| Giant freshwater stingray | |
|---|---|
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Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Order: | Myliobatiformes |
| Family: | Dasyatidae |
| Genus: | Urogymnus |
| Species: | U. polylepis |
| Binomial name | |
| Urogymnus polylepis (Bleeker, 1852) | |
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| Geographic range (excludes populations not confirmed asextant and/or conspecific)[1] | |
| Synonyms | |
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Thegiant freshwater stingray (Urogymnus polylepis, also widely known by thejunior synonymHimantura chaophraya) is aspecies ofstingray in the familyDasyatidae. It is found in largerivers andestuaries inSoutheast Asia andBorneo, though historically it may have been more widely distributed in South and Southeast Asia. The widest freshwater fish in the world, this species grows up to 2.2 m (7.2 ft) across and can exceed 300 kg (660 lb) in weight. It has a relatively thin, ovalpectoral fin disc that is widest anteriorly, and a sharply pointed snout with a protruding tip. Its tail is thin and whip-like, and lacks fin folds. This species is uniformly grayish brown above and white below; the underside of the pectoral andpelvic fins bear distinctive wide, dark bands on their posterior margins.
Bottom-dwelling in nature, the giant freshwater stingray inhabits sandy or muddy areas and preys on small fishes andinvertebrates. Femalesgive live birth to litters of one to four pups, which are sustained to term by maternally producedhistotroph ("uterine milk"). This species faces heavy fishing pressure for meat,recreation, andaquarium display, as well as extensivehabitat degradation andfragmentation. These forces have resulted in substantial population declines in at least centralThailand andCambodia. As a result, theInternational Union for Conservation of Nature (IUCN) has assessed the giant freshwater stingray asEndangered.
The firstscientific description of the giant freshwater stingray was authored by DutchichthyologistPieter Bleeker in an 1852 volume of thejournalVerhandelingen van het Bataviaasch Genootschap van Kunsten en Wetenschappen. His account was based on a juvenile specimen 30 cm (12 in) across, collected fromJakarta, Indonesia. Bleeker named the new speciespolylepis, from theGreekpoly ("many") andlepis ("scales"), and assigned it to thegenusTrygon (now a synonym ofDasyatis).[2][3] However, in subsequent years Bleeker's description was largely overlooked, and in 1990 the giant freshwater stingray was described again by Supap Monkolprasit andTyson Roberts in an issue of theJapanese Journal of Ichthyology.[4] They gave it the nameHimantura chaophraya, which came into widespread usage. In 2008, Peter Last and B. Mabel Manjaji-Matsumoto confirmed thatT. polylepis andH. chaophraya refer to the same species, and since Bleeker's name was published earlier, thescientific name of the giant freshwater stingray becameHimantura polylepis.[1][5] This species may also be called the giant freshwater whipray, giant stingray, or freshwater whipray.[6]
There is acomplex of similar freshwater andestuarine stingrays in South Asia, Southeast Asia, andAustralasia that are or were tentatively identified withU. polylepis. The Australian freshwaterUrogymnus were described as a separate species,Urogymnus dalyensis, in 2008. The freshwaterUrogymnus inNew Guinea are probablyU. dalyensis rather thanU. polylepis, though confirmation awaits further study.[5]Trygon fluviatilis from India, as described byNelson Annandale in 1909, closely resembles and may beconspecific withU. polylepis.[4] On the other hand, comparison of freshwater whiprayDNA andamino acid sequences between India and Thailand has revealed significant differences.[7] Finally, additional research is needed to assess the degree of divergence amongst populations ofU. polylepis inhabiting variousdrainage basins across its distribution, so as to determine whether further taxonomic differentiation is warranted.[1]
In terms of the broader evolutionary relationships between the giant freshwater whipray and the rest of the familyDasyatidae, a 2012phylogenetic analysis based onmitochondrial DNA reported that it was most closely related to theporcupine ray (Urogymnus asperrimus), and that they in turn formed aclade with themangrove whipray (U. granulatus) and thetubemouth whipray (U. lobistoma). This finding adds to a growing consensus that the genusHimanturasensu lato isparaphyletic.[8]
The giant freshwater stingray has a thin, ovalpectoral 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 largespiracles. 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 sevenpapillae (two to four large at the center and one to four small to the sides) on the floor. The small and rounded teeth are arranged into pavement-like bands. There are five pairs ofgill slits on the ventral side of the disc. Thepelvic fins are small and thin; mature males have relatively largeclaspers.[3][4]
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.[3] At up to 38 cm (15 in) long, the spine is the largest of any stingray species.[9] 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 brownmucus. 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.[3][4][10] The giant freshwater stingray reaches at least 1.9 m (6.2 ft) in width and 5.0 m (16.4 ft) in length,[10] and can likely grow larger (It is not impossible that length is even 10 m (33 ft), and width is5 m (16 ft)).[citation needed] With reports from theMekong andChao Phraya Rivers of individuals weighing 500–600 kg (1,100–1,300 lb), but it is not impossible, that it is 1,500 kg (3,300 lb), or even 2,000 kg (4,400 lb) - it ranks among the largest freshwater fishes in the world.[4][9]
In June 2022, it was reported that a specimen caught in the Mekong river had broken the record for the largest strict freshwater fish ever documented (the largeststurgeon species can far exceed this size, but they areanadromous). The individual weighed 661 lb (300 kg), and was measured at 3.98 m (13.1 ft) long and 2.2 m (7.2 ft) wide.[11]
The giant freshwater stingray is known to inhabit several large rivers and associated estuaries inIndochina andBorneo. In Indochina, it occurs in the Mekong River to potentially as far upstream asChiang Khong in Thailand, as well as in the Chao Phraya,Nan,Mae Klong,Bang Pakong, andTapi Rivers, also found inBueng Boraphet but now completely extinct. In Borneo, this species is found in theMahakam River inKalimantan and theKinabatangan andBuket Rivers inSabah; it is reportedly common in the Kinabatangan River but infrequently caught. Though it has been reported fromSarawak as well, surveys within the past 25 years have not found it there. Elsewhere in the region, recent river surveys inJava have not recorded its presence, despite the island being the locality of the speciesholotype.
Historical records from theGanges River in India, and theBay of Bengal asTrygon fluviatilis are possiblyHimantura fluviatilis, although it has been confirmed as being present in theKaladan andMayu rivers in Myanmar in 2022.[12]
Disjunct populations of the giant freshwater stingray in separate river drainages are probably isolated from one another; though the species occurs inbrackish environments, there is no evidence that it crosses marine waters. This is abottom-dwelling species that favors a sandy or muddyhabitat.[1] Unexpectedly, it can sometimes be found near heavily populated urban areas.[9]
The diet of the giant freshwater stingray consists of small, benthic fishes andinvertebrates such ascrustaceans andmolluscs, which it can detect using itselectroreceptiveampullae of Lorenzini.[9][10] Individuals can often be seen at the edge of the river, possibly feeding onearthworms.[1][13]Parasites documented from this species include thetapewormsAcanthobothrium asnihae,A. etini,A. masnihae,A. saliki,A. zainali,[14]Rhinebothrium abaiensis,R. kinabatanganensis, andR. megacanthophallus.[15] The giant freshwater stingray isviviparous, with the developingembryos nourished initially byyolk and later byhistotroph ("uterine milk") provided by the mother.[6] This species does not appear to bediadromous (migrating between fresh and salt water to complete its life cycle). Observed litter sizes range from one to four pups; newborns measure around 30 cm (12 in) across. Pregnant females are frequently found in estuaries, which may serve as nursery areas. Malesmature sexually at approximately 1.1 m (3.6 ft) across; female maturation size and other life history details are unknown.[1][4]
The giant freshwater stingray is not aggressive, but its sting is sheathed in toxic mucus and is capable of piercing bone.[9] Across its range, this species iscaught incidentally byartisanal fishers usinglonglines, and to a lesser extentgillnets andfish traps.[10][16] It is reputedly difficult and time-consuming to catch; a hooked ray may bury itself under large quantities of mud, becoming almost impossible to lift, or drag boats over substantial distances or underwater.[9] The meat and thecartilage are used; large specimens are cut into kilogram pieces for sale.[6] Adults that are not used for food are often killed or maimed by fishers nonetheless.[16] In the Mae Klong and Bang Pakong Rivers, the giant freshwater stingray is also increasingly targeted bysport fishers and for display inpublic aquariums. These trends pose conservation concerns; the former becausecatch and release is not universally practised and the post-release survival rate is unknown, the latter because this species does not survive well in captivity.[1]
The major threats to the giant freshwater stingray areoverfishing andhabitat degradation resulting fromdeforestation, land development, anddamming. The construction of dams also fragments the population, reducinggenetic diversity and increasing the susceptibility of the resulting subpopulations toextinction.[16] Due to its low reproductive rate, the giant freshwater stingray is not resilient toanthropogenic pressures. In central Thailand and Cambodia, the population is estimated to have been reduced by 30–50% over the past 20–30 years, with declines as severe as 95% in some locations. The size of rays caught has decreased significantly as well; for example, in Cambodia the average weight of a landed ray has dropped from 23.2 kg (51 lb) in 1980 to 6.9 kg (15 lb) in 2006. The status of populations in other areas, such as Borneo, is largely unknown. As a result of documented declines, theInternational Union for Conservation of Nature (IUCN) has assessed this species asEndangered overall, and asCritically Endangered in Thailand.[1][17] In the 1990s, the Thai government initiated acaptive breeding program atChai Nat to bolster the population of this and other freshwater stingray species until the issue of habitat degradation can be remedied. However, by 1996 the program had been put on hold.[16]
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