Resting Traces of Ocypode quadrata Associated with Hydration and Respiration: Sapelo Island, Georgia, USA

Abstract

This report adds resting traces to the behavioral suite of traces made by the modern ghost crab Ocypode quadrata, but with emphasis on their association with hydration and respiration in intertidal environments. Resting traces were observed on sandy upper shoreface surfaces of Sapelo Island (Georgia, USA) immediately after high tide. The best preserved of these traces were impressions of the ventral anatomy of O. quadrata that connected with trackways, the latter indicating movement of tracemakers away from body impressions. Crabs formed the traces by moving from the upper shoreface to the surf zone to rehydrate their gills, which they facilitated by settling into saturated sands following high tide. Ventral surfaces of crabs compacted sediments sufficiently to form body and leg impressions. An ebbing of the tide and corresponding drainage of pore water caused the best-preserved traces. Later observations of this behavior in O. quadrata confirmed the ichnological interpretations. The ichnological significance of the traces is that they hint of more detailed behavioral interpretations for arthropod resting traces in the geologic record and more precise definitions of the Skolithos and Psilonichnus ichnofacies. They also may provide a behaviorally based explanation for the preservation of fossil semi-terrestrial crabs in upper shoreface facies.