Biologia reproductiva de la saraquita Ramnogaster arcuata (Jenyns, 1842) del estuario del Rio de la Plata.
The reproductive biology of Ramnogaster arcuata inhabiting waters of the Rio de la Plata estuary (Argentina - Uruguay) was studied by using macroscopic and histological analysis of ovaries. R. Arcuata specimens were collected from two research cruises of the National Institute of Fisheries Research and Development (INIDEP) carried out in June 1998 and June 1999. During these surveys, bottom salinity and temperature data were also registered. In addition, information about annual catches of this species in the Rio de la Plata, obtained from different INIDEP cruises performed during the last ten years, was analyzed. During winter, a prevalence of R. arcuata individuals was observed in the Rio de la Plata estuary. In summer, this species apparently moves outside the estuary south of the Samborombon bay. The analysis of the maturity stages composition in different sampling sites showed that most of the females collected during June 1998 and 1999 were in spawning stage. Females reached higher size than males and no spatial segregation between sexes was observed. Histological analysis of the ovaries and structure of the oocyte size frequency distribution obtained for gravid females showed that R. arcuata is a batch spawner with indeterminate annual fecundity. A preliminary estimate of spawning frequency indicated a daily percentage of females with postovulatory follicles of 17.5%, which suggests that this species spawns on average once every six days. Batch fecundity ranges from 669 to 2,026 hydrated oocytes for females between 75 to 98 mm total length (TL) in 1998, and from 570 (82 mm TL) to 1,834 (116 mm TL) hydrated oocytes in 1999. During the last sampled year batch fecundity was fitted to a linear function of total length and total weight (without ovary). Mean relative fecundity was 235 hydrated oocytes per female gram (ovary free) in 1998 and 154 hydrated oocytes per female gram (ovary free) in 1999, and no significant relationship was observed between this parameter and female size. Dry weight and diameter of hydrated oocytes ranged between 3.9-6.3 mg and 1153-1387 mu m, respectively and no significant relationship was found between these variables and total length. On the other hand, the comparison between the oocyte dry weight values obtained from different years showed that in 1999 eggs were heavier than in 1998. This characteristic may compensate in part the low batch fecundity values observed in 1999. The results found suggest that reproductive tactic of R. arcuata include some characteristics of species that reproduce in cool waters and others typical of temperate spawners. As in the first ones it possess quite big oocytes, without oil droplets and spawning occurs at low temperatures. On the other hand, R. arcuata is a batch spawner with indeterminate annual fecundity, which is characteristic of species with a long reproductive period.