On a clocklike mechanism timing lunar-rhythmic reproduction inTyposyllis prolifera (Polychaeta)

Summary

In ‘constant’ conditions, individuals of the polychaeteTyposyllis prolifera stolonize up to 18 times each at circalunar intervals (mean ∼31 days). The endogenous mechanism driving this reproduction rhythm exhibits the characteristics of a true ‘circa’-oscillator (circalunar clock):

• 1. Entrainment of cyclic sexual development (and ultimately stolonial swarming) to the lunar cycle has been demonstrated both in the field and in experimental conditions. Animals in a natural population tend to reproduce synchronously during the 2nd quarter phase of the moon. In asynchronous laboratory populations, ‘moonlight’ zeitgeber programs (some successive ‘moonlit’ nights every 30 days) cause entrainment, provided that the zeitgeber signals exceed a critical length (between 2 and 4 days). Above this value, the length of zeitgeber stimuli does not affect the quality of entrainment. The peaks of stolon abundance are phase-set by the cessation of nocturnal illumination (mean at 20 °C: 17.0 days after ‘moonoff’), independent of the length of the preceding ‘moonlit’ period. The temporal pattern of increasing entrainment in the course of repetitive stimulation permits a preliminary insight into the mechanism which causes adjustment of the clock.

• 2. Within the range of 15 to 25 °C, the freerunning period is temperature-compensated (Q ₁₀=1.04). However, coupling between oscillator and overt rhythm is restricted to the (cyclic) initiation of stolonization processes. Stolon release follows at a temperature dependent interval. Changes in temperature thus result in phase-shifts of the overt rhythm of stolon release.

• 3. The oscillator runs essentially independent of the peripheral physiological processes involved in the manifestation of the reproduction rhythm (absence of feedback influences). Temperature and starvation influence the overt rhythm while the endogenous oscillator remains largely unaffected.