Interactions of Patella and macroalgae with settling Semibalanus balanoides (L.)

Biological interactions affecting densities of settling and newly-settled Semibalanus balanoides (L.) have been investigated by manipulative field experiments on the Isle of Man.The effects of sweeping by fucoid clumps of different species and Patella browsing have been compared on moderately-exposed shores. Patella allowed barnacle settlement by preventing growth of competing green algae, but reduced post-settlement densities. Small clumps of Fucus spiralis L., F. vesiculosus L, and F. serratus L. all reduced settlement considerably more so than limpets. F. serratus had the greatest sweeping effect.Interactions between macroalgae and Semibalanus balanoides have been investigated at all levels on sheltered shores and low down on more exposed shores. In the Fucus spiralis and F. vesiculosus zones, post-settlement numbers were higher than in adjacent areas where the canopy was removed. Barnacles did not settle readily in the Ascophyllum zone in either experimental or control areas. Settlement occurred in the upper part of the Fucus serratus zone in experimental areas where the canopy was removed but not in control areas. No settlement occurred in either treatment or control areas lower in the F. serratus zone. At all levels on the shore fucoid canopies seemed to reduce cyprid settlement, but the effect was greatest amongst F. serratus where there was total prevention. High on the shore the effect of enhanced post-settlement survival under the canopy outweighs reduction of cyprid settlement thus there are greater numbers in the controls. Competition with red algal turfs was shown to set the lower limit of the barnacle zone on a vertical pier face.