The hermit crab Calcinus tibicen lives commensally on Millepora spp. in St. John, United States Virgin Islands

The present work describes an association between the hermit crab Calcinus tibicen and milleporine hydrocorals on shallow reefs (<6-m depth) in St. John, US Virgin Islands. In one bay, most colonies of Millepora spp. were occupied by C. tibicen in 2010 (62 %) and 2011 (50 %). In 2011, the association was common along 23 km of the coast of St. John, as well as at several locations around St. Thomas. On average, a colony of Millepora spp. harbored 4 C. tibicen within its branches, but more crabs were found on bigger colonies. During the day, large numbers of C. tibicen were found on Millepora spp., and these crabs frequently (>88 % of trials) returned to the same colony of Millepora spp. when removed and placed on adjacent surfaces. Of the C. tibicen found on Millepora spp. during the day, 48 % left their colonies at night, but most subsequently returned to the same colony as shown by the high site fidelity of tagged crabs (51 % over 5 days). A Y-maze experiment conducted in the laboratory suggested that C. tibicen could detect (and move toward) Millepora spp. on a spatial scale of about 30 cm and under a flow speed of about 5 cm s-1. When tethered on algal turf or sand, 45 % of C. tibicen disappeared over 7 days and presumably were eaten, whereas 15 % disappeared when tethered on Millepora spp. These results demonstrate that the association between C. tibicen and Millepora spp. is temporally stable and widespread, and suggest that hermit crabs seek Millepora spp. to secure a daytime refuge from predators. In the absence of negative fitness consequences for Millepora spp., but demonstrable benefits to C. tibicen, we propose that the Calcinus- Millepora association is commensal.