The role of highly mobile crab predators in the intertidal zonation of their gastropod prey

We experimentally compared the effect of four species of highly mobile predatory crabs on their herbivorous gastropod prey. On northeastern Pacific shores the intertidal distributions of the gastropods, Littorina sitkana (Philippi) and L. scutulata (Gould), overlap broadly with those of two species of mid-shore crabs, Hemigrapsus nudus (Dana) and H. oregonensis, but do not extend below the upper limit of two species of low-shore crabs, Lophopanopeus bellus (Stimpson) and juvenile Cancer productus (Randall), or into the subtidal zone where adult C. productus are abundant. We hypothesized that these gastropods are absent from the low shore despite potentially higher growth and reproductive rates because of the high risk of predation from low-shore and subtidal crabs. To test whether the risk of predation was higher in the low intertidal zone, we tethered 640 adult Littorina sitkana at two tidal levels on four beaches varying in crab abundance. Predation rates varied from 2% to 77% per high tide period At three of the sites the risk of predation was significantly higher at the lower than at the higher tide level. At the fourth site where Cancer productus found shelter under large boulders in the mid-tidal zone, the risk of predation was the same at both tidal levels. Field measurements of per capita consumption rates for the different crab species, test fishing with crab rings, field observations and analysis of the broken shell fragments of the snails showed that predation was primarily by large, subtidal C. productus that moved up into the intertidal to feed. These highly mobile predators aggregate to exploit patches of high prey density in ephemerally favorable habitats, consequently the change in the risk of predation with increasing tidal height is less predictable in both time and space than that for slow moving, low-shore predators.