Mekong freshwater stingray
Encyclopedia
The Mekong freshwater stingray, Dasyatis laosensis, is a species
of stingray
in the family
Dasyatidae, restricted to the Mekong and Chao Phraya River
s in Laos
and Thailand
. Measuring up to 62 cm (24.4 in) across, this ray has an oval pectoral fin disc, a tail with both upper and lower fin folds, and a midline row of spine-like dermal denticles. A characteristic feature of this species is its bright orange underside. The Mekong freshwater stingray preys on invertebrate
s and is aplacental viviparous. It has been assessed as Endangered by the International Union for Conservation of Nature (IUCN), as it is threatened by overfishing
and habitat degradation.
Environmental Biology of Fishes. The type specimen is an immature male 23 cm (9.1 in) across, caught from the Mekong in Chiang Rai Province
, Thailand
. A 1999 phylogenetic analysis, based on cytochrome b
sequences, found that the Mekong freshwater stingray is closely related to an undescribed Dasyatis species from the Gulf of Thailand
.
rivers, the Mekong freshwater stingray has only been recorded from sandy habitat
s in the Mekong River along the Laos-Thailand border, and in the Chao Phraya River
near Chai Nat in central Thailand. The populations of these two rivers are likely isolated from each other.
s. There are 28–38 upper tooth rows and 33–41 lower tooth rows; the teeth of juveniles and females are blunt, while those of adult males are pointed with a central keel. The jaws of adult males also become strongly curved. A row of 5 papilla
e lie across the floor of the mouth, with the outermost pair smaller than the others. The five pairs of gill slit
s are short. The pelvic fins are longer than wide and triangular with rounded corners. The whip-like tail is no longer than twice the disc width and bears 1 (rarely 2) stinging spines on the upper surface. Both dorsal and ventral fin folds are found behind the spine, with the ventral fold 2.5–3 times longer than the upper fold.
A single row of thorn-like dermal denticles runs along the midline of the back and tail; with the largest found at the base of the tail. The number of enlarged midline denticles increases with age. There is also a narrow band of minute granular or pointed denticles on the back, which are confined by a pair of parallel lines drawn backward from the spiracles. The upper surface of the disc and tail are plain brown, with the tail folds much darker. The underside is pale with large, irregular orange-yellow patches and a wide orange-red band around the disc margin. Bright orange ventral coloration also characterizes the red stingray
(D. akajei) of the northwestern Pacific; the two species have some similar meristic counts, but differ in disc shape, denticles, and dorsal coloration. The maximum known disc width of the Mekong freshwater stingray is 62 cm (24.4 in); it can reportedly reach a weight of 30 kg (66.1 lb).
s from the river bed. Like other stingrays, it is aplacental viviparous with litter sizes possibly as small as one pup.
, typically in seine nets or on hook-and-line, and sold fresh for human consumption; fishermen typically break off the tail spine before bringing it to market. However, a greater threat to the survival of this species is widespread habitat degradation within its limited range, from dam
construction, agricultural runoff, and industrial pollution
. Finally, the small juveniles may be collected for the aquarium trade
. Because of these pressures, the International Union for the Conservation of Nature (IUCN) has assessed this species as Endangered. The Thai government initiated a captive breeding program for this and other freshwater stingray species in the 1990s at Chai Nat, but the program has since been placed on hold.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of stingray
Stingray
The stingrays are a group of rays, which are cartilaginous fishes related to sharks. They are classified in the suborder Myliobatoidei of the order Myliobatiformes, and consist of eight families: Hexatrygonidae , Plesiobatidae , Urolophidae , Urotrygonidae , Dasyatidae , Potamotrygonidae The...
in the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Dasyatidae, restricted to the Mekong and Chao Phraya River
Chao Phraya River
The Chao Phraya is a major river in Thailand, with its low alluvial plain forming the centre of the country. It runs through Bangkok, the capital city, and then empties into the Gulf of Thailand.-Etymology:...
s in Laos
Laos
Laos Lao: ສາທາລະນະລັດ ປະຊາທິປະໄຕ ປະຊາຊົນລາວ Sathalanalat Paxathipatai Paxaxon Lao, officially the Lao People's Democratic Republic, is a landlocked country in Southeast Asia, bordered by Burma and China to the northwest, Vietnam to the east, Cambodia to the south and Thailand to the west...
and Thailand
Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...
. Measuring up to 62 cm (24.4 in) across, this ray has an oval pectoral fin disc, a tail with both upper and lower fin folds, and a midline row of spine-like dermal denticles. A characteristic feature of this species is its bright orange underside. The Mekong freshwater stingray preys on invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s and is aplacental viviparous. It has been assessed as Endangered by the International Union for Conservation of Nature (IUCN), as it is threatened by overfishing
Overfishing
Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....
and habitat degradation.
Taxonomy and phylogeny
The Mekong freshwater stingray was first recognized as a new species by Yasuhiko Taki, who included it as "Dasyatis sp." in his 1968 list of Mekong River fishes from Laos. Taki's specimens were subsequently lost, and this ray was not formally described until 1987, by Tyson Roberts and Jaranthada Karnasuta, in the scientific journalScientific journal
In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. There are thousands of scientific journals in publication, and many more have been published at various points in the past...
Environmental Biology of Fishes. The type specimen is an immature male 23 cm (9.1 in) across, caught from the Mekong in Chiang Rai Province
Chiang Rai Province
Chiang Rai is the northernmost province of Thailand. Neighbouring provinces are Phayao, Lampang and Chiang Mai. In the north it borders Shan State of Myanmar and Bokeo of Laos.-Geography:...
, Thailand
Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...
. A 1999 phylogenetic analysis, based on cytochrome b
Cytochrome b
Cytochrome b/b6 is the main subunit of transmembrane cytochrome bc1 and b6f complexes. In addition, it commonly refers to a region of mtDNA used for population genetics and phylogenetics.- Function :...
sequences, found that the Mekong freshwater stingray is closely related to an undescribed Dasyatis species from the Gulf of Thailand
Gulf of Thailand
The Gulf of Thailand , also known in to Malays as Teluk Siam literally meant Gulf of Siam, is a shallow arm of the South China Sea.-Geography:...
.
Distribution and habitat
An exclusive inhabitant of large, freshwaterFreshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
rivers, the Mekong freshwater stingray has only been recorded from sandy habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
s in the Mekong River along the Laos-Thailand border, and in the Chao Phraya River
Chao Phraya River
The Chao Phraya is a major river in Thailand, with its low alluvial plain forming the centre of the country. It runs through Bangkok, the capital city, and then empties into the Gulf of Thailand.-Etymology:...
near Chai Nat in central Thailand. The populations of these two rivers are likely isolated from each other.
Description
The pectoral fin disc of the Mekong freshwater stingray is oval in shape and slightly longer than wide. The tip of the snout protrudes slightly past the disc. The eyes are small and followed by somewhat larger spiracleSpiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s. There are 28–38 upper tooth rows and 33–41 lower tooth rows; the teeth of juveniles and females are blunt, while those of adult males are pointed with a central keel. The jaws of adult males also become strongly curved. A row of 5 papilla
Papilla (fish mouth structure)
The papilla, in certain kinds of fish, particularly rays, sharks, and catfish, are small lumps of dermal tissue found in the mouth, where they are "distributed uniformly on the tongue, palate, and pharynx"...
e lie across the floor of the mouth, with the outermost pair smaller than the others. The five pairs of gill slit
Gill slit
Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of Cartilaginous fish such as sharks, rays, sawfish, and guitarfish. Most of these have five pairs, but a few species have 6 or 7 pairs...
s are short. The pelvic fins are longer than wide and triangular with rounded corners. The whip-like tail is no longer than twice the disc width and bears 1 (rarely 2) stinging spines on the upper surface. Both dorsal and ventral fin folds are found behind the spine, with the ventral fold 2.5–3 times longer than the upper fold.
A single row of thorn-like dermal denticles runs along the midline of the back and tail; with the largest found at the base of the tail. The number of enlarged midline denticles increases with age. There is also a narrow band of minute granular or pointed denticles on the back, which are confined by a pair of parallel lines drawn backward from the spiracles. The upper surface of the disc and tail are plain brown, with the tail folds much darker. The underside is pale with large, irregular orange-yellow patches and a wide orange-red band around the disc margin. Bright orange ventral coloration also characterizes the red stingray
Red stingray
The red stingray is a species of stingray in the family Dasyatidae, found in the northwestern Pacific Ocean off Japan, Korea, and China, and possibly elsewhere. It primarily inhabits shallow, sandy habitats close to shore, and has been known to enter brackish water...
(D. akajei) of the northwestern Pacific; the two species have some similar meristic counts, but differ in disc shape, denticles, and dorsal coloration. The maximum known disc width of the Mekong freshwater stingray is 62 cm (24.4 in); it can reportedly reach a weight of 30 kg (66.1 lb).
Biology and ecology
The diet of the Mekong freshwater stingray consists of invertebrateInvertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s from the river bed. Like other stingrays, it is aplacental viviparous with litter sizes possibly as small as one pup.
Human interactions
The Mekong freshwater stingray is subject to intensive fishing aimed at large bony fishes. It is caught unintentionallyBycatch
The term “bycatch” is usually used for fish caught unintentionally in a fishery while intending to catch other fish. It may however also indicate untargeted catch in other forms of animal harvesting or collecting...
, typically in seine nets or on hook-and-line, and sold fresh for human consumption; fishermen typically break off the tail spine before bringing it to market. However, a greater threat to the survival of this species is widespread habitat degradation within its limited range, from dam
Dam
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...
construction, agricultural runoff, and industrial pollution
Water pollution
Water pollution is the contamination of water bodies . Water pollution occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful compounds....
. Finally, the small juveniles may be collected for the aquarium trade
Fishkeeping
Fishkeeping is a popular hobby concerned with keeping fish in a home aquarium or garden pond. There is also a fishkeeping industry, as a branch of agriculture.-Types of fishkeeping systems:...
. Because of these pressures, the International Union for the Conservation of Nature (IUCN) has assessed this species as Endangered. The Thai government initiated a captive breeding program for this and other freshwater stingray species in the 1990s at Chai Nat, but the program has since been placed on hold.