Abundance of Calcinus tibicen on Millepora spp.: Surveys in 2010 were performed during the day at Tektite and Yawzi Point in Great Lameshur Bay. At each site, 25-m transects were placed parallel to the 2-6 m depth contour and Millepora spp. colonies within 1 m of either side of the transect were censused for size and abundance of C. tibicen. The benthos was also searched for C. tibicen to determine the extent to which they occurred on surfaces other than Millepora spp. Millepora spp. was scored for size and presence of C. tibicen. The sizes of C. tibicen were estimated indirectly through the sizes of the gastropod shells they occupy.

Nocturnal surveys: Surveys were completed at night in 2011 to test the possibility that C. tibicen leave Millepora spp. after dark. This portion of the study was motivated by the strong likelihood that C. tibicen is an omnivorous detritivore and, therefore, eventually would have to leave their Millepora spp. to forage. Presumably, this behavior might best be accomplished at night when they could escape visual fish predators.

Site fidelity: Hermit crabs were collected and grouped by the colony from which they were collected, and their Millepora spp. colonies marked so that tagged crabs could be returned to the same colony. Site fidelity was expressed as the percentage of tagged hermit crabs remaining on each colony over the census period. Results table.

Behavioral response: To evaluate the capacity for C. tibicen to find Millepora spp., two experiments were conducted. First, a displacement experiment was conducted in 2010 and 2011, in which C. tibicen were removed from Millepora spp. colonies during the day, placed on adjacent algal turf, and observed for 5 min to determine whether they returned to the same Millepora spp. Second, a laboratory experiment was conducted in 2011 in which C. tibicen were placed in a Y-maze and presented with a choice of Millepora spp. versus seawater.

Results of Y-Maze Choice:
Millepora: 29
Other: 14
Total: 4

Predator avoidance: To gain insight into the mechanistic basis of the Millepora- Calcinus association, a tethering experiment was conducted in which the losses of secured C. tibicen were determined on Millepora spp. versus sand/algal turf.

day_night.csv