The effect of the SW Atlantic burrowing crab Chasmagnathus granulatus on the intertidal razor clam Tagelus plebeius

The deposit feeding burrowing crab Chasmagnathus granulatus and the suspension feeding burrowing clam Tagelus plebeius occur sympatrically in the intertidal flats of the south western Atlantic estuarine environments. Common ecological theory predicts negative effects of deposit feeder bioturbation on co-occurring suspension feeders up to complete exclusion of the latter. By means of descriptive and experimental approaches performed at the Mar Chiquita coastal lagoon (37°32′S–57°19′W; Argentina) we show that within beds of C. granulatus (i) frequency of clam shell damage is higher, (ii) shell geometry shows proportionally shorter shells, (iii) clam condition is lower and, (iv) shell growth rates are lower. The negative effect of crabs increases towards lower tidal level. Changes in shell geometry are only detectable in damaged shells, suggesting that this pattern results from direct physical effects by crabs. The negative effect of crabs on clam condition and growth is mediated by indirect mechanisms. Burrowing and migrating of crabs may disrupt clam feeding and/or may force clams to invest more energy into escape movements and burrow rebuilding, thus leaving less energy for growth. Our study indicates that the adaptation of T. plebeius to a highly dynamic intertidal environment enables the clam to coexist with this strong bioturbator, too. The additional stress inflicted by C. granulatus does affect the clam negatively, but is not sufficiently high to eliminate it from the system.