Tide and wind control of megalopal supply to estuarine crab populations on the Portuguese west coast
Square mesh traps placed under natural boulder cover were used to obtain in situ set- tlement rates of intertidal brachyurans along a rocky coast within the Cascais area, Central Portugal. Megalopae of all intertidal species known to occur in this region were present in the samples. The settlement variability within different spatial scales, i.e. hundreds, tens and unit metres, were tested for the most abundant species, Pachygrapsus marmoratus, using a nested analysis of variance design. A significant contrast was found between areas 1.5 km apart, while no differences were detected at lower spatial scales. Spatial heterogeneity is likely to depend on coastal hydrological patterns rather than smaller-scale processes such as substrate selection. Fine time series settlement data were obtained for P. marmoratus and Xantho incisus, which were analysed for periodic patterns and cor- respondence to tide range and wind speed components. Across-shore wind forcing was found to be the major factor affecting settlement for both species, although in inverse ways. While onshore winds favour settlement of P. marmoratus, it is the relaxation of the onshore component or contrary offshore forcing that is apparently transporting the megalopae of X. incisus ashore. We predict that the verti- cal distributions of X. incisus and P. marmoratus megalopae are different within the inner shelf and that wind-driven upwelling and downwelling compensating coastal flows are largely determining their temporal abundance variation. In the case of P. marmoratus, there is also a secondary but sig- nificant effect of tides on the abundance of megalopae. An increasing number of settlers on traps was verified during periods of higher tide range. This seems to be particularly marked during spring tides around the new moon, but there is no statistical support for such a trend.