AVIAN PREDATION TRACES AND POSSIBLE ANTI-PREDATION STRATEGIES OF THE BLUE LAND CRAB IN THE BAHAMAS: PALEOBIOLOGICAL IMPLICATIONS

The blue land crab Cardisoma guanhumi is the largest land crab on San Salvador and adjacent Bahamian Islands and has fewer predators than other decapods. Whereas collection for food by the islanders (likely beginning with the Lucayan natives) and potential attacks by introduced dogs and rats (and wild hogs in other regions) have put pressure on blue crab populations, large birds are the primary predators on this keystone species of the coastal mangroves. Surveys in recent years revealed irregular openings on empty carapaces of C. guanhumi (many >10 cm in length), most of which are missing parts or all of the appendages. Typically, a single hole ranging from 4 mm to >10 mm occurs at the center or near the posterior edge of the carapace. Most openings lead to large cracks and some exhibit incipient fragmentation. Minor peeling on the underside suggests an “exit wound” effect, but this is less well developed than analogous punctures on molluscan shells (Belichnus isp.) due to a thinner and more brittle nature of the brachyuran exoskeleton. These marks are interpreted as beak impact traces, which likely incapacitated the prey. Of the ten species of ardeids present on the island, the great blue heron (Ardea herodias) and yellow-crowned night heron (Nyctynassa violacea) are the most likely tracemakers. In addition, a variety of stilts and seagulls are capable of feeding on smaller C. guanhumi individuals. It is assumed that many exoskeletons that sustained greater damage during predation have been fragmented and pose challenges for interpretation. Site selection of some decapod burrows beneath limestone clasts or dense mangrove pneumatophores is a common response to predation in supratidal wetlands. Furthermore, a bend in the upper segment of the blue crab burrow, typically located within 10 cm from the entrance, may be intended in part to avoid penetration by long-beaked birds. These findings have implications for interpreting similar traces on brachyuran carapaces in the sedimentary record, as well as for the design and morphological evolution of land crab burrows.