Sensory and reflex physiology underlying cheliped flexion behavior in hermit crabs

Summary

• 1. Hair sensilla and proprioceptors of the hermit crab cheliped were analyzed with respect to a) morphology and anatomy, b) responses to tactile, chemical and joint position or movement stimulation and c) resistance reflexes evoked by proprioceptors. This was part of a larger study to determine the role of these sense organs in cheliped flexion behavior.

• 2. Hair sensilla on the propus and dactyl are innervated by units which responded to tactile or chemical (glycine) stimuli (Fig. 3). Chemosensory responses were both phasic (Fig. 3) and tonic (Fig. 4).

• 3. The CP2 chordotonal organ differs from previous descriptions of pereiopod proprioceptors in two respects: a) it does not attach to the stretcher muscle apodeme and is restricted to the side opposite the apodeme (Fig. 2C), and b) it has an accessory nerve with several sensory neurons which seem to detect dactyl movements via contact with closer muscle fibers (Figs. 2D, 7F). This could be the mechanism responsible for a previously-described bender resistance reflex evoked by dactyl closure (Muramoto and Shimozawa, 1970).

• 4. Resistance reflexes were observed (Figs. 8 and 9) which closely resembled those described for walking legs of crabs. It was concluded that these reflexes do not participate in the ongoing cleaning flexion motor program but, instead, must be centrally inhibited.