Physical factors and neurosecretion in the control of reproduction in Melampus (Mollusca: Pulmonata)

Melampus bidentatus a saltmarsh snail living in the high littoral zone, lays eggs in phase with the moon, three to four times in early summer. Repetitive cycles of aggregation, egglaying, and dispersion were quantified in field studies. Physiological preparedness for breeding begins with an essential winter vernalization (<5°C). Breeding condition (growth of ovotestis, etc.) is induced in early spring only after both photoperiodic (>14 hours per 24 hours) and temperature (>15°C) thresholds are exceeded. Animals collected during winter could be induced to lay eggs if given spring‐like conditions; progressively shorter induction periods were effective as spring approached. Yearround histological studies of neurosecretory cells in the central ganglia of field animals showed staining patterns in two groups of neurons that were closely related to reproductive events. The medium cells (N =7) of the lateral lobe in each cerebral ganglion showed an accumulation of neurosecretory material (NSM) only in April, during the period when the natural photoperiod first exceeds 14 hours. These cells may mediate the light requirement and stimulate early reproductive events, such as ovotestis growth. The giant cells (N = 15) of the visceral ganglia stained heavily only during the reproductive season. They showed repetitive cycles of accumulation and subsequent depletion of NSM in phase with egg‐laying cycles. These neurons, signalled by some unknown but semilunar‐related cue, apparently synthesize and store NSM prior to each egglaying; release of this material may initiate the behavioral and physiological events of egg deposition.