The effects of beach charasteristics on ghost crab (Ocypode quadrata) size and distribution

Background/Question/Methods

Sandy beaches are dynamic environments inhabited by specific biota structured by physical forces. These environments are commonly exposed to natural and anthropogenic induced changes, such as storage, transport, exchange of sand, erosion and renourishment.  The ghost crab, Ocypode quadrata has been used as a bio-indicator of the health of sandy beaches based on anthropogenic and natural impacts, but there is a lack of research on how the species’ density, distribution and activity are affected by sediment movement. In this study, the coastline of Cape San Blas, Florida was evaluated for accretion or erosion in order to identify a possible correlation with ghost crab density and distribution. Three research sites were identified and classified as accreting, eroding or stable based on historical sediment movement patterns, and sampled three times a week for five consecutive weeks. For each sampling event, ghost crab burrows were counted and measured on three 30m by 1m plots per site, designated as littoral, berm and dune strata. Beach width was measured from the mean high water mark to the dune. Burrow count and size were compared within and between sites, based on beach width, time and strata with a basic fit model and a two tailed Fisher’s test. 

Results/Conclusions

The Fishers’ test reflected a significant difference when comparing strata between eroding and stable sites with a Pr (>| t |) value of  0.001 for both dunes. There was also a significant difference in size distribution for littoral area in these sites, with values of values of 0.001(eroding) and 0.01 (stable). The eroding site had a higher burrow count than the stable.  There was a negative correlation between beach width and burrow size. There was a gradient in vertical size distribution: smaller crabs appeared to concentrate close to the water line on the littoral area and the largest place themselves close to the dunes. These results indicate that ghost crabs may prefer more dynamic beach environments, and respond to size-specific needs by burrow placement on the beach.