The Influence of Larval Supply and Settlement on the Population Structure of Barnacles

The abundance and distribution of settlers, recruits, and adults of the barnacle Semibalanus balanoides were monitored in the high, mid, and low intertidal zones at two adjacent, but physically and biologically different sites (A and B) on a rocky intertidal shore in Nova Scotia following a rare occurrence of ice scouring. Availability of planktonic cyprid larvae also was measured at site A. There was a strong concordance between cyprid availability and settler density over the main settlement period among intertidal heights, where each was inversely related to intertidal height. Differences in cyprid availability with intertidal height were related to immersion time as the larvae, in general, were uniformly distributed at the water—substratum interface. Settlement rate was strongly correlated with cyprid availability in the low intertidal zone at site A, where most settlement occurred. In all zones at both sites, recruit density at the end of the main settlement period was a positive function of settler density over this period: on average, 79% of the variation in recruit density was explained by settler density over the main settlement period. Adult density and total settler density also tended to be strongly related: 86% (mid zone at site A) and 73% (high zone at site B) of the variation in adult density was explained by total settler density, except where postrecruitment mortality (i.e., predation) was high (e.g., only 1% of the variation in adult density was explained by total settler density in the low zone at site A). Local hydrodynamics, which influences supply and distribution of planktonic larvae, may dictate the distribution of settlers and, ultimately, adults on this shore.