Intertidal estuarine areas located between southern Brazil and northern Argentinean Patagonia are characterized by extended beds of the burrowing crab Chasmagnathus granulatus. Their activity leads to profound changes on the structure, quality and dynamics of sediments, affecting the entire benthic community and creating a patchy distribution of resources for other species. In this thesis I analyzed how the presence of C. granulatus crab beds affects the habitat use and the interactions with both the estuarine crab Cyrtograpsus angulatus and with juvenile fishes. The study was performed in adjacent areas with (“crab beds”) and without crab burrows in intertidal mudflats of the Mar Chiquita coastal lagoon (Argentina, 37 32’ S, 57 19’ W) from 2001 to 2005.

In the first chapter, I studied the habitat use of C. angulatus comparing abundances, sizes, sexual proportions, incidence of autotomies, and diet between areas with and without C. granulatus burrowing beds. The results indicate that C. angulatus is more abundant in areas without C. granulatus burrows. Only rarely does C. angulatus venture inside C. granulatus beds. Those that venture were always the larger crabs and in higher proportion males. After a long rainy period in which C. granulatus was absent from these areas, the pattern of habitat use of C. angulatus changed. During this period C. angulatus showed higher abundance in the areas where C. granulatus previously constructed burrows, and there were no differences between areas in any of the measured parameters. In the second chapter, I experimentally tested the hypothesis that in absence of C. granulatus, C. angulatus will build burrows, and that both species have similar effects on sediment characteristics, infauna and meiofauna abundances. The results show that C. angulatus does not use C. granulatus burrows but they built their own burrows when C. granulatus is excluded. Both species were similar in their effects on sediment characteristics and infauna abundances, but their effects on meiofauna were not similar. The interaction between C. granulatus and C. angulatus may be a good example of competitive exclusion when the shared resource is the access to surface soft-sediment.

In the third chapter I evaluate differences in habitat use by juvenile fishes above soft sediments inhabited by C. granulatus. There were more fish species in crab bed areas. Abundance of benthivores and planktivores fishes were also higher in crab beds during flood tide, but during ebb tide planktivorous fishes were more abundant outside crab bed areas while benthivores did not vary between areas. In the fourth chapter I evaluate whether the presence of this crab affects the predator-prey interaction between juvenile fishes and their benthic prey. Gut content and benthic prey selection by juvenile fishes inside and outside crab beds were evaluated, and predation effect was experimentally contrasted between areas using fish exclosures. The results shows that in crab beds, the percentage of fish with empty guts was lower and the number of polychaetes consumed by fish higher than outside crab beds. The silverside Odontesthes argentinensis and the catfish Pimelodella laticeps fed on larger polychaetes outside than inside crab bed areas, while the white mouth croaker Micropogonias furnieri preyed upon larger polychaetes inside crab beds. In addition, field experiments shows that fish predation decreases polychaete abundances only in crab beds. These results suggest that crab bioturbation affect habitat use by juvenile fishes and facilitate fish predation on benthic prey.

These results indicate that C. granulatus has important effects on the distribution and abundances of nektonic organisms. These effects can be mediated by direct interactions, as those observed between C. granulatus and C. angulatus, as well as by indirect interactions through modifications of sediment characteristics and infauna, as those observed with juvenile fishes.