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Shells of the ribbed mussel Aulacomya atra from the Patagonian gulfs of San Matı´as and SanJose´ (Argentina) are commonly bored by polydorids and covered by serpulids and crustose coralline algae. As the co-occurring mussel Mytilus edulis is neither bored by polydorids nor covered by coralline algae, we hypothesize that early or previous calcareous growth would favour larval settlement and boring by polydorids. Generalized Linear Model analyses showed that polydorid boring is indeed positively correlated with the abundance of serpulids and crustose coralline alga. Infestation by polydorids is higher in larger, older ribbed mussels due to their correspondingly higher available area for larval settlement. Increasing polydorid infestation adversely affects the condition index of A. atra. Fil: Diez, María Emilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; Argentina Fil: Vázquez, Nuria Natalia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; Argentina Fil: da Cunha Lana, P.. Universidade Federal do Paraná; Brasil Fil: Cremonte, Florencia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; Argentina

Structure and temporal variability of macrobenthic associations at vegetated and unvegetated sites of a Spartina alterniflora marsh were analyzed over a 14 mo period in the euhaline, highenergy sector of Paranagua Bay (southeastern Brazil). The vegetated site was characterized by significantly higher numbers of species, mean densities and faunal dominance. The presence of plant cover affects epifaunal grazers, such as the gastropods Nentina virginea, Littorina flava and the isopod Sphaeromopsis mourei, besides a number of infaunal dominants associated with the root-rhizome mat, such as the polychaetes Isolda pulchella and N e r e ~ s ollgohalina Seasonal patterns of change were evident at both sites, with higher macrofaunal densities coinciding seasonally with higher availability of plant litter and lower predation pressure. Macrobenthic populations were generally more persistent at the vegetated site, despite short-term or seasonal vanations. At the unvegetated site, intense and unpredictable population shifts were exhibited by the numerical dominants Kalljapseudes schubarti and Corophium acherusicum. Differences in trophic organization were also evident, with a marked dominance of suspension-feeders at the unvegetated site. Our results suggest that (1) much of the spatial and temporal structure of macrobenthic associations in euhaline, high-energy flats of a subtropical estuary depends on the presence of plant cover, together with seasonal fluctuations in detrital input and predation pressure, and (2) knowledge of the interaction between plant cover and time is a prerequisite for useful data interpretation or hypotheses formulation In such environments.