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Shell beds of the stout razor clam Tagelus plebeius in life position are common in the intertidal areas of several SW Atlantic estuaries (36° to 39° S). In this study we investigate the effect of this habitat structure on the benthic community, focusing on: (1) species which use this habitat as a refuge, (2) characteristics of the internal and external shell sediment and the abundance of macroinfauna and meiofauna in each one, (3) the effect of removing or introducing life-position shells on the abundance of macroinfaunal and meiofaunal organisms, (4) the effectiveness of shells as predator excluders and (5) sedimentary balance in shell beds and the role of superficial life-position shells. Eighteen macrofaunal species were encountered inhabiting the shell cavities; the amphipod Corophium insidiosum, the gastropod Heleobia australis and the polychaete Laeonereis acuta were the most abundant. Water and organic matter content were higher in the inner shell sediment than in the sediment sampled out, side, but the distribution of phi-grain size values did not differ significantly between them. Except for nematodes, all the meiofaunal taxa and C. insidiosum showed higher abundance in the inner shell sediment. Removal of life-position shells in a shell bed negatively affected the abundance of C. insidiosum and the polychaete Heteromastus similis. Shell introduction in a mud flat had a positive effect on the abundance of almost all the taxa encountered. Two predator exclusion experiments indicate that these shells protect the fauna living within the shells and the underlying shell infauna from motile surface predators. Observations of shorebird feeding behavior showed that they rarely peck into the shell cavities. Daily measurements of the protruding length of shells demonstrated that these beds are exposed to erosive regimes, and shell removal in a shell assemblage destabilizes the sediment. The evidence suggests that superficial shells positively affect the diversity and abundance of organisms.

Role of Holocene beds of the stout razor clam Tagelus plebeius in structuring present benthic communities