Effects of bioturbation by the fiddler crabLeptuca speciosa(Ives, 1891) (Decapoda: Brachyura: Ocypodidae) on mangrove peat in Barnes Sound, Florida, USA

Abstract

AbstractBioturbation, which includes burrowing and foraging behaviors, is an important component of the functional role of fiddler crabs (Brachyura, Ocypodidae) within mangrove forests because it modifies sediment properties and composition of mangrove substrates. In this study, fiddler-crab population density and burrow architecture were measured to evaluate the influence of bioturbation by the fiddler crab Leptuca speciosa (Ives, 1891) on mangrove peat from Barnes Sound, Florida, USA. Measurements of burrow architecture were used to estimate the contribution of fiddler-crab burrowing to the bioturbation of mangrove peat. Comparisons were made between the total organic matter in bioturbated sediments, including feeding and burrowing pellets. A population density of 12 burrows m–2 was measured with no significant trends in the spatial distribution of fiddler crabs within the site. Although the deepest burrow depth was 18 cm, results show the upper 5 cm of the peat surface was consistently burrowed by crabs. Fiddler crabs were estimated to increase the total below-ground air-peat surface area m–2 by 5% and accounted for 22% of the excavated volume of mangrove peat per year. Fiddler crabs will thus rework the entire peat substrate within five years. Because data on carapace geometric mean size suggested that a juvenile population of L. speciosa was sampled, the bioturbation rate of the peat substrate will accelerate once this population matures. Feeding pellets had significantly lower percentages of total organic matter (P < 0.01) than other bioturbated peat samples, suggesting that fiddler-crab foraging behaviors significantly (P < 0.01) decrease the organic composition of surficial peats. These results imply that fiddler crab burrowing extends the depth of the taphonomically active zone thus enhancing mangrove peat decomposition and changing the bioavailability and distribution of organic matter in mangroves.