Order

Myliobatiformes

  Class

Elasmobranchii

No. in FishBase

Genera : 6 | Species : 28 Eschmeyer's Catalog of Fishes

  Environment

Fresh : Yes | Brackish : No | Marine : No

  Division

Peripheral/diadromous

  Aquarium

none

  First Fossil Record

     

  Remark

Neotropical freshwater stingrays of the family Potamotrygonidae presently comprise three genera (Potamotrygon, Paratrygon and Plesiotrygon) and at least 18 species. The family has an extensive taxonomic history dating back to 1834, but was known long before formal taxonomic recognition (Castex, 1963a). Potamotrygonids are much maligned and feared because of their venomous caudal stings, but pose little or no threat if not stepped on or directly interfered with. The Potamotrygonidae is the only living chondrichthyan family restricted to freshwater habitats. Potamotrygonid stingrays are clearly monophyletic, sharing unique morphological and physiological specializations, including a pelvis with a greatly expanded anterior median process (pre-pelvic process), blood with low concentrations of urea, and reduction of the rectal gland (Garman, 1877; Thorson et al., 1983a). They are generally medium to large sized batoids, ranging from about 25 cm in disc width or length to more than 100 cm in adults of some species. Species of Potamotrygon have moderately stout and short tails, usually shorter than disc length, whereas Paratrygon and Plesiotrygon have slender, filiform and whiplike tails (in Paratrygon the tail is very long in juveniles but absolutely reduced in adults, and in Plesiotrygon it is much longer than disc length but also stout at base). The dorsal surface of the disc and tail are usually covered with many denticles, thorns and tubercles. The caudal sting (or serrated spine) is a rigid dermal derivative, located on the dorsal surface of the tail, containing small lateral serrations directed toward its base and an acute distal tip. The sting is reduced and situated closer to the tail base in Paratrygon, but well developed and located farther posteriorly in both Plesiotrygon and Potamotrygon. The stings contain longitudinal grooves to conduct venom produced in special glands at their bases, and are continuously worn, shed and replaced; up to four stings may be present in one individual. Additionally, many species have enlarged, non-venomous spines on disc margins or over tail, sometimes in numerous rows. The disc is usually slightly longer than wide (especially in Paratrygon), and covers most of the pelvic fins posteriorly (but less in Plesiotrygon). Dorsal and caudal fins are absent, but membranous skin folds (finfolds), with rudimentary internal radial elements, occur in Potamotrygon on both upper and lower tail midlines posterior to caudal stings (only ventral finfold is present in Plesiotrygon, and finfolds are absent in Paratrygon). The eyes are moderately large in Potamotrygon, but smaller and less protruded in Paratrygon and Plesiotrygon. Oral teeth are small with short cusps (more prominent in adult males), in usually less than 50 rows in either jaw (large specimens of Plesiotrygon may have more than 60 rows), and set in quincunx. Most potamotrygonid species have colorful dorsal arrangements, including spots of various dimensions, ocelli, reticulate patterns, and vermiform markings, which are generally species-specific, and grey, brown or black background coloration. Potamotrygonids are ovoviviparous (aplacentally viviparous), and the developing embryos are nourished by uterine milk secreted by trophonemata (Thorson et al., 1983b). Both uteri are functional and appear to be synchronous. Gestation may be restricted to certain stations or occur throughout the year, and the number of young produced in each gestation varies among species, but is usually from two to seven. Potamotrygonids only occur in South American rivers that drain into the Atlantic Ocean or Caribbean Sea. They are are conspicuously absent, however, from the São Francisco basin in northeastern Brazil, rivers that drain into the Atlantic from the Atlantic rainforest of northeastern and southeastern Brazil, the upper Paraná basin, and rivers south of the La Plata river in Argentina. Generally, most potamotrygonid species have distributions restricted to a single basin or river system, with only a few species present in more than one basin (e.g. Potamotrygon motoro, Potamotrygon orbignyi). Some species are even restricted to a single river (e.g. Potamotrygon leopoldi). This high endemism has led recent workers to express concern that some species may be endangered (Compagno & Cook, 1995; Rosa & Menezes, 1996), or are at least clearly vulnerable at present (two species are cited on the IUCN Red List as "data deficient"). Potamotrygonids are generally not consumed as food, but are commercialized in increasing quantity by the aquarium trade, and are only seldomly bred in captivity for commercial purposes. Further studies on their population structure and dynamics are therefore very much needed. The literature concerning potamotrygonids is vast. After the revision of Garman (1913), the next taxonomic study to treat the entire family is that of Rosa (1985a). Anatomical descriptions can be found in Garman (1877, 1913), Thorson & Watson (1975), Rosa (1985a), Nishida (1990), Taniuchi & Ishihara (1990), and Lovejoy (1996). Their phylogenetic relationships have been treated by Rosa (1985a), Dingerkus (1995), Lovejoy (1996) and McEachran et al. (1996). Accounts of their natural history are presented by Castex (1964a) and Castello (1975). Physiological aspects of potamotrygonids are summarized in Thorson et al. (1983a), and the reproductive biology of certain species by Thorson et al. (1983b), Teshima & Takeshita (1992), and Lasso et al. (1997a). Their historical biogeography has been studied by Brooks et al. (1981), Lovejoy (1996, 1997), Lovejoy et al. (1998), and Lundberg (1998). Recent descriptions of fossil remains (mostly isolated denticles) can be found in Deynat & Brito (1996) and Lundberg (1997). Recent taxonomic and historic compilations are those by Compagno & Cook (1995) and Zorzi (1995), and the extensive literature on potamotrygonid parasites is summarized in Brooks & Amato (1992). The following account is mostly based on original work by the first author, supplemented by information from Rosa (1985a) concerning type specimens of Potamotrygon deposited in museums in Argentina. This account differs from the revision of Rosa (1985a) in relation to the fate of various nominal species, and is also more conservative, recognizing fewer species. At least five new species have been found and are not included here (these are presently being described elsewhere). Taxonomic work on the family is constrained by the fact that many species of Potamotrygon are poorly described, lack adequate material, or present much intraspecific variation in coloration. This is compounded by generally overlapping meristic and morphometric features among species of Potamotrygon (both other genera are monotypic). Therefore, the approach adopted here has been to not recognize certain species that are inadequately defined at present, or that lack adequate material for proper characterization (e.g. Potamotrygon dumerilii,Potamotrygon humerosa). The current account is a working summary of the family, one that is subject to modifications as more collecting and research is conducted. This family is treated as it has traditionally been used in the literature, i.e. restricting it to Potamotrygon, Paratrygon and Plesiotrygon only, and refrain from including other genera that have recently been hypothesized as being their sister-taxa (amphi-American Himantura and Taeniura; Lovejoy, 1996; McEachran et al. 1996). Note that DW is disc width, and DL is disc length, which are standard measurements in stingrays (total length is usually not used as the distal tip of the tail is frequently missing in specimens).

  Etymology

Greek, potamos = river + Greek, trigon = with three angles ( Ref. 45335).

  Reproductive guild

bearers

  Typical activity level

  Main Ref

de Carvalho, M.R., N. Lovejoy and R.S. Rosa 2003

  Coordinator

de Carvalho, M.R., N. Lovejoy and R.S. Rosa 2003

  Deep Fin Classification