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Giant Freshwater Stingray

Urogymnus polylepis (Bleeker 1852)

Behavior

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Intraspecific communication has not yet been studied in this species, but similar species of stingrays have been documented to communicate by generating and sensing species- and context-specific electrical impulses. Stingrays have an extensive electrosensory system that includes many receptive organs known as Ampullae of Lorenzini. There are pores all over the stingray’s body leading to canals under the skin. Each pore is full of many sensory receptor cells. The arrangement of the pores allows the stingray to detect movement of prey and predators via the electrical fields that these movements generate. Stingrays can also perceive their environment visually, although this species does not rely on this sense as much because they are found in areas that are dark and full of sediment. Like other stingray species, giant freshwater stingrays also have well-developed senses of smell and hearing, as well as a lateral line system for detecting vibrations in the water

Communication Channels: visual ; electric

Perception Channels: visual ; tactile ; acoustic ; vibrations ; chemical ; electric

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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
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Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Conservation Status

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Due to rapidly declining numbers of giant freshwater stingrays, the IUCN Red List has declared this species to be endangered. In Thailand, captive breeding efforts are underway to rebuild the population, though rates of survival in captivity are very low. Scientists are working to tag the remaining stingrays in order to understand their movement patterns and improve conservation efforts, but sufficient results are still lacking.

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
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Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Life Cycle

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Little is known about the development of giant freshwater stingrays. However, it is likely similar to that of a related species, Dasyatis sabina (Atlantic stingray). In Atlantic stingrays, development takes approximately 12 weeks. For the first 4 to 6 weeks, the embryo elongates but there is no head or body development. After 6 weeks, gills begin to grow and the fins and eyes begin to develop. The tail and spine appear shortly before hatching and at birth, stingrays look like miniature adults. The average disc width of newly hatched young is 30 cm.

Development - Life Cycle: indeterminate growth

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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
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Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Benefits

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When fishermen try to catch giant freshwater stingrays, they will whip their tail (bearing its large, serrated, venomous spine) to try and get away. These spines have been reported to be strong enough to go through wooden boats. There have been no reports of unprovoked attacks.

Negative Impacts: injures humans (bites or stings, venomous )

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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
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Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Benefits

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Giant freshwater stingrays are a food source in some Asian cities, even though it is now illegal to fish for this endangered species. They are also collected for aquariums and are a popular sport fish.

Positive Impacts: food ; ecotourism

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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
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Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Associations

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Giant freshwater stingrays act as predators of smaller animals living on or in the river floor. A large number of cestode species (tapeworms) have been described from the intestine of this species, which is thought to be their only host. The mode of transmission for these tapeworms is currently unknown.

Commensal/Parasitic Species:

  • Rhinebothrium kinabatanganensis (Class Cestoda, Phylum Platyhelminthes)
  • Rhinebothrium megacanthophallus (Class Cestoda, Phylum Platyhelminthes)
  • Rhinebothrium abaiensis (Class Cestoda, Phylum Platyhelminthes)
  • Acanthobothrium asnihae (Class Cestoda, Phylum Platyhelminthes)
  • Acanthobothrium etini (Class Cestoda, Phylum Platyhelminthes)
  • Acanthobothrium masnihae (Class Cestoda, Phylum Platyhelminthes)
  • Acanthobothrium saliki (Class Cestoda, Phylum Platyhelminthes)
  • Acanthobothrium zainali (Class Cestoda, Phylum Platyhelminthes)
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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
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Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Trophic Strategy

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Giant freshwater stingrays generally feed on river bottoms. Their mouth contains two jaws that act like crushing plates, and small teeth to continue chewing up food. Their diet consists mainly of benthic fishes and invertebrates.

Animal Foods: fish; mollusks; aquatic or marine worms; aquatic crustaceans

Primary Diet: carnivore (Piscivore , Eats non-insect arthropods, Molluscivore , Vermivore)

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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
author
Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Distribution

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Giant freshwater stingrays are found in the large river systems of Thailand, including the Mekong, Chao Phraya, Nan, Bang Kapong, Prachin Buri, and Tapi River basins. They are also found in the Kinabatangan River in Malaysia, as well as the island of Borneo (in the Mahakam River).

Biogeographic Regions: oriental (Native )

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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
author
Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Habitat

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This species is typically found over sandy bottoms in large rivers, at depths of 5 to 20 meters. Many females are found in estuaries and it is thought that they give birth in brackish waters, though the reason for this is not currently known. There are no records of this species from fully marine habitats.

Range depth: 5 to 20 m.

Habitat Regions: tropical ; freshwater

Aquatic Biomes: benthic ; rivers and streams; coastal ; brackish water

Other Habitat Features: estuarine

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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
author
Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Life Expectancy

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There is little information regarding the lifespan of this species. However, other members of the genus Himantura have been reported to live 5 to 10 years in the wild. This species fares poorly in captivity, due to the difficulties associated with providing proper food and space.

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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
author
Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Morphology

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Like other stingray species, giant freshwater stingrays are characterized by their large oval shape and long tails. Individuals have been recorded to reach weights of 600kg and lengths of 300 cm in total length (one third of which was contributed by the tail). The tail is very smooth on the dorsal side, but the ventral side has a spine with saw-like serrations and an associated venom gland. Two pelvic fins are found on either side of the tail, with the primary trait distinguishing males and females being the presence of a clasper on each male pelvic fin. These penis-like structures release sperm during copulation. This species’ oval shape is formed by the pectoral fins, which extend anteriorly to join with the snout. The pectoral fins contain 158-164 pectoral radials, which are small bone-like structures that support the large fins. Overall, the body is relatively flat. The mouth is located on the underside of the disk and is comprised of two jaws filled with small teeth, and lips covered with small papillae, which are similar to taste buds. Two parallel rows of gill slits are found posterior to the mouth. The coloration displays the countershading pattern is typical of many aquatic animals. The dorsum is darkly colored, preventing predators swimming above them from seeing them against the sand, while the lighter colored belly obscures an individuals body outline from predators below, due to incoming sunlight.

Range mass: 600 (high) kg.

Range length: 300 (high) cm.

Other Physical Features: ectothermic ; bilateral symmetry ; venomous

Sexual Dimorphism: sexes alike

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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
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Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Associations

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Adult giant freshwater stingrays, as the largest organisms in their habitats, have few natural predators. Their countershaded color pattern and sedentary lifestyle offer even small individuals protection from larger predators, as they are able to blend in with the river bottom. When swimming above the bottom, their light-colored underside allows them to be hidden from predators, due to the sunlight from above. This species also has a strong, serrated, venomous spine on its tail, which can be used in self defense. The only major predator of giant freshwater stingrays is humans. In Thailand, fishermen seek out these animals for food, personal aquariums, and sport.

Known Predators:

  • humans (Homo sapiens)

Anti-predator Adaptations: cryptic

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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
author
Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Reproduction

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Female stingrays appear to choose mates by using their electrosensory system to detect sex-specific electrical signals produced by males. Once a female has mated, they leave the male and reside with other females in brackish waters until they give birth. Males have a clasper (a copulatory structure that holds sperm) attached to each pelvic fin. Having multiple claspers allows a male stingray to impregnate a female with one clasper and then seek out another female to impregnate with the remaining clasper. Male stingrays produce and store sperm throughout the year to ensure that they have sufficient sperm for use during the mating season.

Mating System: polygynous

Very little is known regarding the reproductive cycle of giant freshwater stingrays in the wild. Captive breeding efforts have indicated that pregnant females give birth to 1 to 2 offspring per breeding event. However, other information from captive breeding programs is sparse and most of these programs have been discontinued.

Breeding interval: Frequency of breeding in giant freshwater stingrays is currently unknown

Breeding season: The seasons in which fertilization and birth occur are currently unknown

Range number of offspring: 1 to 2.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); ovoviviparous

Male stingrays provide no parental investment to their offspring after mating. Females give birth to live young in estuarine birthing grounds. They then take care of their newly hatched young until they are roughly one-third the size of the female, at which point they are considered mature and will move to completely freshwater habitats.

Parental Investment: female parental care ; pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female)

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Thompson, K. 2012. "Himantura chaophraya" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Himantura_chaophraya.html
author
Kelsey Thompson, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Biology

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Despite its enormous size, rivalling the Mekong giant catfish as largest freshwater fish in the world, the giant freshwater stingray is elusive and understudied. It is thought to mature at around 110 cm across and gives birth to live young of 30 cm across (4). It is venomous but uses its sting in self-defence as it preys upon invertebrates and relatively small fish (2).
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Conservation

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Further research into this incredible fish's biology and status is necessary. The Australian government plan to form a national recovery team who will attempt to compile information on the distribution, abundance and ecology of the giant freshwater stingray (4).
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Description

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Awe-inspiringly large, the giant freshwater stingray lives up to its name, possessing a venomous sting and a large whip-like tail (2). It is brown on the upper surface of its broad, thin, disc-shaped body, and paler beneath with a ring of black around the edge. It has a large snout but small eyes (3).
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Habitat

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Inhabits the sandy bottoms of large rivers and estuaries (1).
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Range

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Thought to occur in most large rivers of tropical Australia, as well as the Fly River basin, New Guinea, the Mahakam River basin, Borneo and several rivers in Thailand (4).
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Status

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The giant freshwater stingray is classified as Vulnerable (VU A1bcde + 2ce) on the IUCN Red List (1).
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Threats

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In 1992, Thai fisherman reported having caught 25 individuals, but the following year this figure fell to just three, hinting at a rapid decline (4). Threats in Thailand are from poor habitat management, including the destruction of the forest canopy which leads to drought upstream and flooding downstream during the monsoon, as well as dam building which prevents migratory fish breeding successfully, and therefore reduces available prey. In Australia the main threat to this species is thought to be silt from uranium mines which contains heavy metals and radio-isotopes. However, it is not known how threatening this is to these fish (4). Throughout its range, the giant freshwater stingray is at risk from both direct and incidental fishing, habitat destruction and range fragmentation leading to inbreeding depression (2).
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Diagnostic Description

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Absence of ventral and dorsal skin folds on the tail, and the broad greyish to blackish marginal band on the ventral surface of the disc. The dorsal surface of the disc is brown (Ref. 27732). Spiral valve with 21 turns; dorsal surface with uniform brown or grey coloration (Ref. 12693). Lacking caudal fin; with long whip-like tail (Ref. 43281).
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Recorder
Armi G. Torres
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Life Cycle

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Exhibit ovoviparity (aplacental viviparity), with embryos feeding initially on yolk, then receiving additional nourishment from the mother by indirect absorption of uterine fluid enriched with mucus, fat or protein through specialised structures (Ref. 50449). Distinct pairing with embrace (Ref. 205). Young are born at about 30 cm disc width (Ref. 6871).
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Migration

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Potamodromous. Migrating within streams, migratory in rivers, e.g. Saliminus, Moxostoma, Labeo. Migrations should be cyclical and predictable and cover more than 100 km.
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Susan M. Luna
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Morphology

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Dorsal spines (total): 0; Dorsal soft rays (total): 0
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Biology

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Inhabits sandy bottoms in estuaries and large rivers (Ref. 12693). Feeds on benthic invertebrates (Ref. 12693) and fishes (Ref. 32457). Ovoviviparous (Ref. 50449). Caught occasionally by demersal gillnet and longline fisheries operating in riverine and estuarine areas. Utilized for its meat and possibly its cartilage (Ref.58048). Marketed fresh, with large individuals being sold in cut pieces by the kilogram (Ref. 12693). Size reaches to about 500 cm TL. Threatened due to over harvesting and pollution (Ref. 58490).
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Importance

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fisheries: minor commercial; aquaculture: experimental; aquarium: potential
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Giant freshwater stingray

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The giant freshwater stingray (Urogymnus polylepis , also widely known by the junior synonym Himantura chaophraya) is a species of stingray in the family Dasyatidae. It is found in large rivers and estuaries in Southeast Asia and Borneo, though historically it may have been more widely distributed in South and Southeast Asia. One of the largest freshwater fish in the world, this species grows upwards of 1.9 m (6.2 ft) across and may reach 600 kg (1,300 lb) in weight. It has a relatively thin, oval pectoral 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 and pelvic 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 and invertebrates. Females give live birth to litters of one to four pups, which are sustained to term by maternally produced histotroph ("uterine milk"). This species faces heavy fishing pressure for meat, recreation, and aquarium display, as well as extensive habitat degradation and fragmentation. These forces have resulted in substantial population declines in at least central Thailand and Cambodia. As a result, the International Union for Conservation of Nature (IUCN) has assessed the giant freshwater stingray as Endangered.

Taxonomy and phylogeny

The first scientific description of the giant freshwater stingray was authored by Dutch ichthyologist Pieter Bleeker in an 1852 volume of the journal Verhandelingen van het Bataviaasch Genootschap van Kunsten en Wetenschappen. His account was based on a juvenile specimen 30 cm (12 in) across, collected from Jakarta, Indonesia. Bleeker named the new species polylepis, from the Greek poly ("many") and lepis ("scales"), and assigned it to the genus Trygon (now a synonym of Dasyatis).[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 and Tyson Roberts in an issue of the Japanese Journal of Ichthyology.[4] They gave it the name Himantura chaophraya, which came into widespread usage. In 2008, Peter Last and B. Mabel Manjaji-Matsumoto confirmed that T. polylepis and H. chaophraya refer to the same species, and since Bleeker's name was published earlier, the scientific name of the giant freshwater stingray became Himantura polylepis.[1][5] This species may also be called the giant freshwater whipray, giant stingray, or freshwater whipray.[6]

 src=
Annandale's Trygon fluviatilis may represent an Indian population of giant freshwater stingray.

There is a complex of similar freshwater and estuarine stingrays in South Asia, Southeast Asia, and Australasia that are or were tentatively identified with U. polylepis. The Australian freshwater Urogymnus were described as a separate species, Urogymnus dalyensis, in 2008. The freshwater Urogymnus in New Guinea are probably U. dalyensis rather than U. polylepis, though confirmation awaits further study.[5] Trygon fluviatilis from India, as described by Nelson Annandale in 1909, closely resembles and may be conspecific with U. polylepis.[4] On the other hand, comparison of freshwater whipray DNA and amino acid sequences between India and Thailand has revealed significant differences.[7] Finally, additional research is needed to assess the degree of divergence amongst populations of U. polylepis inhabiting various drainage 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 family Dasyatidae, a 2012 phylogenetic analysis based on mitochondrial DNA reported that it was most closely related to the porcupine ray (Urogymnus asperrimus), and that they in turn formed a clade with the mangrove whipray (U. granulatus) and the tubemouth whipray (U. lobistoma). This finding adds to a growing consensus that the genus Himantura sensu lato is paraphyletic.[8]

Description

 src=
Preserved giant freshwater stingray, showing the characteristic shape of its disc.

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 (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 of gill slits on the ventral side of the disc. The pelvic fins are small and thin; mature males have relatively large claspers.[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 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.[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, and can likely grow larger.[10] With reports from the Mekong and Chao Phraya Rivers of individuals weighing 500–600 kg (1,100–1,300 lb), it ranks among the largest freshwater fishes in the world.[4][9]

Distribution and 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, and Tapi Rivers, also found in Bueng Boraphet but now completely extinct. 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 (the latter two as Trygon fluviatilis) have similarly not been corroborated by any recent accounts.[1]

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.[1] Unexpectedly, it can sometimes be found near heavily populated urban areas.[9]

Biology and ecology

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.[9][10] Individuals can often be seen at the edge of the river, possibly feeding on earthworms.[1] Parasites documented from this species include the tapeworms Acanthobothrium asnihae, A. etini, A. masnihae, A. saliki, A. zainali,[11] Rhinebothrium abaiensis, R. kinabatanganensis, and R. megacanthophallus.[12] The giant freshwater stingray is viviparous, with the developing embryos nourished initially by yolk and later by histotroph ("uterine milk") provided by the mother.[6] This species does not appear to be diadromous (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. Males mature sexually at approximately 1.1 m (3.6 ft) across; female maturation size and other life history details are unknown.[1][4]

Human interactions

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 is caught incidentally by artisanal fishers using longlines, and to a lesser extent gillnets and fish traps.[10][13] 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 the cartilage 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.[13] In the Mae Klong and Bang Pakong Rivers, the giant freshwater stingray is also increasingly targeted by sport fishers and for display in public aquariums. These trends pose conservation concerns; the former because catch 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 are overfishing and habitat degradation resulting from deforestation, land development, and damming. The construction of dams also fragments the population, reducing genetic diversity and increasing the susceptibility of the resulting subpopulations to extinction.[13] Due to its low reproductive rate, the giant freshwater stingray is not resilient to anthropogenic 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, the International Union for Conservation of Nature (IUCN) has assessed this species as Endangered overall, and as Critically Endangered in Thailand.[1][14] In the 1990s, the Thai government initiated a captive breeding program at Chai 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.[13]

References

  1. ^ a b c d e f g h i j Vidthayanon, C.; Baird, I. & Hogan, Z. (2016). "Urogymnus polylepis". IUCN Red List of Threatened Species. 2016: e.T195320A104292419. doi:10.2305/IUCN.UK.2016-3.RLTS.T195320A104292419.en.
  2. ^ Bleeker, P. (1852). "Bijdrage tot de kennis der Plagiostomen van den Indischen Archipel". Verhandelingen van Het Bataviaasch Genootschap van Kunsten en Wetenschappen. 24: 1–92.
  3. ^ a b c d Last, P.R.; Compagno, L.J.V. (1999). "Myliobatiformes: Dasyatidae". In Carpenter, K.E.; Niem, V.H. (eds.). FAO identification guide for fishery purposes. The living marine resources of the Western Central Pacific. Food and Agriculture Organization of the United Nations. pp. 1479–1505. ISBN 978-92-5-104302-8.
  4. ^ a b c d e f Monkolprasit, S.; Roberts, T.R. (1990). "Himantura chaophraya, a new giant freshwater stingray from Thailand" (PDF). Japanese Journal of Ichthyology. 37 (3): 203–208. Archived from the original (PDF) on 2013-10-20. Retrieved 2010-04-12.
  5. ^ a b Last, P.R.; Manjaji-Matsumoto, B.M. (2008). "Himantura dalyensis sp. nov., a new estuarine whipray (Myliobatoidei: Dasyatidae) from northern Australia". In Last, P.R.; White, W.T.; Pogonoski, J.J. (eds.). Descriptions of new Australian Chondrichthyans. CSIRO Marine and Atmospheric Research. pp. 283–291. ISBN 978-0-1921424-1-2.
  6. ^ a b c Froese, Rainer and Pauly, Daniel, eds. (2018). "Urogymnus polylepis" in FishBase. October 2018 version.
  7. ^ Sezaki, K.; Begum, R.A.; Wongrat, P.; Srivastava, M.P.; SriKantha, S.; Kikuchi, K.; Shihara, H.; Tanaka, S.; Taniuchi, T.; Watabe, S. (1999). "Molecular phylogeny of Asian freshwater and marine stingrays based on DNA nucleotide and deduced amino acid sequences of the cytochrome b gene". Fisheries Biology. 65: 563–570.
  8. ^ Naylor, G.J.P. (1992). "The phylogenetic relationships among requiem and hammerhead sharks: inferring phylogeny when thousands of equally most parsimonious trees result" (PDF). Cladistics. 8 (4): 295–318. doi:10.1111/j.1096-0031.1992.tb00073.x. hdl:2027.42/73088. S2CID 39697113.
  9. ^ a b c d e f Lovgren, S. (2008). "Giant River Stingrays Found Near Thai City". National Geographic News. National Geographic Society. Retrieved December 20, 2013.
  10. ^ a b c d Last, P.R.; White, W.T.; Caire, J.N.; Dharmadi; Fahmi; Jensen, K.; Lim, A.P.F.; Manjaji-Matsumoto, B.M.; Naylor, G.J.P.; Pogonoski, J.J.; Stevens, J.D.; Yearsley, G.K. (2010). Sharks and Rays of Borneo. CSIRO Publishing. pp. 208–209. ISBN 978-1-921605-59-8.
  11. ^ Fyler, C.A.; Caira, J.N. (2006). "Five new species of Acanthobothrium (Tetraphyllidea: Onchobothriidae) from the freshwater stingray Himantura chaophraya (Batoidea: Dasyatidae) in Malaysian Borneo". Journal of Parasitology. 92 (1): 105–125. doi:10.1645/GE-3522.1. PMID 16629324. S2CID 24074236.
  12. ^ Healy, C.J. (2006). "Three new species of Rhinebothrium (Cestoda: Tetraphyllidea) from the freshwater whipray, Himantura chaophraya, in Malaysian Borneo". Journal of Parasitology. 92 (2): 364–374. doi:10.1645/GE-560R.1. PMID 16729696. S2CID 1002099.
  13. ^ a b c d Fowler, S.L.; Cavanagh, R.D. (2005). Sharks, Rays and Chimaeras: The Status of the Chondrichthyan Fishes. IUCN. pp. 348–349. ISBN 978-2-8317-0700-6.
  14. ^ Compagno, L.J.V.; Cook, S.F. (2000). "Himantura polylepis (Thailand subpopulation)". IUCN Red List of Threatened Species. 2000.old-form url

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Giant freshwater stingray: Brief Summary

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The giant freshwater stingray (Urogymnus polylepis , also widely known by the junior synonym Himantura chaophraya) is a species of stingray in the family Dasyatidae. It is found in large rivers and estuaries in Southeast Asia and Borneo, though historically it may have been more widely distributed in South and Southeast Asia. One of the largest freshwater fish in the world, this species grows upwards of 1.9 m (6.2 ft) across and may reach 600 kg (1,300 lb) in weight. It has a relatively thin, oval pectoral 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 and pelvic 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 and invertebrates. Females give live birth to litters of one to four pups, which are sustained to term by maternally produced histotroph ("uterine milk"). This species faces heavy fishing pressure for meat, recreation, and aquarium display, as well as extensive habitat degradation and fragmentation. These forces have resulted in substantial population declines in at least central Thailand and Cambodia. As a result, the International Union for Conservation of Nature (IUCN) has assessed the giant freshwater stingray as Endangered.

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