The Mysterious, Endangered River

Sharks (Glyphis spp.)

Rare discovery. This small shark, feared extinct as no specimens were reported
since the 19th Century, was discovered in the Kinabatangan River of Sabah,
in northern Borneo. It is a new, as-yet un-named species of river shark
(Glyphis sp., possibly Glyphis species B), that is presently in the process
of being described by Sarah Fowler and Leonard J.V. Compagno.
Photo courtesy Sarah Fowler, Darwin Project and IUCN Shark Specialist Group.

Most people would be surprised to discover that there are such things as "river sharks". After all, sharks live in the ocean, right? Many shark enthusiasts know about the Bull Shark (Carcharhinus leucas), a large, heavy-bodied, saw-toothed species that is notorious for attacking people and swimming far up tropical rivers. But only advanced students of shark biology have ever heard of the river sharks.

Perhaps the most familiar (I cannot say "best known", because almost nothing is known about it, biologically) is the Ganges Shark (Glyphis gangeticus), which has a reputation as a man-eater in India. Its ferocious reputation may not be warranted, however, due to confusion with the Bull Shark, which also occurs in India's Bombay, Cochin, and Hooghly rivers. Less familiar is the Speartooth Shark (Glyphis glyphis), in which the tips of the anterior lower teeth are serrated and flare outward , arrowhead-like, before coming to a sharp point (a tooth tip type known technically as "hastate"). There are at least four other species of river sharks, only one of which has been formally named.

Lingual (left, identifiable by the nutritive pore on the root) and labial (right) faces of a lower anterior tooth from a Speartooth Shark (Glyphis glyphis), showing the hastate (arrowhead-like) crown tip characteristic of the teeth of this species.

The six known river sharks are definitely rare and probably endangered. Collectively, their distribution, life history, and ecology is a virtual cipher. In this page, I try to summarize what little is known about the mysterious creatures.

Characteristics of River Sharks

The river sharks (genus Glyphis) are members of the family Carcharhinidae, and thus share the basic characteristics of the group.

In general, all river sharks feature the following field characteristics:

a short, broadly rounded snout (its length less than the mouth width)
small, wide-spaced nostrils
small, dark eyes
broad, serrated upper teeth
very short labial furrows (lip grooves), restricted to the jaw corners
a broad dorsal fin with the mid-base closer to the base of the pectoral fins than those of the pelvic fins, and
an anal fin with a deeply notched posterior margin

River sharks are very similar in overall morphology to whaler sharks of the genus Carcharhinus, but can be distinguished from them by the following suite of characteristics:

Cusps of lower teeth protrude prominently when mouth is closed
Second dorsal fin is 1/2 to 3/5 the height of the first dorsal fin
Origin of second dorsal fin slightly anterior to origin of anal fin
Precaudal pit is longitudinal rather than crescent-shaped

Identification of River Sharks

Following summarizes features that may be useful for identifying river sharks of the genus Glyphis:

Species	Head Shape in Lateral View	Total Tooth Count	Lower Anterior Teeth	Total Vertebral Count	Diplo- spondylous Vertebral Count
Glyphis gangeticus	High and Broad	32-37/ 31-34	Smooth, Non-hastate	169	?
Glyphis glyphis	High and Broad	26-29/ 27-29	Serrate Tips, Hastate	?
Glyphis siamensis	High and broad	29/29	?Weakly serrated, low basal cusplets	209
Glyphis species A	High and Broad	33-55/ 32-55	Serrate Tips, Hastate	148-217	54
Glyphis species B	High and Broad	60-63/ 60-63	Serrated Tips, Non hastate	196-205	43-51
Glyphis species C	Flat and narrow	32-24/32-34	Non-hastate, no basal cusplets	147-148	65-68

Pt 2

Species	Free Rear Tip of First Dorsal Fin	Second Dorsal Fin Height	Fin Markings
Glyphis gangeticus	Anterior to Pelvic Fin Origin	1/2 that of First Dorsal Fin	All Fins Plain, Without Dusky Margins
Glyphis glyphis	Anterior to Pelvic Fin Origin	3/5 that of First Dorsal Fin	All Fins Plain, Without Dusky Margins
Glyphis siamensis	Anterior to Pelvic Fin Origin	1/2 that of First Dorsal Fin	All Fins Plain, Without Dusky Margins
Glyphis species A	Opposite Pelvic Fin Origin	3/5 that of First Dorsal Fin	All Fins with Black or Dusky Edges
Glyphis species B	Anterior to Pelvic Fin Origin	3/5 that of First Dorsal Fin	Fins Plain and Light, Except for Dark Patch on Pectoral Bases and Dusky Tip on Hypocaudal Lobe
Glyphis species C	Opposite Pelvic Fin Origin	3/5 that of First Dorsal Fin	All Fins Plain, Without Dusky Margins

Clearly, these species are not yet adequately differentiated. Examination of additional specimens representing the full spectrum of growth stages and geographic distribution is needed to resolve the full range of variation of proportional measurements and meristic counts.

Diversity and Distribution of River Sharks

At least six species of river sharks are known:

1) Ganges Shark
Glyphis gangeticus (Müller & Henle, 1839)
Definitely known from the Hooghly-Ganges river system, West Bengal, India, and likely from the vicinity of Karachi, Pakistan.

2) Speartooth Shark
Glyphis glyphis (Müller & Henle, 1839)
Uncertain. Speartooth-like sharks occur in Borneo, New Guinea, and Queensland, Australia, but it is uncertain at present if any of these are the true Glyphis glyphis. The holotype (original specimen on which the description is based) has no capture locality listed for it.

3) Irrawady River Shark
Glyphis siamensis (Steindachner, 1896)
The only known specimen is from the Irriwaddy River, near Rangoon. Until very recently, this species was regarded as an aberrant specimen and possible synonym of (the same species as) the Bull Shark, but shark systematist Leonard J.V. Compagno now considers it a member of the genus Glyphis, distinct from the other known species.

4) Glyphis species A
[Not yet described]
Known from two specimens from estuarine waters of the lower reaches of the Bizant River in Queensland, Australia, where it occurs along with the Bull Shark. It is also known from the Alligator River system of the Northern Territory, where it occurs with Glyphis species C. Intriguingly, Glyphis specimens from the Adelaide River, Northern Territory, that have provisionally been identified as this species, display very different vertebral counts (148 total vertebrae, versus 217 in specimens from the Bizant River).

5) Glyphis species B
[Not yet described]
Known from a single specimen from Borneo. Recently, several small Glyphis have been collected from the Kinabatangan River of Sabah, in northern Borneo. These may be the same species as Glyphis species B. The species is presently in the process of being described by Sarah Fowler and Leonard J.V. Compagno.

6) Glyphis species C
[Not yet described]
Only nine specimens have ever been collected - an immature female taken about 100km up the Adelaide River in 1989, an adult male taken some 60km up the South Alligator River in 1996, and in 1999, five females and two males from the East, West and South Alligator Rivers, in brackish water, with salinity ranging from 6 to 26 ppt. This species also occurs alongside Glyphis species A.

Glyphis species C, from the Alligator River system of the Northern Territory of Australia. Photograph courtesy of the Museum and Art Gallery of the Northern Territory, Darwin

Natural History of River Sharks

The following table summarizes what is known of the natural history of river sharks:

Species	Length (cm)	Diet	Reproduction	Habitat
Glyphis gangeticus	Max: possibly to 204 Maturity, males: 178 At Birth: 56-61	Fish?	Live-bearing, probably with yolk-sac placenta	Freshwater, possibly estuarine; no confirmed records from saltwater
Glyphis glyphis	Max: at least 100	Fish?	Live-bearing, probably with yolk-sac placenta	Inshore, possibly freshwater
Glyphis siamensis	Max: at least 63	Fish?	Live-bearing, probably with yolk-sac placenta	Freshwater
Glyphis species A	Max: at least 131, possibly 200-300	Fish?	Live-bearing, probably with yolk-sac placenta	Freshwater, possibly estuarine
Glyphis species B	Max: at least 100	Fish?	Live-bearing, probably with yolk-sac placenta	Freshwater
Glyphis species C	Max: to at least 147, possibly to 200	Fish?	Live-bearing, probably with yolk-sac placenta	Freshwater (salinities from 6-26 ppt)

Ecology of River Sharks

Little is known of the ecology of river sharks.

The small eyes and teeth of these sharks suggest that they are primarily fish-eaters that have adapted to hunting in cloudy estuarine and river waters. Interestingly, the eyes of the Ganges Shark are tilted upward rather than laterally or downward, as in most carcharhinids. This ocular orientation suggests that the Ganges Shark may swim along the bottom scanning the water above it for potential prey back-lit by the sun. In turbid waters, it is easy to imagine how the Ganges Shark may occasionally mistake a human for prey, resulting in an accidental bite; fortunately, the small teeth of this shark are less formidable than the larger, broader dentition of the sympatric (occurring in the same range) Bull Shark, which is probably responsible for most — if not all — attacks blamed on the Ganges Shark. Given the limited visibility of many tropical rivers and estuaries, it seems likely that hearing, olfaction, and electroreception may play a more significant role in predation than vision.

Virtually nothing is known of reproduction in river sharks. All are believed to be live-bearing, probably nourishing the developing pups via a yolk sac placenta, as in other carcharhinids (except the Tiger Shark, Galeocerdo cuvier, which is ovoviviparous and which I believe should be placed in its own family). A 178-cm specimen of Ganges Shark was apparently mature and late fetuses or newborn specimens are 56 to 61 cm long. Size at birth or maturity is unknown for any other Glyphis species.

All river sharks appear to be able to tolerate low to reduced salinity environments. The Ganges Sharks is known only from freshwater habitats, but it may occur in brackish water as well; there are no marine records for this species. The confused identity and lack of type locality of the Speartooth Shark preclude any statement about its preferred habitat; it is probably an inshore estuarine and possibly freshwater species. The Irrawaddy River Shark is known only from the holotype, which appears to have been caught in freshwater. Glyphis species A may inhabit fresh to brackish waters, but this is uncertain. All records of Glyphis species B are from freshwater, but it, too, may prove to be euryhaline (able to tolerate a broad range of salinities). Glyphis species C is known only from freshwater, with salinities ranging from 6 to 26 parts per thousand.

River sharks in tropical rivers of northern Australia seem to be adapted to waters with very low dissolved oxygen content compared with the Bull Shark, which occurs in many of the same rivers. This may be a mechanism for ecological separation between the river sharks and the Bull Shark. It may also protect adult and, especially, juvenile river sharks of this region against predation by the Bull Shark, which has a predilection for feeding on the young of other sharks.

Conservation of River Sharks

River sharks are exceedingly rare. The Ganges Shark is known from only three museum specimens, all collected in the 19th Century. After an extensive search in the Ganges River over the past decade, a few additional specimens were caught in 1996. The species is regarded as critically endangered by the IUCN. The Speartooth Shark is known only from the stuffed holotype (original specimen), which was collected in the early 19th Century and now appears to be lost. Status of the Speartooth Shark is unknown, due to confusion with other Glyphis species from Borneo, New Guinea, and northern Australia. The validity of the Irrawaddy River Shark is in question, as it too, is known only from the original description and it is not known if the holotype is still in existence. It's status is likewise unknown. Glyphis species A is known from only a few specimens from rivers in northern Queensland and the Northern Territory of Australia. This species is regarded as critically endangered by Australia's Environment Protection and Biodiversity Act (EPBA) of 1999. Glyphis species B is known from a single small specimen collected in "Borneo" (no further location data is given), however several small specimens collected in the Kinabatangan River of Sabah, in northern Borneo, may be the same species. Its status is unknown. Glyphis species C is known from about 20 specimens from rivers in northern Queensland and the Northern Territory of Australia. This species is listed as endangered under Australia's EPBA.

River sharks may be particularly vulnerable to fishing pressure or habitat changes. Like other sharks, they probably feature a life history cycle characterized by long gestation, slow growth, delayed maturity, and small litter size. As such, their populations can be quickly devastated by even relatively low levels of exploitation, such as sport angling or gill netting. Like other sharks, Glyphis species probably exhibit a very slow rate of genetic change, making them unable to adapt to anthropogenic (human-caused) environmental changes. Humans living or working near tropical rivers or estuaries of the Austral-Asian region may degrade river shark habitat through pollution (sewage, detergents, rubbish, and run-off from agriculture, mining, or manufacturing processes) and so-called "management" (clearing mangrove swamps, redirecting water flow, building dams, etc.). These rivers and estuaries are absolutely vital to the continued survival of river sharks, which cannot readily colonize or adapt to new habitats. If much of their specialized habitat is ruined by human activity, these sharks will almost certainly become extinct.

River sharks are not always recognized for what they are: extremely rare, little known wildlife. So few specimens are available for study that the loss of even one is a great blow to scientific and conservation efforts. In 1986, a Glyphis was caught by an angler in fresh water about 60 km upstream the South Alligator River, in Australia's Northern Territory. Although the jaws were retained, this priceless specimen was eaten by the angler who caught it, resulting in the loss of much valuable data.

Here's hoping that, as more and more people become aware of them, the drive to protect and conserve river sharks — and their habitats — will grow. River sharks may not be particularly dangerous or flashy, but they are rare creatures that add to the diversity and mystery of our world.