Reassignment of the African Freshwater Stingray, Potamotrygon garouaensis, to the Genus Dasyatis, on Physiologic and Morphologic Grounds

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Abstract
Three specimens of the freshwater stingray, Potamotrygon garouaensis Stauch and Blanc, from the Benue River at Makurdi, Nigeria, did not have the prepelvic process diagnostic of the family Potamotrygonidae, nor did they have the extremely low urea concentration in their body fluids characteristic of that family in South America. All relevant morphologic parameters examined agreed with the diagnoses given for Dasyatis by Garman and by Bigelow and Schroeder. We recommend reassignment of the Garoua stingray from the genus Potamotrygon to Dasyatis. Both African and South American fossil rays ascribed to the genus Potamotrygon are also likely to be members of the genus Dasyatis or another genus of the family Dasyatidae. The extensive speciation that has occurred among South American potamotrygonid freshwater rays, as well as their radical departure from the osmoregulatory features of the rest of the elasmobranchs, suggests a relatively long residence in fresh water. On the other hand, the Garoua stingray and other dasyatid rays that appear to be permanent residents of fresh water may have invaded freshwater systems relatively recently. The former deal with their osmotic problems like freshwater osteichthians; the latter like other euryhaline elasmobranchs.

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