Manipulation by the crab claw is dependent upon chordotonal organ afference

Abstract

In crabs, the propus-dactylus (PD) chordotonal organ consists of about 80 bipolar sensory neurons whose dendrites are inserted into an elastic strand that spans the distalmost joint of chelae (claws) and walking legs. Movement of the dactyl to a closed position elongates the strand, while opening the dactyl shortens the strand. In response, the sensory neurons signal movements and positions. Is the crab dependent upon afference from the PD organ for voluntary control of the dactylus of the chelae? We quantified the manipulations of a standard food item by Dungeness crabs Cancer magister with intact chelipeds. Next, crabs were fed after the distal end of the elastic strand of the PD organ was detached (dPD) from the dactylus of both chelae or following a sham operation. All crabs fed avidly, but, in contrast to preoperative and sham crabs, statistical analysis revealed that dactyl-dependent manipulation (gripping, scraping, grasping, pull- ing, carrying, and cutting) by dPD crabs significantly decreased; crabs rarely used the dactyl fol- lowing detachment of the elastic strand. Manipulations not dactyl-dependent that involved the chelipeds and walking legs to aid feeding (cradling, stabilizing, and guiding) significantly increased. Physiological recordings indicate that the position-sensitive neurons of the PD organ fire tonically following detachment, but the firing rate is greater than that characteristic for the dactyl open position. Results support the hypothesis that feedback from the PD organ is required for the initiation and execution of precise contractions of the opener and closer muscles during directed movements of the dactyl. J. Exp. Zool. 279:579n586, 1997. © 1997 Wiley-Liss, Inc. Chordotonal organs (CO) are proprioceptors that provide feedback regarding joint movement and position in arthropod limbs. Those of Decapoda Crustacea have been subjects of considerable ana- tomical and electrophysiological study. Each or- gan consists of either an elastic strand or sheet in which are imbedded sensory neuron endings (Whitear, '62). The organs have been named with the initials of the joint they monitor. For instance, the organ spanning the propus-dactylus joint is referred to as the PD organ (Burke, '53). The elec- trophysiological responses of CO neurons fall into two general categories: large units that fire phasically during movements and small units that fire tonically to maintained positions of joints (Wiersma and Boettiger, '59; Wiersma, '59). Out- put by CO has been shown to mediate resistance (Bush, '62, '65; Evoy and Cohen, '69; Spirito et al., '72), assistance (Vedel, '80, '82; DiCaprio and Clarac, '81), and intersegmental reflexes (Vedel et al., '75; Bush et al., '78; Clarac et al., '78).