Rock type affects settlement, recruitment, and zonation of the barnacle Chthamalus anisopoma Pilsbury

The settlement, recruitment, and zonation of the barnacle Chthamalus anisopoma (Pilsbury) on two shores were investigated in the northern Gulf of California. Its upper vertical limit is, on average, 25 cm lower on a basalt shore than on a nearby granite shore, probably because of greater post-settlement mortality on basalt than on granite near the upper limit of its distribution. This difference in mortality may be related to the thermal capacity of each rock type. Results of a reciprocal transplant experiment showed that: 1. (1) there was consistently greater settlement and recruitment on granite stones than on basalt stones at both shores, suggesting that larvae select granite over basalt substrata for settlement; 2. (2) overall, there was more settlement and recruitment to experimental stones at the basalt shore than at the granite shore, suggesting a larger pool of potential settlers at the basalt shore; and, 3. (3) there was no difference between recruitment on granite stones at the granite shore and that on basalt stones at the basalt shore. That is, there was no difference in recruitment between shores on surfaces natural to each shore. These results demonstrate the importance of effects specific to different shores and substrata to intertidal community structure.