Annual fecundity assessment for the Rio skate Rioraja agassizi (chondrichthyes: arhynchobatidae) endemic to a neotropical area (Southeastern Brazil)

Abstract

Descriptors: Oviparity, Egg case, Rajoid, Southwestern Atlantic, Reproductive biology. Descritores: Oviparidade, Capsulas ovigeras, Rajoideo, Atlântico sudocidental, Biologia reprodutiva. The Rio skate Rioraja agassizi (MULLER; HENLE, 1841) is an endemic species distributed over a restricted neotropical area extending from Rio de Janeiro, Brazil, to Uruguay and southern Argentina (ODDONE et al., 2006, 2007a,b). This species inhabits coastal waters from the shore down to 130 m (FIGUEIREDO, 1977), feeding on benthic organisms, mainly crustaceans and teleosts (MUTO et al., 2001). ODDONE et al. (2007a) noted that male R. agassizi attained sexual maturity at 320 mm and females at 400 mm of total length (TL). From samples collected off Santos, southeast Brazil, ODDONE et al. (2007b) observed that mean female total length was significantly higher than that of males. Additionally, the largest male and female specimens had a TL of 549 mm and 472 mm, respectively. Southward, between Uruguay (34°S) and northern Argentina (42°S), COLONELLO et al. (2007) also recorded that males mature at TL of 400-500 mm and females at 500-550 mm. Off Puerto Quequen (Argentina), ESTALLES et al. (2009) noted that the smallest adult male and the smallest adult female were of 485 mm and 530 mm TL, respectively, while all males and females were adult over 590 mm and 629 mm, respectively. Therefore, in the three areas quoted, female R. agassizi were larger than the males - thus providing another instance of the sexual dimorphism in size recorded for rajoid species (see MELLINGER, 1989). Further, latitudinal differences in size were observed between specimens from these areas, probably due to environmental factors, a phenomenon first recorded by LELOUP and OLIVEREAU (1951) in oviparous elasmobranch species and which has been frequently reported in rajoid species (CAPAPE et al., 2004, 2007 a). Such a pattern has also been observed in the egg cases extracted in utero from female R. agassizi , that were larger off Puerto Quequen (ESTALLES et al., 2009) than those off Southern Brazil (ODDONE et al., 2006). ODDONE et al. (2007a) noted that reproductive activity was continuous for specimens from southeastern Brazil, with vitellogenesis, ovulation, egg-laying and presence of sperm in seminal vesicles being observed all the year round. Additionally, ODDONE et al. (op. cit.) suggested an annual cycle for R. agassizi , with reproductive activity peaking at least once a year. Similar patterns were observed by ESTALLES et al. (2009) who noted a peak of reproductive activity in spring for specimens collected off Puerto Quequen. From studies related to the brown ray Raja miraletus LINNAEUS, 1758 and the rough ray R. radula DELAROCHE, 1809 caught in Tunisian waters (CAPAPE; QUIGNARD, 1975), and more recently related to the thornback ray R. clavata LINNAEUS, 1758 from off the Languedocian coast (CAPAPE et al., 2007b), two categories of oocytes have been distinguished: translucent oocytes and yolky oocytes. Among the latter, three batches of yolky oocytes were considered, i.e ., one batch of large oocytes generally ready to be ovulated, and two batches of large and small developing oocytes, respectively. Each batch exhibited oocytes similar in size and mass. CAPAPE and QUIGNARD (1975) estimated the fecundity of rajid species based on maximal numbers of small,