The sesarmid Perisarma guttatum is the most abundant crab species inhabiting the lower mangrove areas at Inhaca Island, and presumably an ecologically important species in this habitat. Among the scarce available information on its population ecology, this species is known to follow a semilunar rhythm of larval release, with breeding probably extended year-round. In this study, a population of P. guttatum was examined, from which estimates of sexual maturity were carried out for both sexes, and breeding activity was monitored to describe short-term variation of reproductive intensity. In an attempt to relate reproductive cycles with recruitment patterns of young, a systematic random sampling design was used to quantify and describe the juvenile population. Reproductive parameters in adults and density estimates for juveniles were obtained from samples taken at 4-d intervals over a 3-mo period. Reproductive activity increased over the study period. Females released larvae around the new and full moon, but lunar variation of the ovigerous ratio with peaks preceding full moon periods indicated that different breeding groups may account unevenly for the reproductive output of the population. Very young recruits made up the larger fraction of the juvenile population, and their density increased from the beginning to the end of the sampling season. Density variation pattern of first-crab stages followed fortnight cycles, but slow growth prevented recruitment pulses to be tracked in size-frequency distributions. Polymodal distributions corresponded to recent recruits and older juveniles originating from previous settlement seasons. Growth rates of identified age groups suggest that sexual maturity in females is likely to be achieved after 2 years or more.
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Vol. 22 • No. 1