Behavioral Biology of Aplysia: A Contribution to the Comparative Study of Opisthobranch Molluscs

The book argues that simple behavioral modifications in Aplysia californica can be explained by identifiable changes at specific synapses within well-mapped circuits. It opposes the view that learning requires explanations at a level of abstraction beyond cellular neurophysiology. By documenting habituation and sensitization in the gill-withdrawal reflex, Kandel demonstrates that behavioral plasticity is not a mysterious emergent property but a measurable biological process. The monograph thus consolidates a reductionist research program: if a memory trace exists, it should be locatable in the nervous system.

Behavioral Biology of Aplysia (1979) followed Kandel's Cellular Basis of Behavior (1976) and was partly a consolidation and extension of that earlier synthesis. A 1979 review in The Quarterly Review of Biology noted that the chapters on the nervous system “draw heavily on the work of the author and his associates” and are “partial repeats” of the 1976 book, but that new material emerges on peripheral-central interactions, development, and species-specific behavior. The 1979 book is therefore less a standalone theoretical breakthrough than a focused restatement of the Aplysia program for a broader comparative audience.

Kandel (1979) argued that the peripheral nervous system primarily mediates local responses to strong stimuli and that its contribution could be minimized by using weak stimuli in experiments. Subsequent research on the gill-withdrawal reflex challenged this view. As summarized in later literature, evidence from multiple laboratories suggested that the peripheral nervous system is sufficient to mediate the behavior, that the central nervous system has not been shown to be necessary, and that the circuitry is more complex than the Kandel model accounted for. The current view is closer to the reverse of Kandel's 1979 position: the peripheral system handles weak to moderate stimuli, while the central system responds selectively to strong stimuli.

The book helped establish the framework of using identified neurons in a simple invertebrate to study the cellular basis of learning. The gill-withdrawal reflex and its habituation and sensitization became canonical examples of synaptic plasticity. This framework—locate a behavior, map its circuit, identify the cells, and measure changes at their synapses—became a template for cellular studies of memory. Kandel's Nobel Prize in 2000 was awarded for this line of research, and the 1979 monograph stands as a key mid-point consolidation of the program before its later molecular elaborations.

Contemporary reviews were positive about the book's synthesis while noting limitations. A review in Science (1979) by Frank Krasne treated it as a significant statement of the Aplysia program. A longer review in The Quarterly Review of Biology called it “excellent” and “superlative reading” for students of vertebrate behavior, but also criticized the classification and systematics chapters as “overly long and tedious” and noted occasional factual errors and a “condescending” tone in early chapters. The reviewer also observed that the book was in part a repetition of Kandel's 1976 work, suggesting it was seen as a focused restatement rather than a wholly new contribution.