Movements of anemones symbiotic with hermit crabs: Effects of light, gravity, flow and conspecifics

Anemones, Calliactis tricolor (Le Sueur, 1817), are common symbionts associ ated with several genera of hermit crabs on the South East and Gulf Coasts of the United States. Anemones are reported to be mutualistic symbionts. Cr abs protect anemones from anemone predators and anemones protect crabs by s tinging crab predators. Previous studies show protection afforded by anemon es is dependent upon their location in relation to the aperture of the shel l. We conducted studies of anemone responses to physical factors (light, gr avity, and flow) and conspecifics to assess the role that anemone behavior plays in determining location on the shell. Anemones collected on crabs fro m offshore and from crabs in near-shore environments were induced to attach individually to upright cylinders and to shells positioned as if occupied by hermit crabs. Movements of individuals were followed for 48 h. Frequency analysis was performed to determine if anemone movements were random. Anem ones do not show a locomotory response to simple variations in direction of light. Most anemones (70-90% of over 200 individuals) display negative geo taxis in static conditions and move to high points on cylinders and shells. Most anemones show positive geotaxis in continuous flow conditions compara ble to those generated by a crab walking. The presence of a second anemone on a shell results in a more even distribution of anemones because anemones cannot occupy the same spot and interfere with each others' movements. Ane mone distribution is dependent upon movement behavior of the anemone in res ponse to gravity, flow and conspecifics. The anemone collecting and packing behavior reported for crabs of the genus Dardanus may be adaptive because a crab that packs its shell with anemones has a better chance of receiving protection than a crab that hosts only a single anemone.