An unusual, dwarf new species of Neotropical freshwater stingray, Plesiotrygon nana sp. nov., from the upper and mid Amazon basin: the second species of Plesiotrygon (Chondrichthyes: Potamotrygonidae)

Abstract

A new species of the relatively poorly known Neotropical freshwater stingray genus Plesiotrygon Rosa, Castello & Thorson, 1987 is described from the main channel and smaller tributaries (Ríos Itaya and Pachitea) of the upper Amazon basin in Peru. The first specimen to be collected, however, was from much farther east in Rio Solimões in 1996, just down-river from Rio Purus (specimen unavailable for this study). Plesiotrygon nana sp. nov., is a very distinctive and unusually small species of freshwater stingray (Potamotrygonidae), described here mostly from three specimens representing different size classes and stages of sexual maturity. Plesiotrygon nana sp. nov., is distinguished from its only congener, P. iwamae Rosa, Castello & Thorson, 1987, by numerous unique features, including: dorsal coloration composed of very fine rosettes or a combination of spots and irregular ocelli; very circular disc and snout; very small and less rhomboidal spiracles; short snout and anterior disc region; narrow mouth and nostrils; denticles on dorsal tail small, scattered, not forming row of enlarged spines; adult and preadult specimens with significantly fewer tooth rows; fewer caudal vertebrae; higher total pectoral radials; very small size, probably not surpassing 250 mm disc length or width, males maturing sexually at around 180 mm disc length and 175 mm disc width; distal coloration of tail posterior to caudal stings usually dark purplish-brown; and features of the ventral lateral-line canals (hyomandibular canal very narrow, infraorbital and supraorbital canals not undulated, supraorbital and infraorbital loops small and narrow, supraorbital loop very short, not extending posteriorly to level of mouth, jugular and posterior infraorbital canals short, not extending caudally to first gill slits, subpleural loop very narrow posteriorly; absence of anterior and posterior subpleural tubules). To provide a foundation for the description of P. nana sp. nov., morphological variation in P. iwamae was examined based on all type specimens as well as newly collected and previously unreported material. Two specimens topotypic with the male paratype of P. nana sp. nov., referred to here as Plesiotrygon cf. iwamae, are also reported. Relationships of the new species to P. iwamae are discussed; further characters indicative of Plesiotrygon monophyly are proposed, but the genus may still not be valid. Plesiotrygon nana sp. nov., is commercialized with some regularity in the international aquarium trade from Iquitos (Peru), an alarming circumstance because nothing is known of its biology or conservation requirements.

Systematic implications of brain morphology in potamotrygonidae (Chondrichthyes: Myliobatiformes)

“…The peduncles of the olfactory tracts (near the lateral pallia) are wider in Potamotrygon , a characteristic that seems to differentiate this genus from the other three. Our data, however, do not permit specific inferences about the relationships between species of Potamotrygon , or if whether Plesiotrygon is nested within Potamotrygon as indicated by molecular studies (e.g., Marques, ; Toffoli et al, ; see discussion in Carvalho and Ragno, ).…”

“…Potamotrygonids share derived characters with other stingrays (Myliobatiformes), including the absence of thoracic ribs and presence of a second synarcual cartilage and caudal stinging spines (Nishida, ; McEachran et al, ). The family is currently composed of four genera, Paratrygon Duméril, 1865, Potamotrygon Garman, , Plesiotrygon Rosa, Castello and Thorson, , and Heliotrygon Carvalho and Lovejoy, , and some 30 valid species (Carvalho et al, ; Carvalho and Lovejoy, ; Carvalho and Ragno, ; Loboda and Carvalho, ; Fontenelle et al, ).…”

Potamotrygon marquesi, a new species of neotropical freshwater stingray (Potamotrygonidae) from the Brazilian Amazon Basin

“…Four genera are presently recognised as valid within the subfamily (Carvalho et al, ; Carvalho & Lovejoy, ): Potamotrygon Garman 1877, Paratrygon Duméril 1865, Plesiotrygon Rosa et al, and Heliotrygon Carvalho & Lovejoy, . Paratrygon is currently monotypic (although a recent taxonomic revision recognised seven new species within the genus) (Loboda, ), Heliotrygon and Plesiotrygon have two valid species each (Carvalho & Lovejoy, ; Carvalho & Ragno, ; Rosa et al, ) and Potamotrygon is the most diverse genus, including 35 valid species (Carvalho et al, , ; Silva & Carvalho, , ; Loboda & Carvalho, ; Fontenelle et al, ; Silva & Carvalho, , Carvalho, 2016a,b; Fontenelle & Carvalho, ).…”

Revisão taxonômica do complexo Potamotrygon scobina Garman, 1913 (Chondrichthyes: Myliobatiformes: Potamotrygonidae), com inferências biogeográficas

“…D também apresenta menor número de dentes que P. scobina, assim como um palato quadrado menos robusto e dentículos dérmicos mais abundantes, ....................................................................................... (Carvalho & Lovejoy, 2011;Carvalho et al 2004). Plesiotrygon, por sua vez, apresenta uma única cartilagem angular (Carvalho et al 2004, Carvalho & Ragno, 2011. O gênero Potamotrygon demonstra uma variação de estados quanto a este caráter.…”