Adult Interference with Postlarvae in Soft Sediments: The Pontoporeia‐Macoma Example

In the Baltic Sea, the presence of dense populations of the deposit—feeding amphipods Pontoporeia affinis and P. femorata has been hypothesized to inhibit recruitment of the bivalve Macoma balthica. Experimental tests with P. affinis and newly settled M. balthica in small, flow—through aquaria corroborated this hypothesis. Survival of small M. balthica spat decreased with increasing density of Pontoporeia affinis. The increase in mortality was strong enough potentially to explain the field observation that M. balthica is generally absent or rare in areas with dense populations of Pontoporeia spp. Three hypotheses as to the mechanism for the decreased M. balthica survival were tested. The results wee inconsistent with hypotheses of suffocation after burial or starvation as the mechanism of death, but corroborated the hypothesis of direct physical injury to M. balthica spat following contact with P. affinis adults. In the presence of P. affinis, spat survival was lower in shallow than in deep sediment; encounters between the two species should be more frequent in shallow sediment. Adult P. affinis were found to crush newly settled M. balthica spat, presumably ingesting the remains. M. balthica spat are not, however, common enough to be an important food source for P. affinis. It is suggested that while interference with larval recruitment by adults is likely to be an important structuring factor in marine soft bottom communities, it is unlikely to depress community biomass below carrying capacity, as suggested by Peterson (1979). Virtually all communities contain species with brood protection, the young of which are less susceptible to interference by adults. Where such interference is strong, species with brood protection are likely to become dominant, as are Pontoporeia spp. over large areas in the Baltic Sea.