The intertidal burrowing crab Chasmagnathus granulata is one of the most abundant macro-invertebrate species on southwestern Atlantic estuaries. This crab is a deposit feeder that excavates large open burrows producing extensive assemblages (crab beds) of high densities of crabs (60 burrows/m2). Sediment comparisons between crab beds and areas without burrows showed that they enhance humidity, organic matter content and penetrability of the substrate. However, field experiments showed that funnel shape burrows trap clay-silt sediment that is then removed during low tide to surface (2.kg of dry sediment per m2 per day), resulting in a surface deposit of more cohesive sediment. Therefore, this bioturbator species stabilizes the substrate. This is different from subtidal bioturbators that directly move sediment into transport or other intertidal bioturbators, which remove disaggregated sediment that is then easily transported.

The efficiency of different burrow shapes in the trapping of sediment and detritus was also evaluated with field experiments. Burrow shapes did not influence the trapping of material when current velocities were high. However when burrows of different shapes were compared in low current velocities, the most common burrow shapes at field (width-depth ratio = 1) were the ones with higher capture of detritus in the form of small pieces of estuarine plants, and therefore with high percentage of organic matter. Burrows are distributed in the upper part of estuaries and saltmarshes, covering extensive areas between the marsh vegetation and the open estuary. These extensive burrow beds may also be considered a large retention area, reducing the amount of organic matter exported or imported from marshes.

Results of field experiments and periodical sampling showed that bioturbation by C. granulata also affects benthic community, particularly they negatively affect densities of the polychaetes L. acuta and Heteromastus similis and the micro-spatial (cm) distribution of nematods. Burrow beds also generate spatial heterogeneity that affects habitat use and feeding success of migratory shorebirds. Charadrius falklandicus forages more and with better efficiency inside crab beds, but Tringa flavipes and T. melanoleuca feed more frequently and Calidris fuscicollis has better efficiency in areas without crabs. This interaction between burrowing crabs and shorebirds must be considered in conservation endeavors.

All these results show that this crab plays an important ecological role in SW Atlantic estuaries from south Brazil to north Patagonia (Argentina). The same effects on sediment dynamics, retention of detritus and habitat use for shorebirds may be occurring with large assemblages of burrowing crustaceans in other estuaries of the world.