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

Shell beds of the stout razor clam Tagelus plebeius in life position are co mmon in the intertidal areas of several SW Atlantic estuaries (36 degrees t o 39 degrees S). In this study we investigate the effect of this habitat st ructure on the benthic community, focusing on: (1) species which use this h abitat as a refuge, (2) characteristics of the internal and external shell sediment anti 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 ro le of superficial life-position shells. Eighteen macrofaunal species were e ncountered 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 inn er shell sediment than in the sediment sampled outside, but the distributio n of phi-grain size values did not differ significantly between them. Excep t for nematodes, all the meiofaunal taxa and C. insidiosum showed higher ab undance in the inner shell sediment. Removal of life-position shells in a s hell bed negatively affected the abundance of C. insidiosum and the polycha ete Heteromastus similis. Shell introduction in a mud flat had a positive e ffect on the abundance of almost all the taxa encountered. Two predator exc lusion experiments indicate that these shells protect the fauna living with in the shells and the underlying shell infauna from motile surface predator s. Observations of shorebird feeding behavior showed that they rarely peck into the shell cavities. Daily measurements of the protruding length of she lls demonstrated that these beds are exposed to erosive regimes, and shell removal in a shell assemblage destabilizes the sediment. The evidence sugge sts that superficial shells positively affect the diversity and abundance o f organisms.