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

Feeding by host hermit crabs Dardanus pedunculatus on their symbiotic sea anemones Calliactis polypus was investigated using animals collected at Shirahama, Wakayama Prefecture, Japan. In the first experiment, changes in the number of sea anemones on hermit crab shells were recorded in single- and double-crab trials without food and single-crab trials with food. The number of sea anemones significantly decreased under starved conditions. The extent of this decrease per single hermit crab was higher in the double-crab trials than in the single-crab trials. Direct observations and video recordings showed that hermit crabs occasionally removed sea anemones from their own shells, and also from partners' shells in the double-crab trials, and consumed them. In the second experiment, fed and unfed hermit crabs with or without 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 anemones, which indicates some value of the latter as food. We offer some speculations on the course of development of this symbiosis, with predation on sea anemones having played an important initial role.