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Densities of adult infaunal clams Macoma balthica and Mya arenaria in mesohaline sections of Pamlico Sound, North Carolina, USA, declined rapidly between April and June 1991 and 1992, except in places where there was abundant shell material on the sediment or large densities of the mactrid bivalve Rangia cuneata. Survivorship of M. balthica and M. arenaria was examined experimentally in 1 M2 plots containing different types of habitat: widgeon grass Ruppia maritima, live R. cuneata, 'artificial' R. cuneata, shell valves of R. cuneata and clean sediments to determine whether these structures provided a refuge from predation for the clams. M. balthica and M arenaria obtained a substantial refuge from predation by crabs when living in patches containing R. cuneata. Neither species obtained a refuge from predation when in beds of widgeon grass. There were 2 surprising results in these experiments. First, survivorship of the clams was significantly reduced in patches containing both seagrass and R. cuneata compared with patches of only R. cuneata. Second, survivorship was significantly less in patches of live R. cuneata than in patches of 'sham' (artificial) clams or equal numbers of shell valves. Two alternative, but not necessarily exclusive, explanations are offered for these results. (1) Subsurface rhizomes of widgeon grass may reduce the depth to which clams are able to burrow into the substratum leaving them more susceptible to predation by digging crabs. (2) Increased availability of alternative prey in patches of widgeon grass causes predators to spend more time foraging in these patches and, by chance alone, they capture more clams. Alternatively, predators spend more time in the patches with widgeon grass because they obtain a partial refuge from their own predators. The occurrence of a refuge from predation for M arenaria and M balthica living with R. cuneata is a striking example of an associational defense between potentially competing species. Associational defenses such as this may be a common structuring force in soft-sediment systems and future work may help in producing a more synthetic understanding of their importance in aquatic systems.

Refuges from predation and the persistence of estuarine clam populations