Role of post-settlement mortality and 'supply-side' ecology in setting patterns of intertidal distribution in the chthamalid barnacles Chthamalus montagui and C. stellatus

Abstract

The chthamalid barnacles Chthamalus montagui and C. stellatus occupy distinctive distributional patterns on European rocky shores. To explore whether these patterns are determined primarily by larval settlement or by post-settlement mortality, we examined the abundance of each species from cyprid attachment through successive stages in its intertidal life history and thus attempted to determine the critical stage for the establishment of adult patterns of dominance. Sampling was carried out on 4 shores in Cork, SW Ireland. On 2 of the shores, sampling sites were in zones in the upper shore dominated by C. montagui, and on the other 2 shores, sites were in zones in the midshore dominated by C. stellatus. The patterns of cyprid settlement and recruitment of early metamorphs in these zones did not reflect that of adult abundances. Patterns of mortality of the 2 species were determined from photographs. Mortality of recruits between 6 and 12 mo after settlement differed between the 2 zones. Mortality of C. montagui was higher than that of C. stellatus in the mid-shore zone, while the reverse trend was observed in the upper C. montagui-dominated zone, with C. stellatus recruits suffering higher rates of mortality than those of C. montagui. Post-settlement mortality, rather than larval supply, was therefore deemed to be the more significant factor in determining the characteristic patterns of these chthamalid species on SW Ireland shores.