Hermit crabs alter sea anemone placement patterns for shell balance and reduced predation

Abstract

Little attention has been given to where on the snail shell symbiotic sea anemones are placed by hermit crabs. Based on previous studies, the locations of the anemones can greatly affect their ability to protect hermit crabs from octopus predators. In the present study, the anemone placement patterns of Pagurus pollicaris Say were studied to determine whether the crabs place Calliactis tricolor (Le Sueur) anemones on the snail shell randomly, for balance, or for maximum protection from an octopus predator. The hermit crabs were given access to detached anemones under six conditions: (1) shell weight unaltered, (2) weight added to the right side of shell, (3) weight added to the left side of shell (4, 5, and 6 had the same weight conditions as 1, 2, and 3, respectively, but chemical cues from an octopus were present in each). Under all six conditions, the anemones were placed nonrandomly on the gastropod shells by the hermit crabs. In both conditions with shell weight unaltered (1 and 4), hermit crabs placed anemones primarily on the top and right side of the shell. When the octopus was present, however, more anemones were placed bordering the shell aperture (sites best for protection from octopuses) than in the control (1). When weights were added to the shells, the larger anemones were generally placed in locations furthest from the weight, while the smaller anemones were placed more randomly. The chemical presence of an octopus in conditions 5 and 6 did not greatly affect anemone placement patterns. Overall, balance was apparently the most important factor affecting the placement of anemones on shells by this hermit crab.