Coastal ecosystems like the Gulf of Maine face anthropogenic threats including increased water temperature from global climate change, and the introduction of invasive species. The Asian shore crab, Hemigrapsus sanguineus, is an invasive species that has become highly abundant along the east coast of the United States, dramatically altering the balance of the ecosystem and intertidal ecological interactions. As it expanded its range northward into the Gulf of Maine, it encountered native species such as the snail Littorina obtusata. To better understand how this invasive predator might impact L. obtusata populations, we conducted laboratory experiments to determine (1) how snail vulnerability to crushing scales with crab size, (2) what size prey crabs prefer to eat when given a choice, and (3) the feeding rates of different sized crabs on different sized snails. To better predict how climate change may affect this system, we compared these predator-prey relationships at the normal mean summertime water temperature for Massachusetts (16° C) and elevated temperatures (22° C). We also compared these performance metrics in male and female crabs, because many studies of crab feeding habits only use males, despite differences in claw size between the sexes.
Results/Conclusions
Our experiments demonstrated a sex difference in the size scaling of predator and prey. As size increased in snail shells and male crab carapace width, neither prey nor predator gained a disproportionate advantage. In contrast, as size increased in snail shells and female crab carapace width, snails became less vulnerable to crushing. Male crabs also had higher feeding rates, and preferred to eat larger snails than females did. Thus, the two sexes do not exploit the same size food resources, and may impact the prey population in different ways. When given a choice, crabs of both sexes preferred smaller snails than the maximum size they were able to eat. This means that in the wild, smaller snails will likely be at a higher risk of predation. We did not find significant temperature effects in any of our experiments. If increased temperatures due to climate change do not impact Hemigrapsus sanguineus’s foraging success on Littorina obtusata, it could potentially continue to be a successful invader over a wide temperature range in the future.
