Population abundances, growth and natural mortality of the crustacean macroinfauna at two sand beach morphodynamic types in southern Chile

Monthly samplings were carried out during 1994-1996 at an intermediate and at a dissipative sandy beaches of southern Chile (Calfuco and Mehuín, respectively; ca. 39° S), to evaluate the predictions of the swash exclusion hypothesis on the population abundances and life history of the crustacean macroinfauna inhabiting the intertidal of that beaches. It is hypothesized that only true intertidal species (i.e., that directly affected by the swash) will be affected by differences in beach morphodynamic types. That species were the cirolanid isopods Excirolana braziliensis and Excirolana hirsuticauda and the anomuran crab Emerita analoga. On the other hand, the almost supralittoral amphipod Orchestoidea tuberculata should not be affected by differences in beach morphodynamic types. The total population abundances of O. tuberculata and E. analoga were significantly higher at the intermediate beach of Calfuco, that of E. braziliensis and E. hirsuticauda at the dissipative beach of Mehuín. While the abundances of ovigerous females of O. tuberculata and E. analoga were higher at Calfuco, that of the ovigerous females of E. braziliensis and E. hirsuticauda did not differ between beaches. Juveniles of O. tuberculata had higher abundances at Calfuco, while that of the other species peaked at Mehuín. The reproductive period of amphipods and isopods peaked at the same spring-summer months at both beaches. E. analoga also reproduced during spring-summer months, but their abundances peaked at different months. While juveniles of O. tuberculata peaked at similar times at both beaches, those of Excirolana braziliensis and E. hirsuticauda peaked later at Mehuín. Juveniles of E. analoga had very low abundances during the first spring-summer period of this study; during the second one they peaked at the same month (October) at both beaches. The growth curves and growth parameters of O. tuberculata, E. braziliensis and E. hirsuticauda did not differ significantly between beaches, even when O. tuberculata tended to growth faster at Calfuco. The growth curves and growth parameters for E. analoga were different only for females, which had higher growth at the dissipative site. The natural mortality of the four species showed no significant differences between beaches. Thus, most of the results of this study do not support our hypothesis, suggesting that other factors, apart from beach morphodynamics are important in explaining spatial variability in population abundances and population biology of the sandy beach macroinfauna.