''Potamotrygon rex'', the great river stingray, is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

of freshwater stingray belonging to the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Potamotrygonidae River stingrays or freshwater stingrays are Neotropical freshwater fishes of the family Potamotrygonidae in the order Myliobatiformes, one of the four orders of batoids, cartilaginous fishes related to sharks. They are found in rivers in tropic ...

, native to South America in the

Neotropical region The Neotropical realm is one of the eight biogeographic realms constituting Earth's land surface. Physically, it includes the tropical terrestrial ecoregions of the Americas and the entire South American temperate zone. Definition In biogeo ...

. ''P. rex'', along with some other ''Potamotrygon'' species form a sub-species group known as "black stingrays" known by their similar features and their locality to rivers draining the Central Brazilian Shield. Specifically, it is native to the Rio Tocantins in the central Brazilian state of Tocantins. It is quite a large organism, weighing up to 20 kg (44 pounds). Its appearance is dark brown to blackish with distinct clusters of yellow-orange spots around similarly colored

ocelli A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...

on its dorsal side. Distinguishing features include broad pelvic-fin apices, lack of labial grooves, irregular double row of dorsal tail thorns, and having two angular cartilages associated with each hyomandibula.

Distribution

Native to the Neotropical regions of the mid and upper Rio Tocantins in the central Brazilian state of Tocantins. So far, it has not been found outside of these regions.De Carvalho, Marcelo R., 2016: Potamotrygon rex, a new species of Neotropical freshwater stingray (Chondrichthyes: Potamotrygonidae) from the middle and upper rio Tocantins, Brazil, closely allied to Potamotrygon henlei (Castelnau, 1855). Zootaxa 4150 (5): 537-565

Habitat and ecology

''Potamotrygon rex'' is a neotropical freshwater stingray in the family Potamotrygonidae from the middle and upper Rio Tocantins, Brazil. It is benthopelagic, meaning that it lives at the bottom of the river. ''Potamotrygon rex'', ''P. henlei'', ''P. leopoldi'' and another new ''Potamotrygon'' species from the Rio Tapajós form a species-group (the "black stingrays") occurring in rivers draining the central Brazilian shield, characterized by their black to dark brown but highly ornate dorsal color, wide pelvic fins with broadly convex apices, among other features. Potamotrygon rex is the 25th species of ''Potamotrygon'', and highlights that very large new species of fishes that still await discovery and description in the Neotropical region.

Morphology

The genus ''Potamotrygon'' consists of freshwater stingrays that can be found in the rivers of South America. ''Potamotrygon rex'' is a large stingray species with a circular, almost oval-shaped disc. The top of the disc is very dark brown with yellow-orange spots in round clusters across the entirety of the disc. However, the density of these spots can vary from individual to individual and some exhibit one or two larger spots at the center of the clusters. The bottom of the disc can be dark grey to dark brown with yellowish-white spots. The density of these spots can also vary heavily based on the individual. The largest adult female was found to be 111 cm (3.6 ft) from anterior edge of the disc to the end of the tail, 75 cm (2.5 ft) from the anterior edge to the posterior edge of the disc, and 69 cm (2.3 ft) wide. Adult males are much smaller at around 82.5 cm (2.7 ft) from anterior edge of the disc to the end of the tail, 52.6 cm (1.7 ft) from the anterior edge to the posterior edge of the disc, and 47.5 cm (1.6 ft) wide. Individuals larger than 100 cm can weigh up to 20 kg (44lbs). Like the rest of the species in the genus, ''Potamotrygon rex'' has venomous barbs at the end of its tail. Beginning at the base of the tail there are two rows of irregularly placed, semi-erect, sharp thorns that go down the length of the tail ending at the venomous barb. The stingray has a venom that causes painful inflammation at the site of the sting. The venom has been found to be more concentrated in females and more effective as the stingray ages.

Conservation

Due to lack of knowledge and research on the species, there is limited information regarding the widespread distribution and population of ''Potamotrygon rex''. Because of this, there are no current conservation efforts regarding the specific species of ''Potamotrygon rex''. However, all species of freshwater stingray belonging to the genus ''Potamotrygon'' are listed in CITES Appendix III, which combats the illegal international trade of such animals. The widespread protection of all ''Potamotrygon'' species is a basic step in the conservation of freshwater stingrays. Currently, other species of the ''Potamotrygon'' genus, ''Potamotrygon motoro'' and ''Potamotrygon falkneri'', have been genetically analyzed to suggest the hybridization of such species in conservation efforts.Cruz, Cruz. “First Identification of Interspecies Hybridization in the Freshwater Stingrays Potamotrygon Motoro and P. Falkneri (Myliobatiformes, Potamotrygonidae).” Conservation Genetics, vol. 16, no. 1, Springer Netherlands, Feb. 2015, pp. 241–45, doi:10.1007/s10592-014-0642-8. Considering the close morphological, geographic, and genetic similarities regarding ''Potamotrygon'' genus, similar studies may be applied to ''Potamotrygon rex''.

References

{{Taxonbar, from=Q27242242 rex