Amphipods as potential prey of the Asian shore crab Hemigrapsus sanguineus: Laboratory and field experiments

• Hemigrapsus sanguineus ate amphipods in laboratory microcosms.
• Sediment reduced consumption rate.
• Male and female crabs ate similar numbers of amphipods.
• Large crabs ate fewer amphipods than small crabs.
• Hemigrapsus sanguineus did not decrease amphipod numbers in field experiments.

The invasive crab Hemigrapsus sanguineus has a generalist diet, consuming primarily macroalgae and sessile and sedentary invertebrate prey. To assess the dietary importance of motile prey, predation by H. sanguineus on the amphipod Hyale plumulosa was examined in laboratory and field experiments. Male and female crabs 7–27 mm in carapace width were offered H. plumulosa in laboratory microcosms with and without sediment (sand and rocks) present. Despite sex differences in claw morphology, male and female H. sanguineus consumed similar numbers of amphipods. Large crabs of both sexes consumed fewer H. plumulosa than small crabs, and consumption was lower when sediment was present. Crab abundance was then manipulated in the field in randomized complete block experiments in the lower rocky intertidal zone. In three 14-day experiments, H. sanguineus had no negative effect on amphipod abundance. In fact, excluding crabs from cages led to lower amphipod densities. The results of the 2-species predator–prey laboratory experiments did not transfer to the natural environment, suggesting that complex interactions exist between H. sanguineus and motile amphipod prey in rocky intertidal areas.