Fiddler crabs (Uca thayeri, Brachyura: Ocypodidae) affect bacterial assemblages in mangrove forest sediments

To clarify the relationship between the detritivore fiddler crab, Uca thayeri, and sediment bacteria, we quantified morphotype richness, abundance and evenness of these microorganisms inside the crabs' guts and in mangrove-associated sediments in Sisal, Mexico (21 degrees 9' N, 90 degrees 1' W) from July to September 2008. Increased bacterial richness and abundance were observed in mesocosm experiments when nutrients were added to the sediment or in the absence of fiddler crabs. Thus, crab disturbance seems to play a role in shaping the bacterial assemblage by reducing richness and abundance just as nutrient limitation does. Crabs can also play a second role by harboring a subset of bacterial morphotypes inside their gut. We exposed sterile sediment to fiddler crabs and found that viable cells were expelled from the crab's gut and proliferated in previously sterile substratum. The bacterial community is thus structured by the foraging behavior of fiddler crabs since it benefits some bacteria and restricts others. By agar plating we have obtained conservative results, yet the data suggest that the crab influences the bacterial assemblage in two ways by allowing inoculation of the sand from the gut and reducing bacteria diversity through disturbance when foraging on sediments.