BURROWING AND SWASH BEHAVIOR OF THE PACIFIC MOLE CRAB HIPPA PACIFICA (ANOMURA, HIPPIDAE) IN TROPICAL SANDY BEACHES

Abstract

To investigate factors associated with the distribution of an intertidal crab, Hippa pacifica Dana, which is common in the swash zones of reflective subtropical and tropical sandy beaches, we examined the effect of sediment size on burrowing rates of a wide size range of individuals. We also examined aspects of swash behavior in field conditions, including drift time, distance and velocity of individual crabs. Hippa pacifica burrowed very rapidly (in 0.3 to 2.7 s) in laboratory and field conditions. Burrowing time increased significantly with increasing crab size in all sediments tested but did not differ between male and female crabs nor between ovigerous and non-ovigerous crabs. There were no significant differences in burrowing times among five sediments ranging from very fine sand to gravel, suggesting that this species is a sediment generalist whose distribution is not limited by sediment size. Individual crabs released in the swash zone of a reflective beach oriented, swam and/or drifted for 1.2 to 3.8 seconds before contacting substrate and burrowing. Drift times were similar for all sizes of crabs. However, drift distance (ranging 0.10 m to 1.16 m) and drift velocity (ranging 0.11 m/s to 0.45 m/s), decreased with increasing crab size, suggesting a possible mechanism for size-specific zonation patterns reported in this species. Burrowing times observed in H. pacifica in this study were faster than reported for other sandy beach macrofauna, including tropical forms. This burrowing ability, in combination with the characteristics of substrate generalist and well-oriented swash behavior, appear to be a key to the ability of H. pacifica to inhabit reflective coarse sand beaches.