Dados sobre a biologia reprodutiva e aspectos básicos da ecologia da raia de água doce Potamotrygon orbignyi (Chondrichthyes: Potamotrygonidae) ocorrente na baía de Marajó – Pará

Abstract

Potamotrygon is one of the genera of freshwater ray that has approximately 20 described species and many of these are being exploited as ornamental fish in the Amazon. Despite this, studies on these animals are still scarce in this region. In order to provide data on the biology and some aspects of the ecology of Potamotrygon orbignyi, collections were made on the islands of Cotijuba and Colares, Marajó Bay, between May 2000 and March 2002. The collected material was photographed, weighed, labeled, gutted and fixed with 10% formalin solution while still in the field. Later, the material was washed and analyzed. A total of 350 specimens of P. orbignyi were captured, which corresponded to 42.7% of the total number of freshwater rays captured during this study. The sex ratio among the captured specimens was 1:1.1 in favor of females, but there was a large discrepancy between the number of juveniles (98%) and the number of adults (1.14%) / subadults (0.86%). The comparative analysis of the reproductive apparatus of adults and juveniles revealed that in adult males the most striking features were: the sectioning of the seminal vesicles and the size and stiffening of the clasperes. Adult females showed individualized uteri and vagina, with clear delineation between such structures, and greater development of the ovaries and nidamentary glands. Five coloration patterns that characterize this species in the region were observed and ranged from discrete dots on the brown surface of the dorsal region of the disk, to the formation of polygonal figures well delimited by dark lines. The data indicate that probably Potamotrygon orbignyi uses the beaches of Colares and Cotijuba islands as a nursery area and possibly presents a differentiated spatial distribution according to ontogenetic development, as observed for the middle Negro River (Araújo, 1999).