Geographical variation in natural history of the sandy beach isopod Excirolana hirsuticauda Menzies (Cirolanidae) on the Chilean coast

Monthly samplings were carried out during 1999–2000 at two sandy beaches of the Chilean coast (El Apolillado 29 S and La Misio´n 39 S), to test the hypothesis that population abundances, reproductive biology and life history characteristics of the cirolanid isopod Excirolana hirsuticauda Menzies are affected by latitude. Sediment samples were collected at 2m spaced levels along two transects separated 2m apart and extending from the uppermost beach levels down to those levels where no more isopods were found. The isopods were sexed and measured for growth studies through the analysis of month length frequency distribution. The fecundity of ovigerous females was analysed and the relationship between this and the size of the females was estimated with analyses of linear regression and compared with ANCOVA. The population abundances of E. hirsuticauda were higher at the northern beach of El Apolillado (up to 48 000–50 000 ind.m 1) as compared to the southern beach of La Misio´ n (up to 13 000 ind.m 1). The reproductive period occurred during early summer (December–January) at both beaches. The fecundity of females increased significantly with body size at both beaches. Results of covariance showed that for a similar body size, fecundity was significantly higher at El Apolillado. Body size of the largest male, smallest and largest ovigerous females and body size at maturity of ovigerous females were larger at La Misio´ n. The intramarsupial stages of the ovigerous females of this beach were significantly larger than that of the ovigerous females from the northern site. The growth and mortality rates were higher at the southern site. It is concluded that population abundances, reproductive biology and growth of E. hirsuticauda change along the Chilean coast.