The relationship of juvenile southern mole crabs Hippa australis Hale (Crustacea : Anamura : Hippidae) to surficial swash water-circulation over several diurnal spring-tide cycles during winter conditions on a micro-tidal sandy beach

Abstract

The location of the southern mole crab Hippa australis Hale within the swash zone at North Beach, Western Australia, was studied with respect to tidal levels, wave run-up, beach morphology, and wave energy conditions over a 4-day period. H. australis undertake distinct tidal migrations within the ground-water resurgence zone on the sandy beachface where characteristics of the water circulation system alter with tidal fluctuations. They locate themselves high in the swash zone at low tide and low in the swash zone at high tide. Mole crabs avoid high wave energy conditions as indicated by their scarcity during storms at North Beach. Their immediate reappearance in higher number as storm surges subside further indicates that they avoid storms by burrowing deep into the substratum. The position of mole crabs at any time during the tidal cycle may be related to the opportunities of feeding in the swash zone.