Behavioral responses of the Japanese freshwater crab, Geothelphusa dehaani, to the leech, Myxobdella sinanensis

Abstract

Myxobdella sinanensis Oka, 1925 (Arhynchobdellida, Praobdellidae) is endemic to Japan, and was recently observed attaching to the freshwater Japanese crab, Geothelphusa dehaani White, 1847 (Decapoda, Potamidae), revealing that M. sinanensis is a facultative parasite of G. dehaani. Although they both inhabit headwater streams in Japan, their interactions and behavioral responses have not been investigated before. We hypothesized that G. dehaani reacts to M. sinanensis through (1) chemical cues, and (2) visual cues or physical contact in addition to the chemical cues. Two experiments were conducted in a container in which G. dehaani could hide under a rock on the bottom covered with sediment. The duration of activity outside the rock and distance moved before and after introducing rearing water of M. sinanensis, or live M. sinanensis, was measured. No statistical difference was observed in movement distance, behavior, and time outside the rock in G. dehaani, indicating limited sensitivity to chemical cues. G. dehaani remained beneath the rock for a statistically shorter period after M. sinanensis was introduced into the container, indicating its dependence on visual and/or physical cues. It showed altered behavior by remaining stationary and escaping, confirming importance of visual and contact stimuli to escape the ectoparasite.