FIGURE 10: Small, free-living specimen of Plesiotrygon nana sp. nov. MZUSP 57642, juvenile or preadult female 912 mm TL, 124 mm DL, 118 mm DW. Characters that allow it to be identified as P. nana include its dorsal color pattern, dark purplish-brown caudal whip, proportionally smaller spiracles and nasoral region, and ventral lateral-line canals.
Photo: M. Toledo-Piza. | DOI: 10.1590/S0031-10492011000700001
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.
Key-Words: Plesiotrygon iwamae; Taxonomy; Morphology; Myliobatiformes; Rio Solimões; Río Ucayali; South America.
Etymology: The specific epithet nana is in reference to its dwarf size (from the Latin nanus). Gender feminine.
Common name: Dwarf Antenna Ray (as in aquarium literature; also known as "black-tailed" antenna ray).
Geographic distribution: As far as is known, Plesiotrygon nana occurs in the upper Río Amazonas basin of Peru, both in smaller tributaries and in the main Río Amazonas channel, and in the lower course of Rio Solimões in Brazil, just down-river from the mouth of Rio Purus (MZUSP 57642). The small female paratype is from Río Itaya, a small tributary of Río Nanay (near Iquitos). The male paratype was collected in Río Amazonas slightly farther south, near Tamshiyacu, and the holotype was found in Río Pachitea, a triburay of Río Ucayali near Puerto Inca (Figure 16). This species may be expected to occur in Río Napo and other regions of the upper Río Amazonas inhabited by P. iwamae. Plesiotrygon nana is not restricted to the main Amazonas channel as previously thought (accounts from the aquarium fish trade conflict).
Marcelo R. de Carvalho and Maíra Ragno. 2010. 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). Papéis Avulsos de Zoologia (São Paulo). 51(7):101-138. DOI: 10.1590/S0031-10492011000700001