COMPARATIVE INDICATIVE MEANING OF DECAPOD BURROW COMPONENTS: FOCUS ON LOW-SUPRATIDAL CARBONATES

A variety of land crab species spend large parts of their life cycle in supratidal burrows, making them potentially valuable paleoenvironmental indicators. Cardisoma guanhumi (blue land crab) inhabits mangrove and related habitats along the Atlantic margin, from Florida to Brazil, and has been the subject of recent ecological investigations. However, few efforts have addressed the ichnology of this brachyuran, apart from general burrow structure analysis and environmental boundary conditions. We present examples from several sites in the Bahama Archipelago (mangrove and blue hole margins, San Salvador Island; phytodetrital wetland, Eleuthera Island), where blue land crab bioturbation has been investigated using non-invasive geophysical techniques and casting. Due to the requirement for gill immersion in shallow water, the basal sections of C. guanhumi chambers are closely correlated with long-term high-tide level, making them potential indicators of past biotope boundaries (wetland/upland) and sea level position. Its dwelling chambers (=Macanopsis isp.) satisfy the key requirements of an effective sea-level indicator: in situ occurrence, wide regional distribution, limited vertical range, known relationship to tidal/groundwater level index points, ability to be age-dated, and high preservation potential. This crab exhibits physical and trophic traits (molting, diet, scavenging, and food storage) that make its burrow fill superior for radiocarbon dating compared to other supratidal decapods (beach-dwelling ocypodids). Saltmarsh peat bioturbated by small ocypodids (Uca spp.) may provide additional sea-level index points, however it has a limited distribution. Preservation of the basal C. guanhumi burrow parts is also greater compared to the upper shafts and mounds of intertidal crustaceans (callianassids, upogebiids). Ultimately, rapid lithification of coastal carbonate deposits potentially improves the preservation of the trace fossil record in these tropical deposits.