Settlement and post-settlement interactions between Semibalanus balanoides (L.) (Crustacea: Cirripedia) and three species of fucoid canopy algae

Abstract

Manipulative field experiments carried out on sheltered rocky shores investigated settlement and post-settlement interactions between Semibalanus balanoides and three species of fucoid canopy algae, Fucus spiralis L., Ascophyllum nodosum (L.) Le Jolis and Fucus serratus L. All three species had a negative effect on settlement and early recruitment of S. balanoides, densities being significantly lower on settlement tiles beneath the canopy compared to cleared areas. Investigation of the mechanism by which settlement and recruitment were inhibited showed the importance of sweeping of algal fronds at all three shore heights. In addition, we found that the F. serratus canopy inhibited settlement not only by its sweeping action but also by limiting the access of cyprids to the substratum. Detailed examination of the sweeping effect in F. serratus showed that newly settled cyprids transplanted beneath a canopy suffered extremely high mortality (between 82 and 97%) over just one high water period. Mortality was significantly reduced for individuals that had settled within a matrix of adults. Metamorphosis of cyprids to spat conferred no additional resistance to sweeping-induced mortality but resistance increased in 6 day old spat. Mortality in S. balanoides after the end of the settlement season was monitored for three months in experimental treatments and controls. Mortality rates were significantly lower under the canopy than in cleared areas in both the F. spiralis and Ascophyllum zones. The overall influence of each canopy species on the development of barnacle populations on sheltered shores is discussed.