DRILLING PREDATION BY CASSID GASTROPODS ON ECHINOID PREY IN A TROPICAL MARINE SETTING

This project reports quantitative surveys of echinoid tests drilled by cassid snails at two shallow-water tropical sites located on the island of San Savador, the Bahamas. Multiple specimens of recently deceased echinoids were collected both in-situ from shallow marine environments and from beach shell assemblages. The frequency, size and location of drillholes attributed to predatory gastropods were recorded. The first collection site, Sand Dollar Beach, located south of Rocky Point on the northwest side of the island (24°06’22” north, 74°31’09” west), was systematically surveyed along a 150 by 150 meter transect that ranged from beach to shallow water (<3 meters). The species Meoma ventricosa and Leodia sexiesperforata were found at this locality. The second collection site, Fernandez Bay, located south of the Cockburn Town on the west side of the island (24°02’07” N, 74°31’32” W), was surveyed along a 1-km north-south transect of intertidal beach. Tripneustes ventricosus and a species of Echinometra were found at this locality. Wet lab experiments and field observations confirm that cassid gastropods prey upon these echinoids, drilling easily identifiably, irregular sub-circular holes in the prey tests. Total numbers of specimens collected were 94 for Meoma ventricosa, 67 for Leodia sexiesperforata, 84 for Echinometra sp., and 21 for Tripneustes ventricosus. Overall drilling frequency was 97.9% for M. ventricosa, 85.5% for L. sexiesperforata, 44.0% for Echinometra sp., and 33.3% for Tripneustes ventricosus. Landmark analysis did not indicate any noticeable site-selectivity in drill-hole location suggesting limited behavioral stereotypy in spatial site of the attack. However, the oral side of the test was preferentially drilled in M. ventricosa (85.9%) and L. sexiesperforata (96.2%), whereas the aboral side was preferentially drilled in Echinometra sp. (73.0%). All analyzed localities suggest extremely high rates of drilling predation, although preferential preservation of drilled tests may have also influenced the observed patterns. The differences in species composition and predator-prey interactions between the two relatively proximal study sites points to the extremely patchy nature of ecological interactions in tropical marine settings.