Development of learning and memory in Aplysia. I. Functional assembly of gill and siphon withdrawal

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Abstract
The marine mollusc Aplysia california provides an excellent preparation with which to examine the development of the neuronal control of behavior for 2 reasons: first, adult Aplysia exhibit a variety of behaviors that are well understood in cellular terms; and second, the development of Aplysia from embryo to adult has been studied in considerable detail. Among the best understood behaviors in Aplysia are the withdrawal responses of the mantle organs (the gill, siphon, and mantle shelf), which exhibit 2 different kinds of behaviors: (1) "spontaneous that are part of a fixed action pattern, a respiratory pumping sequence or the mantle organs, and (2) reflex contractions in response to tactile stimuli. We have examined the development of both of these withdrawal behaviors in juvenile stages 9-12 and found that they are functionally assembled according to different ontogenetic timetables. Spontaneous contractions. As soon as the siphon and gill emerge, in stages 9 and 10, respectively, they each show a high rate of spontaneous contraction that gradually diminishes throughout subsequent stages until it reaches the low rate typical of adults (stage 13). Since the siphon emerges first, it already exhibits a significant decline in its spontaneous activity (e.g., in stage 11) when the gill''s spontaneous activity is at its highest. In addition to a developmental trend in the rate of contractions, there was also a clear developmental progression in the degree of cocontraction of the siphon and gill during spontaneous contractions. In adults, the siphon and gill show a very high degree of cocontraction during spontaneous pumping. However, in juvenile animals, there was a very low degree. Thus, if appears that the siphon and gill withdrawal components of the fixed action pattern become progressively more functionally coupled during juvenile development. Reflex contractions. As soon as the siphon and gill emerge in their respective developmental stages, they exhibit a brisk withdrawal reflex to tactile stimulation of the siphon. Moreover, at each developmental stage, reflex siphon contractions were graded as a function of stimulus intensity, as they are in the adult. Finally, throughout development tactile stimulation of the siphon invariably evoked coincident contractions of both the siphon and the gill, which is characteristic of the adult reflex. Thus, unlike the fixed action pattern that takes several weeks to mature, the defensive withdrawal reflex closely resembles the adult form as soon as the effector organs emerge during juvenile development. Our results show that a reflex and a fixed action pattern, which utilize a common set of effector organs, emerge according to different developmental timetables. It is now possible to begin to test specific hypotheses concerning the developmental assembly of these behaviors on a cellular level.