Abstract
SUMMARY: We studied the bryozoan assemblage encrusting valves of the Patagonian scallop, Psychrochlamys patagonica, in 4 beds distributed along the continental shelf off Argentina to analyse (a) the distribution pattern of bryozoan colonies in different zones of the valves, (b) the influence of interspecific competition on assemblage composition, (c) whether encrusting species display different space utilisation strategies, and (d) whether bryozoan species richness and number of colonies vary in relation to host size. The assemblage was composed of 22 taxa and was dominated by Arachnopusia monoceros and Osthimosia eatonensis, which were at least one order of magnitude more abundant than any other species. Multivariate analyses based on coverage data of multiserial bryozoans separated the beds according to species richness rather than to geographic proximity and showed significant differences in assemblage structure between upper (left) and lower (right) valves and among beds. Competitive interactions occurred almost exclusively on lower valves, and more frequently in peripheral zones than in central zones of these valves. Correlations between valve area and number of bryozoan colonies, coverage and species richness were low but significant. Bryozoans were significantly more frequent, larger, and taxonomically diverse on lower valves than on upper valves. The uniserial colonies of Neothoa cf. chiloensis, the weakest bryozoan competitor, were as frequent in central zones as they were in peripheral zones, and usually spread out along channels on the scallop surface. This fugitive species was partially covered by multiserial colonies, but managed to persist even in peripheral areas of the lower valves.
Highlights
Bivalve shells can be regarded as small habitat-islands (Ward and Thorpe, 1991) because they are discrete pieces of substratum isolated from the neighbouring environment and often represent the only hard surfaces available for prospecting larvae of sessile organisms
Subtidal bryozoan assemblages that encrust hard substrata are often composed of a variety of species competing for space (López Gappa, 1989; Ward and Thorpe, 1989, 1991; Barnes and Clarke, 1995, 1998; Barnes and Rothery, 1996; Barnes et al, 1996; Barnes and Dick, 2000; Barnes and Lehane, 2001) and food (Buss 1980a; Okamura et al, 2001)
The aim of this study is to analyse (a) the distribution pattern of bryozoan colonies in different zones of the two valves of Psychrochlamys patagonica, (b) the influence of interspecific competition on assemblage composition, (c) whether encrusting species display different space utilisation strategies on the valves, and (d) whether bryozoan species richness and the number of colonies vary in relation to host size
Summary
Bivalve shells can be regarded as small habitat-islands (Ward and Thorpe, 1991) because they are discrete pieces of substratum isolated from the neighbouring environment and often represent the only hard surfaces available for prospecting larvae of sessile organisms. Solitary and colonial organisms have different capacities of using space (Jackson, 1977) While both are able to select their habitat during settlement, only colonial animals exhibit directional growth towards more favourable refuges on the substratum (Buss, 1979). Runners are entirely committed to a fugitive strategy, while sheets and mounds are committed to survival within and around their areas of settlement and to maintaining the integrity of their colony surfaces (Jackson, 1979)
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