Geographic Limits and Local Zonation: The Barnaclessemibalanus (Balanus)andchthamalusin New England

The interactions between the intertidal barnacles Semibalanus (Balanus) balanoides and Chthamalus fragilis were examined in order to determine whether the factors which influence local zonation in the intertidal also contribute to the establishment of geographic limits. Both physical and biotic factors influence intertidal zonation at the northern limit of Chthamalus in New England. On sloping surfaces Semibalanus died at all shore levels higher than mid tide level, apparently as a result of desiccation associated with high summer temperatures. Chthamalus settlement occurred at all shore levels above mean tide level, and postsettlement mortality apparently restricts Chthamalus to high shore locations where Semibalanus growth and survival is inhibited. North of the northern limit of Chthamalus, Semibalanus does not suffer summer heat death, so it occupies the zone where Chthamalus would have a refuge from competition further south. The northern limit of Chthamalus is set not by factors directly related to cold acting on Chthamalus. Rather the northern limit appears to be set by cold which allows the dominant competitor to exclude Chthamalus from its refuge zone. South of the northern limit the competitor, Semibalanus, is excluded from the high shore by high summer temperatures.