Barnacle walls as mediators of intertidal mussel recruitment:effects of patch size on the utilization of space

Abstract

Recruitment of the competitively dominant mussel Perumytilus purpuratus to the mid rocky intertidal zone in central Chile depends upon the presence of recruitment-mediators such as mussel clumps, filamentous algae or barnacle shells. The relationship between the lateral walls of adult barnacles and the recruitment of P. purpuratus was investigated in a semi-sheltered rocky habitat at Las Cruces, central Chile. Changes in number of individuals and percent cover of sessile species were recorded for 8 mo (January to September 1988) in different sized patches of bare rock. All patches were surrounded by barnacle beds. Throughout the study, P. purpuratus recruited only on the walls of adult barnacles that formed the patch borders, and never on bare rock. Conversely, barnacles recruited directly on the bare surface of cleared patches. Regressions of numbers of individuals in relation to patch area showed that mussel recruits outnumbered those of barnacles in patches smaller than 35.5 cm2. Above this critical patch area, the number of barnacle recruits was progressively larger than that of mussels. The area of barnacle walls suitable for mussel recruitment increases linearly with increasing patch perimeter, while the area suitable for barnacle recruitment is a function of patch area. The number of recruits was related to the substratum availability in different patch sizes, suggesting that recruitment of barnacles and mussels is not limited by larval abundance. The probability of mussels monopolizing the patches was higher in small than in large patches; the chance for barnacles to monopolize large patches increased with increasing size of patch. Consequently a barnacle-dominated substratum may change to a mussel-dominated one depending upon the relative abundances of small and large patches. A barnacle bed disrupted in several small patches will provide more substratum for mussel recruitment than if only few large patches are produced. Possible effects of patch and predator sizes on intertidal community structure are discussed.