Combined engineering effects of clams and crabs on infaunal assemblages and food availability in intertidal systems

In soft sediments, ecosystem engineers (EEs) may play key roles in modifying habitats and therefore affecting bottom-assemblage species. In the southwestern Atlantic mud flats, 2 EEs coexist: the stout razor clam Tagelus plebeius and the burrowing crab Neohelice (Chasmagnathus) granulata. Clams create small depressions (i.e. millimeters), while crabs build large burrows (i.e. centimeters) generating crab beds covering many hectares. We hypothesized that these differences in the bioturbation scale may have different consequences for infaunal assemblages. We found that (1) microscale sediment-surface heterogeneities created by clams (e.g. holes and surrounding depressions) were related to higher organic-matter content and microphytobenthic biomass (measured as chlorophyll a), (2) abundances of meiofaunal groups (copepods, ostracods, and nematodes) were higher in clam holes than outside at all tidal levels, and (3) habitats with a more heterogeneous structure—such as clam holes inside a crab bed—had a higher food availability and an abundance of several meiofaunal groups (e.g. ostracods, and principally nematodes). Large-scale bioturbation (crab-bed formation) also affected primary producers, infaunal assemblages, and clam distribution, because at the highest intertidal levels clams were absent outside the crab beds. Our results thus demonstrate the differential effects of 2 contrasting EEs on the organization of soft-bottom communities and the key role of microheterogeneities in adding specific structures to already modified systems on a larger scale.