The Ganges shark (Glyphis gangeticus) is a critically endangered species of requiem shark found in the Ganges River (including the Padma River) and the Brahmaputra River of India and Bangladesh. Additionally, genetic evidence has shown that both the Borneo river shark (G. fowlerae) and Irrawaddy river shark (G. siamensis) should be regarded as synonyms of the Ganges shark, expanding the range of the species to Pakistan, Myanmar, Borneo, and Java. The species remains poorly known and is very rare throughout its range. Glyphis are 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.
Taxonomy Formerly, the Borneo river shark (Glyphis fowlerae) and the Irrawaddy river shark (Glyphis siamensis) were considered to represent two other species in the genus Glyphis. They have recently been reclassified as G. gangeticus based on genetic studies, and their scientific names are treated as synonyms. The Borneo river shark is known only from the Kinabatangan River in Borneo. 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. The Irrawaddy river shark is known only from a single museum specimen originally caught at the mouth of the Irrawaddy River in Myanmar, a brackish-water locality in a large, heavily silt-laden river lined with mangrove forests. It was collected in the 19th century and described as Carcharias siamensis by Austrian ichthyologist Franz Steindachner, in Annalen des Naturhistorischen Museums in Wien (volume 11, 1896). However, subsequent authors doubted the validity of this species, regarding it as an abnormal bull shark (Carcharhinus leucas), until in 2005 shark systematist Leonard Compagno recognized it as distinct member of the genus Glyphis. The specimen is a 60-cm-long immature male. It closely resembles the Ganges shark, but has more vertebrae (209 versus 169) and fewer teeth (29/29 versus 32–37/31–34). A possibly undescribed species of Glyphis is known from Mukah in Borneo, as well as Bangladesh. The status of a Borneo specimen from Sampit remains unclear.
Physical appearance G. gangeticus is a little-known species that is yet to be adequately described. 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). The size at birth or maturity is unknown for any other Glyphis species. A typical requiem shark in its external appearance, it is stocky, 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. A longitudinal upper precaudal pit is seen, 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. Its eyes 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. 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. The tooth row counts are 32–37/31–34.
Diagnostic features 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.
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. It is typically found in the middle to lower reaches of a river. One found in 2018 in a Mumbai fish market may have come from somewhere along the banks of the Arabian Sea. 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 genus Carcharhinus. 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.
Habitat and ecology
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. 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), 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 It 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.
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