Scavenger and burrowing features of Hippa pacifica (Dana 1852) on a range of tropical sandy beaches

Abstract

Carrion consumption and scavenging are increasingly recognized for their role in community structure and food web dynamics The Pacific mole crab Hippa pacifica (Dana 1852) is a conspicuous scavenger of the swash zone of exposed sandy beaches on islands of the Pacific and Indo-Pacific regions. Our findings on the population structure of this species on Hawaiian beaches differed with the method of sampling used (baited vs. non-baited). The rapid attraction and aggregation of large crabs to bait stations contrasted with the population samples dominated by small crabs collected in the absence of bait. This result suggested that small individuals were excluded by larger crabs when competing for carrion resources. Burrowing velocity (mm s−1) increased with increasing carapace length, meaning that large individuals were more efficient than small crabs in the harsh hydrodynamic environment of the swash. Burrowing assays to exhaustion in laboratory conditions indicated that the ability of the crabs to cope with the harsh environment of the swash zone is a body size-dependent feature, independent of sex or fecundity state. We simulated the potential effect of the hydrodynamic and sediment instability typical of the swash zone of reflective beaches with a consecutive sequence of removals from the sediment. Following this treatment, the burrowing times of the larger crabs were shorter than those of the small individuals. Crab abundance was not related to sand grain size or to beach morphodynamics. Abundance was correlated with the amount of edible organic matter in the sediments, suggesting that H. pacifica is a highly opportunistic species, able to feed on a wider range of potential food sources than expected.