| Ganges shark Temporal range: Miocene-recent[1] | |
|---|---|
_-_Kawahara_Keiga_-_1823_-_1829_-_Siebold_Collection_-_pencil_drawing_-_water_colour.jpeg) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Division: | Selachii |
| Order: | Carcharhiniformes |
| Family: | Carcharhinidae |
| Genus: | Glyphis |
| Species: | G. gangeticus |
| Binomial name | |
| Glyphis gangeticus (J. P. Müller &Henle, 1839) | |
 | |
| Glyphis gangeticus inhabits the Ganges-Hooghly River system | |
| Synonyms | |
| |
TheGanges shark (Glyphis gangeticus) is acritically endangered species ofrequiem shark found in theGanges River (Padma River) and theBrahmaputra River ofIndia andBangladesh. It is often confused with the more commonbull shark (Carcharhinus leucas), which also inhabits the Ganges River and is sometimes incorrectly referred to as the Ganges shark.[3] The genus is currently considered to contain three recent species;genetic evidence has shown that both the Borneo river shark (G. fowlerae) and Irrawaddy river shark (G. siamensis) should be regarded assynonyms of the Ganges shark, expanding the range of the species toPakistan,Myanmar,Borneo, andJava. The species remains poorly known and very rare.
Formerly the Borneo river shark (Glyphis fowlerae) and the Irrawaddy river shark (Glyphis siamensis) were considered to represent two other species in the genusGlyphis. They have recently been reclassified asG. gangeticus based on genetic studies, and their scientific names are treated as synonyms.[4]
The Borneo river shark is known only from theKinabatangan River inBorneo. It can reach a length of 78 cm (31 in). Only 13 specimens are known to science, all collected in 1996. Expeditions in 2010 and 2011 failed to find any, and while fishermen recognised the shark, they have not been seen for many years.[4]
The Irrawaddy river shark is known only from a single museum specimen originally caught at the mouth of theIrrawaddy River inMyanmar, abrackish-water locality in a large, heavilysilt-laden river lined withmangrove forests. It was collected in the 19th century and described asCarcharias siamensis byAustrianichthyologistFranz Steindachner, inAnnalen des Naturhistorischen Museums in Wien (volume 11, 1896).[5] However, subsequent authors doubted the validity of this species, regarding it as an abnormalbull shark (Carcharhinus leucas), until in 2005 shark systematistLeonard Compagno recognized it as distinct member of the genusGlyphis.[6] The specimen is a 60-cm-long immature male. It closely resembles the Ganges shark, but has morevertebrae (209 versus 169) and fewer teeth (29/29 versus 32–37/31–34).[6][7]
A possiblyundescribed species ofGlyphis is known fromMukah in Borneo, as well asBangladesh. The status of a Borneo specimen fromSampit remains unclear.[4]
G. gangeticus is a little-known species that is yet to be adequately described.[8] Its size at birth is 56 to 61 cm (22 to 24 in), growing to an estimated 178 cm (70 in) at maturity, with a maximum size of about 204 cm (80 in).[9] The size at birth or maturity is unknown for any otherGlyphis species.[6]
A typical requiem shark in its external appearance, it is stocky, with two spinelessdorsal fins and ananal fin. The first dorsal fin originates over the last third of thepectoral fins, with a free rear tip that is well in front of thepelvic 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. A longitudinal upper precaudal pit is seen, but no interdorsal ridge. It is uniformly grey to brownish in color, with no discernible markings.[3]
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.[3]
Its eyes are minute, suggesting that it may be adapted to turbid water with poor visibility, such as occurs in the Ganges River and theBay of Bengal. It has internalnictitating eyelids.[3]
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 spear-like tips and with low cusplets on feet of crowns. Thetooth row counts are 32–37/31–34.[10]
G. gangeticus can be identified by the first few lower front teeth, which have cutting edges along entire cusp, giving the cusps a claw-like shape, and low cusplets. Also, a second dorsal fin that is about half the height of first dorsal is distinct to this species.[11]
The Ganges shark, as its name suggests, is largely restricted to the rivers of eastern and northeastern India, particularly theHooghly River ofWest Bengal, and theGanges,Brahmaputra, andMahanadi inBihar,Assam, andOdisha, respectively. It is typically found in the middle to lower reaches of a river.[12] One found in 2018 in aMumbai fish market may have come from somewhere along the banks of theArabian Sea.[13]
In theory,G. gangeticus could occur in shallow marine estuaries; however, no marine records of the species have been verified to date. 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 genusCarcharhinus.[9]
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.[14]
G. gangeticus is known to inhabit only freshwater, inshore marine, and estuarine systems in the lower reaches of the Ganges-Hooghly River system. Their feeding habits are mostly unknown. The shark's small eyes and slender teeth suggest that it is primarily a fish-eater and is adapted to turbid water.[3] With such limited visibility typical of many tropical rivers and estuaries, other senses − such as hearing, smell, andelectroreception − are likely used for predation.[6] Because its eyes are tilted towards its back rather than to the sides or bottom (as is the case in most carcharhinids), the shark may swim along the bottom and scan the water above it for potential prey back-lit by the sun.[10] However, in the Bay of Bengal,G. gangeticus was found to feed heavily on dasyatidstingrays, which spend much of their time on the bottom.[15]
It is probablyviviparous, with a yolk-sac placenta (speculation through analogy to related species of carcharhinids). The litter size and gestation period are unknown.[3] 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.[6]
Some researchers considerG. gangeticus to beamphidromous, covering more than 100 km (62 mi) in both directions.[16] However, this is not thought to be for breeding, as the case in anadromous and catadromous species.[17] The presence of newborn individuals in the Hooghly River suggests that the young may be born in fresh water.[11]
A specimen photographed in 2011 by natural history journalist Malaka Rodrigo atNegombo 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 expertLeonard Compagno emphasised the need to check the dentition and the dorsal fin proportions to confirm the specimen asG. gangeticus, stating that it could also be one of the four other named species.[18]
G. gangeticus was originally known only from three 19th-century museum specimens, one each in theNational Museum of Natural History in Paris,Natural History Museum, Berlin in Berlin, and theZoological Survey of India inCalcutta.[19] No records exist between 1867 and 1996, and the 1996 records have not been confirmed asG. gangeticus. A specimen collected 84 km (52 mi) upstream of the mouth of the Hooghly River atMahishadal in 2001 was identified asG. gangeticus, but on photographs of the jaw only.[10] IfCarcharias murrayi (Günter, 1887) can be considered a junior synonym of this species, one was found nearKarachi, Pakistan. However, theholotype was apparently lost or misplaced in theBritish Museum of Natural History.[3]
One female specimen was recorded at Sassoon Docks inMumbai, India in February 2016, measuring 266 cm (105 in)total length. It was caught in theArabian Sea.[20]
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.[6]
As only a few specimens exist, naturally little material available is for genetic sampling. However, two websites list records forG. gangeticus:
The Barcode of Life Data Systems (BOLDS) Stats website[21] 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,[22] reported in a 2012 paper on DNA sequencing in shark and ray species.[23]
G. gangeticus is one of 20 sharks on theInternational Union for Conservation of Nature and Natural Resources Red List of endangered shark species. The species is currently classified ascritically endangered. According to the organisation, fewer than 250 Ganges sharks are believed to exist. The need is urgent for a detailed survey of the shark fisheries of the Bay of Bengal.[2][3]
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 affected by human activity. Overfishing, habitat degradation frompollution, increasing river use, and management including the construction of dams and barrages, are the principal threats. Thought to be consumed locally for its meat, the Ganges shark is caught bygillnet, and its oil, along with that of theSouth Asian river dolphin, is highly sought after as a fish attractant.[24] It is also believed to be part of the Asianshark fin trade.[9] After a sighting in 2006, the species was not seen again for over a decade until one was found at a Mumbai fish market in 2016.[25]
The single Irrawaddy river shark specimen stems from an area of intensiveartisanal fishing, mainly gillnetting, but also line andelectrofishing.Habitat degradation may pose a further threat to this shark, includingwater pollution and the clearing of mangrove trees for fuel, construction materials, and other products. The shark may be naturally rare in this area and highly restricted in its range. Despite fishing and scientific surveys in the area, no more Irrawaddy river sharks have been recorded in the 100-plus years since the first.[26]
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, includingG. gangeticus, are protected under Schedule I, Part II A of theWildlife Protection Act of India.[27][28] Doubt exists about the effectiveness of this measure, however, because of difficulties in enforcement. A widespread, albeit widely dispersed, artisanal fishery exists 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 asstingrays andsawfish.[2]
The Ganges shark is widely feared as a ferocious man-eater,[29] but most of the attacks attributed to it are probably the result of confusion with the bull sharkCarcharhinus leucas.[10] 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. Since little is known about the behaviour of genuine freshwater river sharks, and sinceG. gangeticus is critically endangered, contact with humans is very rare.[30] There has been one attack which might be attributable to the Ganges shark, this attack happened in 1868 and was attributed to the shark byJoseph Fayrer.[31]
The biological differences between the Ganges shark and bull shark also point to a lower likelihood of attacks on humans by the Ganges shark.G. gangeticus has much narrower, higher, upper teeth and slender-cusped, less heavily built lower teeth thanC. leucas. 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.[11]
Glyphis: from Greekglyphe, means "carving".[32]
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