Ghost Crab Spatial Distribution - 2016

Fall 2016 survey of Ghost Crab (Ocypode quadrate) distribution along Folly Beach, conducted by the Fall 2016 Graduate-level Marine Invertebrate Zoology Course (College of Charleston) 

Students: Baylye Boxall, Lorenzo Fruscella, Jefferson Canann

Introduction

Ocypode quadrata is a species of ghost crab found on sandy beaches along the Atlantic coasts of North and South America. Its range of distribution extends from Rio Grande do Sul in the Southern Region of Brazil up to Massachusetts, USA (Negreiros-Fransozo et al., 2002). The Atlantic Ghost Crab, O. quadrata, is the dominant predator in the simple, filter-feeding based food chain in which it resides (Wolcott, 1978). A vast majority of the diet for O. quadrata comes from live prey, but they are facultative scavengers and will readily consume nearly any form of organic matter (Wolcott, 1978). Ocypode quadrata have also been noted to exhibit diurnal deposit-feeding behavior which may act as an important adjunct to predatory feeding (Robertson & William, 1981). 

In order to avoid both predation and desiccation during the day, O. quadrata burrow deep into the sand. The size of the openings to these burrows can be used as a proxy for crab size (Turra et al., 2005; Valero-Pacheco et al., 2007). Ocypode quadrata size and population density have been noted to be affected by anthropogenic influences and therefore might act as assessments of human impacts on sandy beach ecosystems (Steiner, 1981; Barros, 2001; Turra et al., 2005; Neves & Bemvenuti, 2006; Aheto et al., 2011). Average Ocypode quadrata burrow diameter and relative population density were surveyed along the length of Folly Beach to make such assessments.

Methods and Approach

Experimental Design

A total of eighteen locations along the coast of Folly Island, South Carolina were surveyed (Figure 1). Sections of coast were sampled at 6 locations roughly a mile apart. At each of the 6 locations, three soutward-walking transects, were conducted for 5 minutes each, parallel to the water line. Every crab burrow opening found along each transect was recorded, its diameter measured (mm), and its distance from the water line recorded (m).

Figure 1: Sampling locations along the coast of Folly Island, South Carolina. Markers on the map indicate the starting positions of each transect.

Data Analysis

Three plots were generated in R Studio illustrating the mean crab burrow opening diameter relative to location, density of openings relative to location, and relationship between density of openings and distance from the water line.

Results and Discussion

With respect to location, the fewest number of crab burrows were observed at transect locations: 13, 14, and 15, in which none were observed at location 13 (Figure 2). Coincidentally, location 13 is about 200 meters south of the Folly Beach Fishing Pier, a popular public beach access location and tourist attraction. Further south toward locations 16, 17, and 18, in the Folly Beach County Park, densities of crab burrows were higher, where less tourist traffic is present. A similar trend was observed on the northern end of the island at locations 1, 2, and 3. Based on these observations, ghost crabs are less likely to create burrows and reside in sections of Folly Beach that have a greater human presence, subsequently higher foot traffic, and frequent disturbance.

Figure 2: Geographical Distribution of Mean Density of Ghost Crab Holes. Locations are numbered from northeast to southwest. The Folly Beach Fishing Pier is located about 200 meters from location 13, where no crab burrows were observed.

At locations 16, 17, and 18, where the highest densities of crab burrows were observed, it was not the area with the largest diameter of burrows (Figure 3). This may indicate that crab burrow size is a density-dependent attribute. Similarly, locations 14 and 15 near the fishing pier had fewer crab burrows but relatively large mean burrow diameters, further confirming a density-dependence favoring smaller burrows at higher densities of crabs and larger burrows and lower densities.

Figure 3: Geographical Distribution of Mean Diameter of Ghost Crab Holes. Locations are numbered from northeast to southwest. The Folly Beach Fishing Pier is located about 200 meters from location 13, where no crab burrows were observed. 

Bibliography and Resources

Aheto, D., C. Asare, E. A. Mensah, & J. Aggrey-Fynn. 2011. Rapid assessment of impacts on exposed sandy beaches in Ghana     using ghost crabs (Ocypode spp.) as ecological indicators. Momona Ethiopian Journal of Science 3(2): 93-103.

Barros, F. 2001. Ghost crabs as a tool for rapid assessment of human impacts on exposed sandy beaches. Biological Conservation 97(3): 399-404.

Negreiros-Fransozo, M., A. Fransozo, & G. Bertini. 2002. Reproductive cycle and recruitment period of Ocypode Quadrata (Decapoda, Ocypodidae) at a sandy beach in Southeastern Brazil. Journal of Crustacean Biology 22(1): 157-161.

Neves, F. M. & C. E. Bemvenuti. 2006. The ghost crab Ocypode quadrata (Fabricus, 1787) as a potential indicator of anthropic impact along the Rio Grande do Sul coast, Brazil. Biological Conservation 133: 431-435.

Robertson, J. R. & P. J. William. 1981. Deposit-Feeding by the ghost crab Ocypode quadrata (Fabricus). Journal of Experimental Marine Biology 56(2-3): 165-177.

Steiner, A. 1981. Recreational impacts on the distribution of ghost crabs Ocypode quadrata fab. Biological Conservation 20(2): 111-122.

Turra, A., M. A. O. Gonçalves, & M. R. Dendai. 2005. Spatial distribution of the ghost crab Ocypode quadrata in low-energy tide-dominated sandy beaches. Journal of Natural History 39(23):2163-2177.

Valero-Pacheco, E., F. Alvarez, L. G. Abarca-Arenas, & M. Escobar. 2007. Population density and activity pattern of the ghost crab, Ocypode quadrata, in Veracruz, Mexico. Crustaceana 80(3): 313-325.

Wolcott, T. G. 1978. Ecological role of ghost crabs, Ocypode quadrata (Fabricus), on an ocean beach: scavengers or predators? Journal of Experimental Marine Biology and Ecology 31(1): 67-82.