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

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

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 Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

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 Borneo (; id, Kalimantan) is the third-largest island in the world and the largest in Asia. At the geographic centre of Maritime Southeast Asia, in relation to major Indonesian islands, it is located north of Java, west of Sulawesi, and eas ...

, 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 Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

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 A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through conv ...

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 Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ...

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

placenta The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mater ...

l connection. The

International Union for Conservation of Nature The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natu ...

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 Pieter Bleeker (10 July 1819 – 24 January 1878) was a Dutch medical doctor, ichthyologist, and herpetologist. He was famous for the ''Atlas Ichthyologique des Indes Orientales Néêrlandaises'', his monumental work on the fishes of East Asia ...

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 Singkawang or Sakawokng in Dayak Salako or San-Khew-Jong ( hak, 山口洋), is a city located in the province of West Kalimantan, on the island of Borneo in Indonesia. It is located at about 145 km north of Pontianak, the provincial capital ...

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, ...

. 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 , image = Seal of Universiti Malaysia Sabah.png , image_size = , caption = Seal , motto = ''Bertekad Cemerlang'' (Malay) Jawi: , mottoeng = Strive to Excel , ...

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 Sabah () is a States and federal territories of Malaysia, state of Malaysia located in northern Borneo, in the region of East Malaysia. Sabah borders the Malaysian state of Sarawak to the southwest and the North Kalimantan province of Indone ...

and

Sarawak Sarawak (; ) is a States and federal territories of Malaysia, state of Malaysia. The largest among the 13 states, with an area almost equal to that of Peninsular Malaysia, Sarawak is located in northwest Borneo Island, and is bordered by the M ...

. 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 Leonard Joseph Victor Compagno is an international authority on shark taxonomy and the author of many scientific papers and books on the subject, best known of which is his 1984 catalogue of shark species produced for the Food and Agriculture Organi ...

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 (''C. fitzroyensis''),

whitecheek shark The whitecheek shark or widemouth blackspot shark (''Carcharhinus dussumieri'') is a requiem shark of the family Carcharhinidae, found in the Indo-West Pacific Ocean between latitudes 34°N and 25°S. It can reach a length of 1 m. It feed ...

(''C. dussumieri''), hardnose shark (''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 slit Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In con ...

s are short. The

pectoral 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 ...

s 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

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 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 ...

bears a deep, crescent-shaped pit at the origin of the upper

caudal 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 ...

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

, 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 Zhoushan Island is the principal and namesake island in the Zhoushan Islands, formerly romanized as the ChusanIslands, an archipelago administered by Zhoushan Prefecture in Zhejiang Province in the People's Republic of China. It is the province ...

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 The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no), * bik, Republika kan Filipinas * ceb, Republika sa Pilipinas * cbk, República de Filipinas * hil, Republ ...

in 1895, and

Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...

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

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

l connection formed from the depleted

yolk sac The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is far ...

. The litter size is six, and the pups are born at close to long. From the available specimens, the length at

sexual maturity Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural definitio ...

can be surmised to be under in males and under in females.

Human interactions

The

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 An artisan (from french: artisan, it, artigiano) is a skilled craft worker who makes or creates material objects partly or entirely by hand. These objects may be functional or strictly decorative, for example furniture, decorative art, s ...

and

commercial fishing Commercial fishing is the activity of catching fish and other seafood for commercial profit, mostly from wild fisheries. It provides a large quantity of food to many countries around the world, but those who practice it as an industry must often ...

. 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