CHEMOSENSORY-BASED CONTEXTUAL CONDITIONING IN HERMISSENDA-CRASSICORNIS

Ln two experiments, the marine mollusk Hermissenda crassicornis was ex posed to context dis crimination training. In one context, defined by the presence of a diffuse chemosensory stimulus (shellfish extract A), brief, unsignaled, unconditioned stimuli (USs; high-speed rotation) w ere presented; in a second context, defined by the presence of shellfi sh extract B, no USs were presented. Animals were then tested (at both 1.5 and 24 h) by exposing them to small pieces of the shellfish meat used to define the two contexts. The latency to strike at the meat ser ved as an index of the context-US association. In Experiment 1, the la tency to strike at the cue associated with rotation was reduced relati ve to both preconditioning strike latencies and the associatively neut ral cue. However, in a two-choice test where the animals could approac h the conditioned or neutral stimulus! the animals regularly avoided t he stimulus paired with rotation. Moreover, if, following conditioning , the animals were presented with an unsignaled rotation in the condit ioned context or the neutral context, the animals exhibited more effec tive defensive clinging (an unconditioned reflex normally elicited by rotation) in the conditioned context, suggesting that it ''prepared'' the animal for the aversive US. In total, these results demonstrate th at Hermissenda is capable of making associations to diffuse background (contextual) stimuli. Moreover, the results suggest that pairing the chemosensory cue with an aversive US elicits a strike response in Herm issenda when the animal is placed in forced contact with the cue and a n active avoidance response when the animal can choose between that cu e and a neutral cue.