MEPS 502:197-206 (2014)  -  DOI: https://doi.org/10.3354/meps10728

ABSTRACT: Using field surveys and stable isotope analyses of beach consumers and prey, we investigated whether the identity of dominant prey items of ghost crabs Ocypode quadrata differed across 3 barrier island beaches along the west coast of Florida, USA. Abundance of potential prey, mole crabs Emerita talpoida and coquina clams Donax variabilis, was determined from sediment cores collected from the swash zone at Anclote Key (ANC), Honeymoon Island (HI) and Cayo Costa (CC) from late April to September 2011. Concurrently, wrack-associated amphipods were gathered from supra-tidal areas, and ghost crabs were captured from these beaches. Stable isotopic signatures (δ¹³C and δ¹⁵N) of amphipods, mole crabs, coquinas and ghost crabs were identified and diets of ghost crabs compared across beaches using stable isotope analysis in R (SIAR) mixing models. Ghost crabs from CC and HI fed primarily on swash zone animals (mean contribution to diet = 67.6 and 68.4%, respectively), while those from ANC mainly consumed wrack-associated amphipods (mean contribution = 55.7%). ANC supported a comparatively low abundance of swash prey, contained moderate amounts of fine-grain sediments and retained a high biomass of seagrass wrack. In contrast, significantly greater abundances of swash prey were found on CC than on other beaches, and a low biomass of wrack was commonly recorded. This new information suggests that wrack may serve as an important marine subsidy and underlie a dietary shift observed for ghost crab consumers on some beaches. In addition to products transported onshore, beach morphology and features of inland habitats may contribute to variability in trophic structure of these subtropical beaches.

KEY WORDS: Food web · Ghost crab · Macroinfauna · Sandy beach · Stable isotope analyses · Subsidy · Swash zone · Wrack