Stage-dependent interactions between intertidal crabs: from facilitation to predation

Abstract

Large parts of the south-western Atlantic soft bottom intertidals are inhabited by the burrowing crab Neohelice granulata (previously known as Chasmagnathus granulatus) and the mud crab Cyrtograpsus angulatus, but adults rarely coexist in the same microhabitat. We describe the influence of burrows of N. granulata on the recruitment dynamic of C. angulatus and the effects of different ontogenetic stages of N. granulata on survival of recruits of C. angulatus. A two summer sampling shows that N. granulata burrows facilitate settlement of both species. To evaluate the mortality of recruits inside burrows, we performed a field experiment with juvenile and adult crab exclusion cages and inclusion of juveniles or adults of N. granulata. The results showed differences in crab sizes between treatments, due to predatory interactions that depended on prey size. When only juveniles of N. granulata were present, the higher mortality of C. angulatus was observed in smaller crabs. However when adults of N. granulata were present, larger juvenile crabs are the ones that suffered the highest mortality. These results show that adults are preying upon larger juveniles (of both species) reducing the mortality of the smaller ones that are preyed by the larger ones. We also measured emigration from these burrowing assemblages using bidirectional pitfall traps which showed that C. angulatus juveniles are leaving the burrows towards the subtidal. Here we demonstrate that N. granulata have positive and negative effects on the settlement of C. angulatus. These results highlight the importance of including stage-dependence relationships to analyse species interaction in marine ecology field works.