Factors influencing aggregation patterns in the sand crab Emerita analoga (Crustacea: Hippidae)

Aggregation patterns in Emerita analoga (in southern California) are delineated with respect to their spatial, daily, and seasonal components. Both abiotic and biotic factors are found to be associated with patterns of aggregation. Spatially, E. analoga aggregates from March through September to a significantly greater degree in the upper one-third area of the wash zone where exposure to wave shock and fish predation are probably decreased. Sand crabs are more aggregated on a daily basis during low tides than at high tides. This may be due to differential rates of migration caused by a decrease in the beach slope angle. Two seasonal peak periods of aggregation are present, one in the early spring, and one in the late summer. These periods occur during the times of highest reproductive female abundance. High seasonal intensities of aggregation probably function to facilitate mating through the maintenance of close proximity between males and females. Visual methods and/or quantitative sampling based on visual observations do not adequately reflect patterns of aggregation in E. analoga.