Contrasting influences of suspension-feeding and deposit-feeding populations of Macoma balthica on infaunal recruitment

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Abstract
Effects of adult density of the bivalve Macoma balthica (L.) on growth and density in juvenile M. balthica (1 to 5 wk) was tested experimentally in the field (exclusion cages) and in the laboratory. Growth of juveniles was reduced in the presence of adults at normal field density in a sublittoral, sheltered, muddy-sand sediment but was not reduced under similar conditions in a sublittoral, wave-exposed, sand sediment. Density of juveniles was not affected by adults in either habitat, or in the laboratory. Growth reduction in juveniles at normal clam densities in the muddy sand habitat (where adults and juveniles are deposit feeders) but not in the sand habitat (where adults are suspension feeders and juveniles deposit feeders) indicates that intraspecific competition between adults and juveniles increases with increasing levels of dietary resource overlap between them. Density and growth of other species, both juveniles and adults, were not affected by the normal density of adult M. balthica. Increased competition between species with greater resource overlap was further indicated by the negative effect on densities of deposit-feeding Hydrobia spp. caused by very high experimental densities of adult M. balthica (deposit-feeding) in the muddy sand habitat, whereas deposit-feeding oligochaetes in the sand habitat reached their highest densities at the highest density of adult M. balthica (suspension-feeding). The laboratory experiments showed that the presence of a nereid polychaete reduced survival of juvenile clams. The different influences of polychaete and adult clams on juveniles suggest that a different level of interaction occurs between the 2 groups, i.e. a direct impact (predation, disturbance, interference competition) of polychaetes and an indirect impact (exploitation competition) of bivalves on juveniles.

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