Impacts of seagrass habitat architecture on bivalve settlement

We investigated the effects of differing spatial scales of seagrass habitat architecture on the composition and abundance of settling bivalves in a su b-tropical seagrass community. The density of newly settled bivalves was ge nerally greater at Thalassia testudinum grass bed edge (( 1 m) compared to interior portions of the bed (> 10 m). Deviation from this generalized patt ern occurred when high densities of newly settled tulip mussels (Modiolus a mericanus) were recorded from the interior of the meadow, associated with a ggregations of adult mussels. Bivalve settling densities appear to reflect settlement shadows of passively delivered larvae, bedload transport of newl y settled individuals from unvegetated regions, as well as gregarious settl ement among adult conspecifics. We also investigated the impact of seagrass patch shape and size on settlement by using artificial seagrass units (ASU ) in separate short-term and long-term experiments. We found a positive rel ationship between ASU perimeter and bivalve abundance, suggesting that larv al encounter rates with seagrass habitat may determine initial settlement p atterns. Using ASUs we also investigated the relative role seagrass epiphyt es play in determining the density of settling bivalves. Results showed gre ater settling densities where epiphytic secondary structure was elevated co mpared to controls, and bivalve density was significantly greater when ASUs were fouled with a natural community of epiphytes, suggesting that both mi crostructure and biofilms positively influenced bivalve settlement. We conc lude that structural components of seagrass habitats increase bivalve settl ement at multiple spatial scales, including epiphytic micro-structure, smal l-scale patch shape and size, and large-scale within habitat differences.