MORPHOMETRY AND INDICATIVE MEANING OF BLUE LAND CRAB (Cardisoma guanhumi) BURROWS

Abstract

A variety of land crab species spend large parts of their life cycle in supratidal burrows, making these structures valuable paleoenvironmental indicators. Specifically, the blue land crab (Cardisoma guanhumi Latreille, 1825) is a keystone species and widely distributed throughout tropical and subtropical regions, however few geologic efforts have been made to assess the biogenic structure produced by this decapod. We present, for the first time, a comprehensive (morphological and environmental boundary) analysis of C. guanhumi burrows using modern examples from a carbonate setting in the Bahamas. Ten burrow casts were collected from both a mangrove and interior blue hole environment on San Salvador Island, Bahamas. Measurements were taken from the casts and 2D photogrammetry and 29 key architectural metrics were analyzed using the Bray-Curtis similarity test to determine morphometric indices representative of the biogenic structures produced by C. guanhumi. Additionally, 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. Burrow sites were investigated to assess the range of groundwater perturbations over several tidal cycles, casts and in situ water samples were used to determine the magnitude of tidal dampening. Finally, the water conditions within C. guanhumi burrow end chambers and by extent within their biotope, are essential to support the physiological needs (ventilation, circulation, molting) of this decapod. Salinity thresholds has been investigated using non-invasive extraction of end chamber and biotope water. Results demonstrate high overall burrow morphology between the 10 cast (>0.80) and specifically, the basal end chamber exhibits the highest similarity index (>0.85). In addition our findings show the open-ocean mean tidal range of 0.57 m is reduced by 40% (~0.30 m) within blue holes and the North Pigeon Creek (NPC) mangrove shoreline. Further dampening to a mean amplitude of 0.10 m was measured at NPC burrow sites. Moreover the large terminal chamber is particularly important due to its preservation potential, repository of vegetation matter and potential skeletal remains, and a reliable water table (~sea-level) indicator. Lastly, the results of this study indicate that the blue land crab thrive in an ideal groundwater salinity range of 20-40‰, however evidence from locations indicates that they can live long enough to burrow into muddy substrates at lower salinities (7-10‰). Our findings demonstrate how a quantitative assessment of modern biogenic structures and their likely counterparts from the sedimentary record (e.g., Macanopsis plataniformis) can help constrain tracemaker identity and aid in paleoenvironmental reconstruction.