Bioecology of the Ghost CrabOcypode quadrata(Fabricius, 1787) (Crustacea: Brachyura) Compared with Other Intertidal Crabs in the Southwestern Atlantic

Abstract

Data sets on the natural dynamics of beach ecosystems arc scarce and fragmentary. Such data arc necessary for implementing more efficient monitoring programs that quantify the dynamics of key ecological attributes on sandy beaches. This article contributes to the bioecology of ghost crabs from subtropical Praia Brava, Itajaí, Santa Catarina. Ocypode quadrata occurs in sandy beaches along the tropical-temperate western Atlantic, from Rhode Island (US) to Rio Grande do Sul (Brazil). During 14 consecutive months, a total of 649 specimens were captured: 255 females (39%), 241 males (37%), and 153 juveniles of undetermined sex (24%). Highest densities were recorded in June and November, with a total of 1,900 burrows distributed along the beach (56.95%) and dunes (43.05%). Sixteen natural diet items were identified for this crab, with a larger participation of Apis spp. (38.97% of relative volume). In the local food web, the ground-burrowing owl Speotyto cunicularia was the main crab predator. This article indicates that the ghost crab represented the second most consumed food item of the owl (29.24%), only surpassed by rodent remains, which occupied 50.32% of its stomach volume. The natural diet and the main predator of the ghost crab had not been observed along the coast of Brazil. Knowledge of natural diet is fundamental for understanding distribution patterns, migrations, and molting cycles. Further information on population structure (abundance peaks in spring and summer; a negative allometric growth pattern), spatial distribution (predominance of females in August and May, and of males in July; size of specimens increases toward higher intertidal levels), sexual proportion throughout the year (reproduction is continuous), form and disposition of burrows (reduced abundance toward the higher tidal levels), and relative importance for the diet of the main predator (both predator and prey arc nocturnal) may be used as instruments for evaluating the occupational impact and for conserving natural dune and beach environments along sandy beach ecosystems. In conclusion, ghost crabs provide an alternative source of food for the ground-burrowing owl, a predator that may represent an important population controlling factor, other than the human-induced impacts on sandy beaches. Although burrowing crabs are the most conspicuous and ecologically important invertebrates of intertidal beaches along tropical and temperate regions, factors that regulate abundance arc still controversial, whereas the biotic community interactions in sandy beaches remain barely known.