HYDRODYNAMIC ENHANCEMENT OF LARVAL SETTLEMENT IN THE BIVALVE MULINIA-LATERALIS (SAY) AND THE POLYCHAETE CAPITELLA SP-I IN MICRODEPOSITIONALENVIRONMENTS

To test whether larval settlement patterns of the opportunistic bivalv e Mulinia lateralis (Say) and the opportunistic polychaete Capitella s p. I are influenced by near-bottom flow, laboratory still-water and fl ume-flow experiments were conducted using a sediment-filled array cons isting of depressions and compartments flush with the flume bottom. Co mpartments were filled with organic-rich mud or a low-organic, glass-b ead mixture of a comparable grain size. Previous flume experiments hav e shown that larvae of both species settle in greater numbers in mud c ompared with glass beads. Depressions create a hydrodynamic environmen t that traps passive particles, permitting tests of the relative impor tance of active selection versus passive deposition of larvae in regio ns of microtopography. In both flow and still water, Capitella sp. I l arvae consistently selected organic-rich mud over glass beads, regardl ess of whether treatments were flush or depressions. Settlement was hi gher, however, in depressions (3.8 cm in diameter and 2.8 cm deep) for a given sediment treatment, particularly in glass bead treatments in flow. In flow and still-water experiments, M. lateralis larvae also ch ose mud over glass beads but, in some instances, higher settlement occ urred in glass bead depressions (a ''poor'' choice) compared to flush mud (a ''good'' choice). These results suggest that near-bottom flow i nfluences settlement distributions of both species (i.e. settlement en hancement in depressions), but the effect may be greater for M. latera lis larvae. Higher settlement generally observed in mud depressions co mpared with glass bead depressions suggests that larvae of both specie s may have been able to ''escape'' from depressions if the substratum was unsuitable, although M. lateralis larvae were poorer swimmers than Capitella sp. I larvae and were more vulnerable to passive entrainmen t and retention in depressions. Similar experiments with smaller depre ssions (9 mm in diameter and 9 mm deep) showed no settlement enhanceme nt in depressions for Capitella sp. I and enhancement in only one of t wo flow experiments with M. lateralis larvae, suggesting that the hydr odynamic, trapping effect may bc scale dependent for both species.