The Borneo shark (''Carcharhinus borneensis'') is a species of

requiem shark Requiem sharks are sharks of the family Carcharhinidae in the order Carcharhiniformes. They are migratory, live-bearing sharks of warm seas (sometimes of brackish or fresh water) and include such species as the tiger shark, bull shark, le ...

, and part of the family Carcharhinidae. Extremely rare, it is known only from

inshore A shore or a shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In physical oceanography, a shore is the wider fringe that is geologically modified by the action of the body of water past a ...

waters around

Mukah Mukah ( muːkəh), historically known as Muka is a coastal town which serves as the capital and the administrative center of the Mukah Division since 1 March 2002. The district also covers an area of with a population about 49,900 in the Muka ...

in northwestern Borneo, though it may once have been more widely distributed. A small, gray shark reaching in length, this species is the only member of its genus with a row of enlarged pores above the corners of its mouth. It has a slender body with a long, pointed snout and a low second dorsal fin placed posterior to the

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

origin. Almost nothing is known about the natural history of the Borneo shark. It is viviparous like other requiem sharks; the females bear litters of six pups, which are provisioned through

gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregna ...

by a placental connection. The International Union for Conservation of Nature last assessed this species as Critically Endangered. While an extant population has since been found, the Borneo shark continues to merit conservation concern given its highly limited range within heavily fished waters.

Taxonomy and phylogeny

Dutch

ichthyologist Ichthyology is the branch of zoology devoted to the study of fish, including bony fish ( Osteichthyes), cartilaginous fish (Chondrichthyes), and jawless fish (Agnatha). According to FishBase, 33,400 species of fish had been described as of Octobe ...

Pieter Bleeker originally described the Borneo shark as ''Carcharias (Prionodon) borneensis'' in an 1858 issue of the

scientific journal In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. Content Articles in scientific journals are mostly written by active scientists such as s ...

''Acta Societatis Regiae Scientiarum Indo-Neêrlandicae''. He based his account on a newborn male long, caught off Singkawang in western

Kalimantan Kalimantan () is the Indonesian portion of the island of Borneo. It constitutes 73% of the island's area. The non-Indonesian parts of Borneo are Brunei and East Malaysia. In Indonesia, "Kalimantan" refers to the whole island of Borneo. In 2019, ...

, Borneo. Later authors have recognized this species as belonging to the genus ''

Carcharhinus ''Carcharhinus'' is the type genus of the family Carcharhinidae, the requiem sharks. One of 12 genera in its family, it contains over half of the species therein. It contains 35 extant and eight extinct species to date, with likely more species y ...

''. Before 2004, only five specimens of the Borneo shark were known, all of them immature and collected before 1937. In April and May 2004, researchers from Universiti Malaysia Sabah discovered a number of additional specimens while surveying the

fishery Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life; or more commonly, the site where such enterprise takes place ( a.k.a. fishing ground). Commercial fisheries include wild fisheries and fish farms, both ...

resources of Sabah and Sarawak. The evolutionary relationships of the Borneo shark are uncertain.

Jack Garrick John Andrew Frank "Jack" Garrick (1928 – August 30, 2018) was a New Zealand ichthyologist. He specialized in elasmobranchs and published many books and articles about shark and ray biology. In 1982, he published a thorough taxonomy on sharks of ...

, in his 1982 morphological study, did not place it close to any other member of the genus. Leonard Compagno in 1988 tentatively grouped it with the

smalltail shark The smalltail shark (''Carcharhinus porosus'') is a species of requiem shark, and part of the family Carcharhinidae. It is found in the western Atlantic Ocean, from the northern Gulf of Mexico to southern Brazil. It inhabits shallow waters close ...

(''C. porosus''),

blackspot shark The blackspot shark (''Carcharhinus sealei'') is a small species of requiem shark in the family Carcharhinidae found in the tropical Indo-West Pacific Ocean between latitudes 24°N and 30°S, from the surface to a depth around . Its length is ...

(''C. sealei''),

spottail shark The spot-tail shark, or sorrah shark (''Carcharhinus sorrah''), is a species of requiem shark, in the family Carcharhinidae, found in the tropical Indo-West Pacific Ocean between latitudes 31°N and 31°S, from the surface to a depth around . ...

(''C. sorrah''),

creek whaler The creek whaler (''Carcharhinus fitzroyensis'') is a common species of requiem shark, and part of the family Carcharhinidae, endemic to northern Australia. It frequents shallow waters close to shore, including estuaries. This small, stocky shark ...

(''C. fitzroyensis''), whitecheek shark (''C. dussumieri''),

hardnose shark The hardnose shark (''Carcharhinus macloti'') is a species of requiem shark, in the family Carcharhinidae, so named because of the heavily calcified cartilages in its snout. A small bronze-coloured shark reaching a length of , it has a slender bo ...

(''C. macloti''), and

Pondicherry shark The Pondicherry shark (''Carcharhinus hemiodon'') is an extremely rare species of requiem shark, in the family Carcharhinidae. A small and stocky gray shark, it grows not much longer than and has a fairly long, pointed snout. This species can be ...

(''C. hemiodon''). The Borneo shark resembles the sharpnose sharks (''Rhizoprionodon'') in certain traits, for example the enlarged pores by its mouth. Nevertheless, other aspects of its morphology firmly place it within ''Carcharhinus''.

Description

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 membrane The nictitating membrane (from Latin '' nictare'', to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision. All ...

s. 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 fin Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two en ...

s 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

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. The

dermal denticle A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as we ...

s are small and overlapping, each with three horizontal ridges leading to marginal teeth. This species is slate-gray above, darkening towards the tips of the dorsal fins and upper caudal fin lobe; some specimens have irregular rows of small, white blotches, which may be an artifact of handling. The underside is white, which extends onto the flanks as a vague pale band. There are faint, lighter edges on the pectoral, pelvic, and anal fin trailing margins. The largest known specimen measures long.

Distribution and habitat

All recent specimens of the Borneo shark have been collected solely from fishery landing sites at

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,

waters in northwestern Borneo. Of the five earlier specimens, four came from Borneo and one from Zhoushan Island in

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

, hinting at a wider historical distribution. This species was also recorded from

Borongan Borongan, officially the City of Borongan ( Waray: ''Siyudad han Borongan''; fil, Lungsod ng Borongan), is a 1st class component city and capital of the province of Eastern Samar, Philippines. According to the 2020 census, it has a populatio ...

in the Philippines in 1895, and Java in 1933; these records cannot be substantiated and there have been no subsequent sightings from these areas.

Biology and ecology

Bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...

es are probably the main food of the Borneo shark. It is viviparous like other requiem sharks, with the developing

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

s 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 long. From the available specimens, the length at sexual maturity can be surmised to be under in males and under in females.

Human interactions

The International Union for Conservation of Nature last assessed the Borneo shark as Critically Endangered. Previously, several fishery surveys within its supposed historical range had failed to find it. The Borneo shark's conservation status remains precarious given its very small range in waters subjected to intensive artisanal and commercial fishing. It is caught by line gear and used for meat, though it has minimal commercial significance.

References

External links

''Carcharhinus borneensis'', Borneo shark
a
FishBase

''Carcharhinus borneensis'' (Borneo Shark)
a
IUCN Red List

Species Description of Carcharhinus borneensis at www.shark-references.com
{{Taxonbar, from=Q893969

Borneo shark The Borneo shark (''Carcharhinus borneensis'') is a species of requiem shark, and part of the family Carcharhinidae. Extremely rare, it is known only from inshore waters around Mukah in northwestern Borneo, though it may once have been more wi ...

Fauna of Borneo Marine fauna of Southeast Asia