Aspects of copulatory behavior and peptide control of egg laying in Aplysia

The atrial gland of the marine mollusc Aplysia is associated with the large hermaphroditic duct of the reproductive system and contains several peptides capable of inducing egg laying. The structure and function of these peptides are briefly reviewed. It has been hypothesized that during copulation the atrial gland of the female is stimulated by penile insertion to release its peptides, which in turn initiate events leading to egg deposition. To test this hypothesis we monitored reproductive activity over periods of weeks in individual, paired, and grouped A. brasiliana. It was found that copulation is not a necessary stimulus for egg laying, because individually housed Aplysia lay more eggs than when they are paired and allowed to copulate. Nor is copulation a sufficient stimulus, because the vast majority of copulations are not followed by egg laying. Simultaneous egg laying and female copulatory behavior were often observed with grouped and paired animals, but these events are probably not causally related. It is concluded that although the atrial gland contains at least three peptides that can induce egg laying, stimulation of this gland during copulation does not normally serve to initiate egg laying.