Predation on symbiont sea anemones by their host hermit crab Dardanus pedunculatus

Feeding by host hermit crabs Dardanus pedunculatus on their symbiotic sea a nemones Calliactis polypus was investigated using animals collected at Shir ahama, Wakayama Prefecture, Japan. In the first experiment, changes in the number of sea anemones on hermit crab shells were recorded in single- and d ouble-crab trials without food and single-crab trials with food. The number of sea anemones significantly decreased under starved conditions. The exte nt of this decrease per single hermit crab was higher in the double-crab tr ials than in the single-crab trials. Direct observations and video recordin gs showed that hermit crabs occasionally removed sea anemones from their ow n shells, and also from partners' shells in the double-crab trials, and con sumed them. In the second experiment, fed and unfed hermit crabs with or wi thout sea anemones were examined for body weight changes. Fed hermit crabs gained weight whereas unfed hermit crabs lost it. The degree of weight loss in unfed hermit crabs was significantly higher in those without sea anemon es, which indicates some value of the latter as food. We offer some specula tions on the course of development of this symbiosis, with predation on sea anemones having played an important initial role.