Predation, Physical Stress, and the Organization of a Tropical Rocky Intertidal Hermit Crab Community

Predation pressure and physical stress are shown to be important selective pressures on hermit crabs in the Bay of Panama, influencing their intertidal distributions, shell preferences, and behaviors. Predation on hermit crabs by shell—crushing teleost fish and crabs is greatest in the low intertidal habitat and decreases in the high intertidal environment where physical stresses are the most intense. Pagurus sp. (undescribed) lives in the low intertidal habitat exclusively and prefers relatively heavy shells with narrow apertures which reduce vulnerability to predation while decreasing resistance to thermal stress. Calcinus obscurus is found in the middle to low intertidal environment, and Clibanarius albidigitus has a middle to high intertidal distribution, partly as a result of active competitive displacement by C. obscurus from the low intertidal habitat. These two speices prefer shells that minimize thermal stress and give relatively little defense against predators. Predation in these crabs is avoided by a behavioral escape response most acutely developed in C. obscurus, while C. albidigitus is superior to C. obscurus in withstanding thermal stress. Habitat and shell resource utilization pattern sin this tropical hermit crab community can be interpreted to represent the result of a complex interaction between predation, physical stress, and competitive pressures.