 55–58 cm (22–23 in) in males and under 61–65 cm (24–26 in) in females.