Glyphis siamensis (Irrawaddy river shark) is a species of requiem shark, belonging to the family Carcharhinidae, known only from a single museum specimen originally caught at the mouth of the Irrawaddy River in Myanmar. A plain gray, thick-bodied shark with a short rounded snout, tiny eyes, and broad first dorsal fin, the Irrawaddy river shark is difficult to distinguish from other members of its genus without anatomical examination. Virtually nothing is known of its natural history; it is thought to be a fish-eater with a viviparous mode of reproduction. The International Union for Conservation of Nature (IUCN) has assessed this species as Critically Endangered, as its distribution is extremely limited and suffers heavy fishing pressure and habitat degradation.
Genetic evidence has shown that both the Borneo river shark (G. fowlerae) and Irrawaddy river shark should be regarded as synonyms of the Ganges shark (G. gangeticus).
The only known specimen 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.
Although traditionally recognized as separate species, genetic evidence has shown that both the Borneo river shark (G. fowlerae) and Irrawaddy river shark should be regarded as synonyms of the Ganges shark (G. gangeticus). All three populations are very poorly known and very rare.
The Irrawaddy river shark is found in the delta of the Irrawaddy River near Yangon, Myanmar, apparently inhabiting brackish water in a large, heavily silt-laden river lined with mangrove forests.
The sole specimen is a 60 cm (24 in) long immature male, suggesting an adult length of 1–3 m (3–10 ft). Like other river sharks, its body is robustly built with a high back that slopes down to a broadly rounded snout shorter than the mouth is wide. The eyes are minute, and the nares are small and widely spaced. The mouth contains 29 tooth rows in the upper and lower jaws, and has short furrows at the corners. The upper teeth are broad, triangular, and upright, with serrated margins, while the lower teeth at the front are more finely serrated with a pair of small cusplets at the base.
The first dorsal fin is broad and triangular, originating over the rear pectoral fin bases with its free rear tip ending in front of the pelvic fin origins. The second dorsal fin is half as tall as the first, and there is no ridge between the dorsal fins. The trailing margin of the anal fin has a deep notch. The coloration is a plain grayish brown above and white below, without conspicuous fin markings. This shark most closely resembles the Ganges shark (G. gangeticus), but has more vertebrae (209 versus 169) and fewer teeth (29/29 versus 32–37/31–34).
The small teeth of the Irrawaddy river shark suggests that it mainly preys on fish, while its small eyes are consistent with the extremely turbid water in which it hunts. Reproduction is presumably viviparous, with the young sustained by a placental connection, as in other requiem sharks.
Intensive artisanal fishing, mainly gillnetting but also line and electrofishing, occurs in the stretch of river where the sole Irrawaddy river shark specimen was caught. Habitat degradation poses a further threat to this shark, including water pollution and the clearing of mangrove trees for fuel, construction materials, and other products. This shark may be naturally rare, which along with its highly restricted range, probable overfishing, and loss of habitat, has led the International Union for Conservation of Nature (IUCN) to list it as Critically Endangered. Despite fishing and scientific surveys in the area, no more Irrawaddy river sharks have been recorded in the hundred-plus years since the first.