Giant freshwater stingray

Appearance

The giant freshwater stingray has a thin, oval pectoral fin disc slightly longer than wide and broadest towards the front. The elongated snout has a wide base and a sharply pointed tip that projects beyond the disc. The eyes are minute and widely spaced; behind them are large spiracles. Between the nostrils is a short curtain of skin with a finely fringed posterior margin. The small mouth forms a gentle arch and contains four to seven papillae on the floor. The small and rounded teeth are arranged into pavement-like bands. There are five pairs of gill slits on the ventral side of the disc. The pelvic fins are small and thin; mature males have relatively large claspers.

The thin, cylindrical tail measures 1.8–2.5 times as long as the disc and lacks fin folds. A single serrated stinging spine is positioned on the upper surface of the tail near the base. At up to 38 cm long, the spine is the largest of any stingray species. There is band of heart-shaped tubercles on the upper surface of the disc extending from before the eyes to the base of the sting; there is also a midline row of four to six enlarged tubercles at the center of the disc. The remainder of the disc upper surface is covered by tiny granular denticles, and the tail is covered with sharp prickles past the sting. This species is plain grayish brown above, often with a yellowish or pinkish tint towards the fin margins; in life the skin is coated with a layer of dark brown mucus. The underside is white with broad dark bands, edged with small spots, on the trailing margins of the pectoral and pelvic fins. The tail is black behind the spine. The giant freshwater stingray reaches at least 1.9 m in width and 5.0 m in length, and can likely grow larger. With reports from the Mekong and Chao Phraya Rivers of individuals weighing 500–600 kg , it ranks among the largest freshwater fishes in the world.

Distribution

The giant freshwater stingray is known to inhabit several large rivers and associated estuaries in Indochina and Borneo. In Indochina, it occurs in the Mekong River to potentially as far upstream as Chiang Khong in Thailand, as well as in the Chao Phraya, Nan, Mae Klong, Bang Pakong, Prachin Buri and Tapi Rivers. In Borneo, this species is found in the Mahakam River in Kalimantan and the Kinabatangan and Buket Rivers in Sabah; it is reportedly common in the Kinabatangan River but infrequently caught. Though it has been reported from Sarawak as well, surveys within the past 25 years have not found it there. Elsewhere in the region, recent river surveys in Java have not recorded its presence, despite the island being the locality of the species holotype. Historical records from Myanmar, the Ganges River in India, and the Bay of Bengal have similarly not been corroborated by any recent accounts.

Disjunct populations of the giant freshwater stingray in separate river drainages are probably isolated from one another; though the species occurs in brackish environments, there is no evidence that it crosses marine waters. This is a bottom-dwelling species that favors a sandy or muddy habitat. Unexpectedly, it can sometimes be found near heavily populated urban areas.

Behavior

The diet of the giant freshwater stingray consists of small fishes and invertebrates such as crustaceans and molluscs, which it can detect using its electroreceptive ampullae of Lorenzini. Individuals can often be seen at the edge of the river, possibly feeding on earthworms. Parasites documented from this species include the tapeworms ''Acanthobothrium asnihae'', ''A. etini'', ''A. masnihae'', ''A. saliki'', ''A. zainali'', ''Rhinebothrium abaiensis'', ''R. kinabatanganensis'', and ''R. megacanthophallus''. The giant freshwater stingray is viviparous, with the developing embryos nourished initially by yolk and later by histotroph provided by the mother. This species does not appear to be diadromous . Observed litter sizes range from one to four pups; newborns measure around 30 cm across. Pregnant females are frequently found in estuaries, which may serve as nursery areas. Males mature sexually at approximately 1.1 m across; female maturation size and other life history details are unknown.

Habitat

The giant freshwater stingray is known to inhabit several large rivers and associated estuaries in Indochina and Borneo. In Indochina, it occurs in the Mekong River to potentially as far upstream as Chiang Khong in Thailand, as well as in the Chao Phraya, Nan, Mae Klong, Bang Pakong, Prachin Buri and Tapi Rivers. In Borneo, this species is found in the Mahakam River in Kalimantan and the Kinabatangan and Buket Rivers in Sabah; it is reportedly common in the Kinabatangan River but infrequently caught. Though it has been reported from Sarawak as well, surveys within the past 25 years have not found it there. Elsewhere in the region, recent river surveys in Java have not recorded its presence, despite the island being the locality of the species holotype. Historical records from Myanmar, the Ganges River in India, and the Bay of Bengal have similarly not been corroborated by any recent accounts.

Disjunct populations of the giant freshwater stingray in separate river drainages are probably isolated from one another; though the species occurs in brackish environments, there is no evidence that it crosses marine waters. This is a bottom-dwelling species that favors a sandy or muddy habitat. Unexpectedly, it can sometimes be found near heavily populated urban areas.The diet of the giant freshwater stingray consists of small fishes and invertebrates such as crustaceans and molluscs, which it can detect using its electroreceptive ampullae of Lorenzini. Individuals can often be seen at the edge of the river, possibly feeding on earthworms. Parasites documented from this species include the tapeworms ''Acanthobothrium asnihae'', ''A. etini'', ''A. masnihae'', ''A. saliki'', ''A. zainali'', ''Rhinebothrium abaiensis'', ''R. kinabatanganensis'', and ''R. megacanthophallus''. The giant freshwater stingray is viviparous, with the developing embryos nourished initially by yolk and later by histotroph provided by the mother. This species does not appear to be diadromous . Observed litter sizes range from one to four pups; newborns measure around 30 cm across. Pregnant females are frequently found in estuaries, which may serve as nursery areas. Males mature sexually at approximately 1.1 m across; female maturation size and other life history details are unknown.