Interaction between a mobile and a sessile species in SW Atlantic mudflats: Lack of negative effects of the mud crab Cyrtograpsus angulatus Dana, 1851 on filtration rate and growth of the infaunal clam Tagelus plebeius (Lightfoot, 1786)

Abstract

Biogenic modifications of benthic environment by organism's activities like feeding, burrowing or excretion have in general negative implications for cohabiting sessile infauna, especially for suspension-feeders. However, the outcome of these interactions depends on several biological and physical characteristics and thus, it is not easily predictable in soft bottom environments. The stout razor clam Tagelus plebeius, an estuarine, infaunal and sessile filter feeder species, coexists in Mar Chiquita coastal lagoon (Argentina, 37° 32′S, 57° 19′W) with the mud crab Cyrtograpsus angulatus, a very mobile benthic consumer. Due to the intense crab activity and consequent habitat transformation, we predicted negative effects on clams' filtration rate and their growth. However, an inclusion experiment showed that T. plebeius filtration rate was higher in enclosures when crabs were present responding to enhanced turbidity and chlorophyll a. Additionally, another experiment showed that mean condition index as well as mean shell growth increment of clams was not negatively affected by crabs, although maximum values of shell growth increments (mm) were smaller in crab inclusions. Our findings suggest that the clams can exploit the increased food availability (i.e., Chl a) generated by resuspension and are also not affected by the disturbance that crabs can make over sediment surface. In conclusion, contrary to the generalized outcome proposed for this kind of interaction, crabs do not have a negative effect on T. plebeius.