Patterns, Mechanisms, Consequences of Variability in Settlement and Recruitment of an Intertidal Barnacle

I examined the settlement or recruitment of the barnacle Chthamalus anisopoma at a variety of scales: temporally, over periods lasting 12 h to 1 mo, and spatially at sites separated by <1 cm to shores separated by 10 km. Both settlement and recruitment were variable at nearly all temporal and spatial scales that I examined. The majority of variability was attributable to differences in settlement or recruitment between sample periods. When only the spatial component was considered, variability decreased with scale. Large, consistent differences in settlement between geographic locations that are at approximately the same tidal height are probably the result of differential larval supply, whereas more subtle but still consistent differences in settlement between adjacent sites may, at least in part, be due to larval behavior. Differences in settlement between close sites were observed only when settlement was relatively high. This observation may be explained as follows. First, Chthamalus settle in pits or small depressions "suitable settlement sites (SSS's)" in the surface of a substrate. Second, within an SSS, space for settlement is restricted. Third, the density of SSS's differ between sites. When settlement is high, settlement differences between locations reflect differences in the number of SSS's, as all are used. However, when settlement is low, settlement will be similar between locations, since SSS's are not limiting. In a series of field experiments using both natural and artificial substrates I found that: (1) Cypris larvae of Chthamalus settled preferentially in the presence of a number of physical and biological factors. (2) Settlement was density dependent but growth and survivorship were not, which suggests exploitative but not interference intraspecific competition. (Although Chthamalus settle gregariously they space themselves out.) (3) Variability in the settlement of Chthamalus affected the ensuing adult population even at the highest settlement densities.