Adult/juvenile interactions of infaunal bivalves: contrasting outcomes in different habitats

Field experiments were used to assess the influence of density vanatlons of adult bivalves on recruitment of juveniles. The generality of these results was tested by running the experiment concurrently at 2 sltes of different sediment grain size and wave exposure. Adults of 2 bivalve species, the deposit-feedlng tellinid Macomona lillana and the suspension-feeding venerid Austrorienus stutchburj~i, were used in the experiment. The experiment consisted of 14 treatments of vanous combinations of densities of live adults or the empty articulated shells of the 2 species. Three species of juvenile bivalve (M. liliana, A. stutchburyi and Nucula hartvigiana) were sampled from the experiment on 3 occasions over a 9 wk period. Treatment effects were consistent over lime, but reflected only small changes In the mean density of juvenile bivalves. The presence of empty articulated shells of adult M. liliana or A stutchburyi, positioned at localities within the sediment which reflected normal living conditions, had no detectable influence on the density of juvenile bivalves. Treatments ~nvolving adult A stutchburyi ~nfluenced only the density of juvenile conspecifics at the muddy-sand site. Treatments involving adult M. liliana produced significant effects on the density of both M. l~liana and N. hartvlgiana juveniles a t both sites. However, these effects occurred in opposite directions a t the 2 sites. At the muddy-sand site, highest juvenile densities were associated with high adult densities, but, at the sandy site, they were associated with the absence of adult M. liliana. The identification of subtle density changes a t the sandy site was unexpected because sediment transport is frequent at this site and associated passive movement of juveniles has the potential to mask local biotic interactions. The change found in the direction of interactions between adult and juvenile bivalves between sltes has irnplications for the generality of such interactions: in particular it highlights the influence that habitat may have on ecolog~cal interactions.

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