Selective and differential avoidance learning in the feeding and withdrawal behavior of Pleuobranchaea californica

1. The marine gastropod mollusk Pleurobranchaea was avoidance conditioned by pairing food stimuli with conditional aversive electric shock. Yoked control specimens received explicitly unpaired food and shock in similar quantities to experimentals. All specimens were tested blind. 2. Experimental animals (N = 7) trained against squid homogenate (conditioned stimulus or CS) acquired an aversion to the squid, as evidenced by withdrawal from the CS (active avoidance learning; Fig. 1) and suppression of feeding behavior (passive avoidance learning; Figs. 2 and 3). The acquired aversion persisted for longer than 1 week, and was not displayed by control animals (N = 7). Statistically significant differences between experimentals and controls tested with squid were obtained in ten out of fifteen post-conditioning comparisons. 3. Experimental animals trained against squid homogenate, as well as control specimens in the same paradigm, showed a weak aversion to a homogenate of the sea anemone Corynactis (Figs. 1-3). The difference between experimentals and controls when tested with Corynactis was statistically significant in only two out of fifteen comparisons, however. It may be concluded that the learned aversion was specific to the CS associated with shock (squid). 4. 42 specimens were subjected to modified differential avoidance conditioning, using various combinations of squid, Corynactis and shrimp as the CS⁺ (the food stimulus paired conditionally with shock) and CS^- (the food stimulus not paired with shock). Differential avoidance learning, evidenced by a statiscally significant difference between responses to the CS⁺ and CS^-, was evident in the withdrawal and feeding behaviors of individual animals, individual experiments on small groups of animals and in the aggregate data from all experiments (Figs. 4 and 5). 5. When data were categorized by the combination of stimuli used as the CS⁺ and CS^-, the strongest differential learning was exhibited by the group in which Corynactis served as the CS⁺ and squid as the CS^- (Figs. 6-10). For certain combinations of food stimuli, differential learning was not obtained. 6. We conclude that the behavioral modification induced by avoidance conditioning of Pleurobran chaea's feeding and withdrawal behaviors is representative of genuine associative learning, and that the learned aversion is specific to the stimulus with which punishment is associated.