Ganges shark

The **Ganges shark** (*Glyphis gangeticus*) is a critically endangered species of requiem shark found in the Ganges River (Padma River) and the Brahmaputra River of Bangladesh and India. It is often confused with the more common bull shark (*Carcharhinus leucas*), which also inhabits the Ganges River and is sometimes incorrectly referred to as the Ganges shark. Unlike bull sharks, which need to migrate to salt water to reproduce, species in the genus *Glyphis* are true river sharks. The genus contains a total of three known species, down from a total of six due to the Borneo river shark and Irrawaddy Rivershark being revealed to be synonymous with the Ganges Shark. Even with this revelation having greatly expanded its range, the species remains very poorly known and very rare.

## Appearance

*G. gangeticus* is a little-known species that is yet to be adequately described. Its size at birth is 56 centimetres (22 in) to 61 centimetres (24 in), growing to an estimated 178 centimetres (70 in) at maturity, with a maximum size of about 204 centimetres (80 in). It is worth noting that the size at birth or maturity is unknown for any other *Glyphis* species, save the speartooth shark, for which adult size has recently been obtained.

A typical requiem shark in its external appearance, it is stocky in build, with two spineless dorsal fins and an anal fin. The first dorsal fin originates over the last third of the pectoral fins, with a free rear tip that is well in front of the pelvic fins. The second dorsal fin is relatively large, but much smaller than the first (about half the height). The anal fin is slightly smaller than the second dorsal fin and the pectoral fins are broad. There is a longitudinal upper precaudal pit, but no interdorsal ridge. It is uniformly grey to brownish in color, with no discernible markings.

Its snout is broadly rounded and much shorter than the width of its mouth. The mouth is long, broad, and extends back and up towards the eyes.

The eyes of the Ganges shark are minute, suggesting that it may be adapted to turbid water with poor visibility, such as occurs in the Ganges River and the Bay of Bengal. It has internal nictitating eyelids, like many requiem sharks.

The upper teeth have high, broad serrated triangular cusps and the labial furrows are very short. The lower front teeth have long, hooked, protruding cusps with unserrated cutting edges along the entire cusp, but without spearlike tips and with low cusplets on feet of crowns. The tooth row counts are 32–37/31–34, respectively.

### Diagnostic features

Ganges sharks can be identified by the first few lower front teeth, which have cutting edges along entire cusp, giving the cusps a clawlike shape, and low cusplets. Also, a second dorsal fin that is about half the height of first dorsal is distinct to this species.

## Distribution

The Ganges shark, as its name suggests, is largely restricted to the rivers of eastern and northeastern India, particularly the Hooghly River of West Bengal, and the Ganges, Brahmaputra, and Mahanadi in Bihar, Assam, and Odisha, respectively. However, its range has seemingly been extended to Borneo and the Irrawaddy River system thanks to genetic research. It is typically found in the middle to lower reaches of a river.

In theory, *G. gangeticus* could occur in shallow marine estuaries; however there are no verified marine records of the species to date, though the discoveries of it being present in such areas as Borneo and the Irrawaddy Rivers may indicate it otherwise can occur there. Originally the species was assigned a wide range in the Indo-West Pacific, but this was found to be mostly based on other species of requiem sharks, particularly members of the genus *Carcharhinus*.

It should be noted that most literature records and specimens labelled as this species are in fact bull sharks (*Carcharhinus leucas*) or other Carcharhinid species. An extensive 10-year search produced only a few specimens, caught in 1996 in the Ganges River, further highlighting this species' rarity.

## Habitat, Biology and Ecology

The Ganges shark is known to inhabit only freshwater, inshore marine, and estuarine systems in the lower reaches of the Ganges-Hooghly River system as well as (according to genetic research), the Irrawaddy River and Borneo. The feeding habits of this requiem shark are mostly unknown. The shark’s small eyes and slender teeth suggest that it is primarily a fish-eater and is adapted to turbid, murky water. With such limited visibility typical of many tropical rivers and estuaries, other senses − such as hearing, smell and electroreception − are likely used for predation. Because its eyes are tilted towards its back rather than to the sides or bottom (as is the case in most carcharhinids), it is thought that the shark may swim along the bottom and scan the water above it for potential prey back-lit by the sun. However, in the Bay of Bengal, *G. gangeticus* was found to feed heavily on dasyatid stingrays, which spend much of their time on the bottom.

### Reproduction

The Ganges shark is probably viviparous, with a yolk-sac placenta (speculation through analogy to related species of carcharhinids). The litter size and gestation period are unknown. However, their life history cycle is probably similar to other river sharks, characterized by long gestation, slow growth, delayed maturity, and small litter size. These factors make the Ganges shark populations vulnerable to even relatively low levels of exploitation, such as sport angling or gill netting.

### Possibility of migration

Some researchers consider Ganges sharks to be amphidromous, covering more than 100 km in both directions. However, this is not thought to be for breeding, as the case in anadromous and catadromous species. The presence of newborn individuals in the Hooghly River suggests that the young may be born in fresh water.

A specimen photographed in 2011 by natural history journalist Malaka Rodrigo at Negombo fish market in Sri Lanka prompted researcher Rex de Silva to speculate on whether the species could occasionally be carried south of its normal range by ocean currents. However, only the head of the shark appears in the photo. Leading shark expert Leonard Compagno emphasised the need to check the dentition and the dorsal fin proportions in order to confirm the specimen as a Ganges shark, stating that it could also be one of the three other named species.

### Specimens

The Ganges sharkwas originally known only from three nineteenth century museum specimens: one each in the Muséum national d’Histoire Naturelle, Paris, Humboldt Museum, Berlin and Zoological Survey of India, Calcutta. No records exist between 1867 until 1996, and the 1996 records have not been confirmed as *G. gangeticus*. A specimen collected 84 km upstream of the mouth of the Hooghly River at Mahishadal in 2001 was identified as *G. gangeticus* but on photographs of the jaw only. If *Carcharias murrayi* (Günter, 1887) can be considered a junior synonym of this species, one was found near Karachi, Pakistan. However, the holotype was apparently lost or misplaced in the British Museum of Natural History.

### Molecular biology

Glyphis species, like other sharks, exhibit a very slow rate of genetic change. This makes them even more vulnerable to becoming extinct, as they are unable to adapt to the rapid and extreme changes caused by humans to their environment.

As only a few specimens exist, naturally there is little material available for genetic sampling. However, two websites list records for Ganges sharks:

The Barcode of Life Data Systems (BOLDS) Stats website lists one record:
Public Records: 0
Specimens with Barcodes: 1
Species With Barcodes: 1

The NCBI Taxonomy database has one record of mitochondrial genetic material (1,044 base pairs of linear DNA):
*Glyphis gangeticus* bio-material GN2669, reported in a 2012 paper on DNA sequencing in shark and ray species.

## Conservation

The Ganges shark is one of 20 sharks on the International Union for Conservation of Nature and Natural Resources Red List of endangered shark species. The species is currently classified as Critically Endangered. There is an urgent need for a detailed survey of the shark fisheries of the Bay of Bengal.

### Major threats

River sharks are thought to be particularly vulnerable to habitat changes. The Ganges shark is restricted to a very narrow band of habitat that is heavily impacted by human activity. Overfishing, habitat degradation from pollution, increasing river use and management, including construction of dams and barrages are the principle threats. Thought to be consumed locally for its meat, the Ganges shark is caught by gillnet and its oil, along with that of the Ganges and Indus river dolphins, is highly sought after as a fish attractant. It is also believed to be part of the Asian shark fin trade, though this is uncertain.

### Conservation actions

In 2001, the Indian government banned the landing of all species of chondrichthyan fish in its ports. However, shortly afterwards this ban was amended to cover only 10 species of chondrichthyans. These, including Ganges sharks, are protected under Schedule I, Part II A of the Wildlife Protection Act of India. There is doubt about the effectiveness of this measure, however, because of difficulties in enforcement. There is a widespread, albeit widely dispersed, artisanal fishery for both local consumption and international trade. Compagno (1997) recommends an in-depth survey of fishing camps and landing sites, along with a sampling program in the Ganges system to determine the current status of this shark along with other gangetic elasmobranchs such as stingrays and sawfish.

## Human interaction

The Ganges shark is widely feared as a ferocious man-eater. But most of the attacks attributed to it are probably the result of confusion with the bull shark (*Carcharhinus leucas*). This is likely because bull sharks are known to travel long distances into freshwater systems and may co-exist in the same waters as the Ganges shark. But since little is known about the behaviour of genuine freshwater river sharks, and since the Ganges shark is critically endangered, contact with humans is very rare.

The biological differences between the Ganges shark and bull shark also point to a lower likelihood of attacks on humans by the Ganges shark. The Ganges shark has much narrower, higher, upper teeth and slender-cusped, less heavily built lower teeth than the bull shark does. Such small sharp teeth are more suitable for fish-impaling and less useful for dismembering tough mammalian prey than the stout teeth of the bull shark.

## In Popular Culture

Because of its rarity, the Ganges shark is not often heard of outside India, though it was mentioned briefly in an episode of Shark Week focusing on its relative, the bull shark. It was also mentioned as the most critically endangered shark species in 2015 on Discovery.com's Shark Week page.