Photoperiodic control of the albumen gland by neurosecretory canopy cells in the pond snail Lymnaea stagnalis.

Abstract

The pond snail Lymnaea stagnalis displays clear photoperiodism; it lays more eggs in long-day conditions than in short to medium-day conditions. The lateral lobes (LLs) are paired small budding structures of the cerebral ganglia and regulate egg laying as a coordinating center between reproduction and body growth, and LL ablation causes gigantism and attenuates egg laying. Our previous study demonstrated that the canopy cell (CC) in LL photoperiodically changes the excitability; it excites more in long-day conditions than in medium-day conditions. Here, we examined the function of CC in egg laying by the microsurgery. As a result, in long-day conditions, snails without CCs exhibited smaller albumen gland somatic index (ASI, proportion of the albumen gland/soft body) and smaller total number of egg masses laid than intact snails. In contrast, in medium-day conditions, there were no significant differences in ASI among intact, sham and CC ablation groups. Since the albumen gland produces galactogen-rich perivitelline fluid and supplies eggs with it, the CCs seem to promote egg mass production via acceleration of the perivitelline fluid production in long-day conditions. It is noteworthy that CC ablation had no effects on gonadal somatic index (GSI, proportion of the ovotestis/soft body). Besides, CC ablation caused slight increase in shell size and body weight. These imply that CCs are at least partially responsible for gigantism and decrement of egg laying caused by LL ablation, and we revealed another regulator in photoperiod-dependent egg laying in L. stagnalis. Our findings contribute to understanding neuroendocrine mechanisms underlying molluscan photoperiodism.