INTERTIDAL ZONATION AMONG CONGENERS - FACTORS REGULATING DISTRIBUTION OF PORCELAIN CRABS PETROLISTHES-SPP (ANOMURA, PORCELLANIDAE)

The porcelain crabs Petrolisthes cinctipes (Randall) and P. eriomerus Stimpson are sympatric on exposed rocky shores in the northeastern Pacific. These species often exhibit a striking pattern of non-overlapping vertical distributions, with P. eriomerus inhabiting low intertidal and subtidal areas and P. cinctipes occurring in the middle and high intertidal. Transplant experiments revealed that P. eriomerus was unable to tolerate displacement as little as 0.3 m above its normal limit, while in contrast its congener showed good survival and growth when caged at lower levels. The 2 species were equally susceptible to desiccation; however, P. eriomerus was extremely sensitive to thermal stress during periods of aerial exposure, and thermal tolerance varied inversely with size. Abiotic factors also control the distribution of P. cinctipes; this species was competitively dominant in laboratory experiments, suggesting the lower limit is not due to interactions with its congener. Rather, this distribution is linked to sudden changes in substrate composition, involving an active avoidance of rocks resting on sand or silt as demonstrated through field manipulation of substrate beneath rock patches. Patterns of resource (space) partitioning beneath rocks are largely a passive consequence of physiological constraints and substrate preferences by adults, maintained in part through gregarious settlement by megalopae both species.