MANGROVE CRABS AS ECOSYSTEM ENGINEERS: THE INFLUENCE OF BURROWING BY FIDDLER CRABS (LEPTUCA SPECIOSA, IVES 1891) ON MANGROVE PEAT, BARNES SOUND, FLORIDA

Mangrove crabs are ecosystem engineers in tidally influenced mangrove wetlands. Burrowing by these crabs modifies the amount and particle size distribution of organic matter (OM), drainage, redox conditions, and nutrient availability within mangrove soils. Of these burrowing crabs, deposit-feeding fiddler crabs (Brachyura: Ocypodidae: Uca) are more dominant in South Florida than their leaf-shredding counterparts (Brachyura: Sesarmidae), which dominate Indo-West Pacific mangroves. To evaluate the influence of burrowing by the fiddler crab Leptuca speciosa (Ives, 1891) on mangrove peat from Barnes Sound, Florida, we assess L. speciosa population density and burrow architecture. In each m² quadrat (n = 7), we use burrow counts to estimate crab population density and paraffin wax to cast 3 – 4 burrows. Seven pitfall traps were used to capture crabs for carapace size measurements, which we use to calculate carapace geometric mean size (CGMS). Five dimensions of burrow architecture were measured and analyzed from 25 complete burrow casts: total burrow depth (TBD), total burrow length (TBL), horizontal length (HL), burrow volume (BV), and diameter of the burrow openings (BD). We further use these measurements to estimate the contribution of burrowing by L. speciosa to the bioturbation of mangrove peat. We measure a population density of 11.86 (± 4.98) burrows / m² and an average CGMS of 7.99 (± 1.99) mm. Based on Kruskal-Wallis tests, there is no difference in burrows / m² (H = 6, df = 6, p = 0.42) nor in CGMS of captured L. speciosa (H = 19.09, df = 13, p = 0.12) among quadrats. We observe no differences among burrow architecture. Our results show that on average the upper 5.39 cm (deepest BD = 17.98 cm) of the peat surface is burrowed by L. speciosa. Thus, we calculate that L. speciosa increases the total below ground air-peat surface area per m² by 4.82 % and accounts for 6.23 x 10^-2 % of excavated volume mangrove peat per day. These results suggest that fiddler crab burrowing extends the depth of the taphonomically active zone, which likely enhances mangrove peat decomposition and changes the quality, bioavailability, and distribution of OM in mangroves.