ADULT JUVENILE INTERACTIONS OF INFAUNAL BIVALVES - CONTRASTING OUTCOMES IN DIFFERENT HABITATS/

Field experiments were used to assess the influence of density variati ons of adult bivalves on recruitment of juveniles. The generality of t hese results was tested by running the experiment concurrently at 2 si tes of different sediment grain size and wave exposure. Adults of 2 bi valve species, the deposit-feeding tellinid Macomona liliana and the s uspension-feeding venerid Austrovenus stutchburyi, were used in the ex periment. The experiment consisted of 14 treatments of various combina tions of densities of live adults or the empty articulated shells of t he 2 species. Three species of juvenile bivalve (M. liliana, A. stutch buryi and Nucula hartvigiana) were sampled from the experiment on 3 oc casions over a 9 wk period. Treatment effects were consistent over tim e, but reflected only small changes in the mean density of juvenile bi valves. The presence of empty articulated shells of adult M. liliana o r A. stutchburyi, positioned at localities within the sediment which r eflected normal living conditions, had no detectable influence on the density of juvenile bivalves. Treatments involving adult A. stutchbury i influenced only the density of juvenile conspecifics at the muddy-sa nd site. Treatments involving adult M. liliana produced significant ef fects on the density of both M. liliana and N. hartvigiana 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 we re associated with the absence of adult M. liliana. The identification of subtle density changes at the sandy site was unexpected because se diment transport is frequent at this site and associated passive movem ent of juveniles has the potential to mask local biotic interactions. The change found in the direction of interactions between adult and ju venile bivalves between sites has implications for the generality of s uch interactions: in particular it highlights the influence that habit at may have on ecological interactions.