TIDAL FLOODING AND THE DISPERSAL OF MELAMPUS COFFEUS ON MANGROVE PEAT SUBSTRATES, KEY LARGO, FLORIDA

Detritivorous coffee bean snails (Melampus coffeus) play an integral role in facilitating organic matter decomposition on exposed peat surfaces in mangrove swamps during low tides. Because M. coffeus breathes air, tidal flooding could restrict their dispersal and shield peat surfaces from detritivory; thus, increasing leaf mat thickness and peat particle size. We investigated the impact surface exposure has on M. coffeus dispersal in mangrove swamps at Barnes Sound, Key Largo, FL, during two tidal stages: spring (SLT) and neap (NLT) low tides.

Snail abundance, shell length (mm), and mean water level (WL) were measured in six meter² quadrats in a basin site (B) with thick leaf mats (19-37 stacked leaves), which is generally flooded, and a fringe site (F) with thin leaf mats (0-4 stacked leaves), where the peat surface is generally exposed. Two-tailed T-tests were performed to compare mean shell length between and within sites at SLT and NLT.

At both sites, snails sought refuge on mangrove aerial roots during inundated NLT (WL_F: 13.8cm; WL_B: 16.7cm) but were on exposed peat surfaces during SLT (WL_F: 0cm; WL_B: 7.4cm). A higher snail abundance was observed in the fringe site (SLT: 73 snails/m², µ=14.1mm; NLT: 49 snails/m², µ=13.1mm) than the basin site (SLT: 29 snails/m², µ=13.4mm; NLT: 19 snails/m², µ=13.5mm). Between sites, snails were significantly larger in the fringe site during SLT (p<0.01) than the basin site, but there was no difference during NLT (p=0.19). Snails were significantly larger in SLT than NLT within the fringe site (p<0.01), but there were no differences within the basin site (p=0.69).

Proffitt et al. (1993) reported M. coffeus seek refuge from high tides by climbing mangrove roots after foraging on peat surfaces during low tides. Our study indicates that M. coffeus dispersal during low tides depends on tidal stage: SLT allow access to the peat surface for detritivory; whereas NLT shield the peat surface and restrict snail dispersal concomitant with increased vertical migrations on aerial roots. Thus, widespread access to the peat surface could result in larger snails and thinner leaf mats.