Fiddler crabs (Uca spp.) are one of the most conspicuous groups of bioturbating animals in mangrove forests, due to their colourful appearance and often high density on the sediment surface. They have been shown to dramatically alter the environment in which they live Robertson, 1986 and Dye and Lasiak, 1987. Fiddler crabs are surface deposit-feeders, which inhabit cylindrical burrows penetrating to a depth of 20–30 cm. The burrows act as mating place and refuge from predators during low tides as well as shelter during high tide (Warren, 1990). The burrowing activity of Uca and other burrow dwelling crabs (e.g., grasids) efficiently displace sediment particles (Botto and Iribarne, 2000), resulting in a more oxidized environment with a lower sulfide content (Morrisey et al., 1999).
In the present study, we examined the impact of fiddler crabs (Uca spp.) on sulfur and iron biogeochemistry in deposited shrimp pond waste used as a growth substratum for mangrove trees. Combined measurements of CO2 production, sulfate reduction rates and iron reduction rates were conducted in experimental plots over 1 year, which enabled us to identify the effects of burrowing fiddler crabs on the dominant diagenetic pathways in the sediments. Furthermore, two-dimensional distributions of sulfate reduction rates and sulfur and iron species were obtained on a millimetre-scale in sediment immediately surrounding the burrows to quantify the immediate effect of burrow structure on local anaerobic carbon oxidation and sulfide oxidation in shrimp pond waste and mangrove sediment.