The mechanism of semilunar emergence rhythm of the marine midge, Pontomyia oceana, was investigated. We used night light (1 lux) to entrain the emergence of the midge. Night light of 4 nights or more can effectively synchronize the semilunar emergence. Moreover, the night light has to be given about 10 days after fertilization to be effective. A batch of fertilized eggs of the marine midge emerge in two semilunar cycles, a second round of night-light treatment is necessary to synchronize the second peak of emergence. We also investigate whether the semilunar rhythm is dependent on the daily rhythm of the midge. Using different day length, from 20 to 28 hours per day, with equal light and dark periods, we want to know if the midges are counting numbers of light-dark cycles, or are independent of light-dark cycles, in determining their semilunar emergence. The results were intermediate between the two hypotheses. We suggest that the midge was not affected by light-dark cycle for the first 14 days of their life, afterward they count 15 light-dark cycles before emergence. This also explains how the second emergence peak occurs about 45 days after fertilization.

Two peaks of eclosion dates, about 15 days apart, occur in the same batch of fertilized eggs in the marine midge, Pontomyia oceana. Two hypotheses, the variable adaptive peaks and the bet-hedging hypotheses, were proposed as the ultimate factor of the polymorphic phenomenon. They were tested by experiments controlling feeding amount and photoperiod, as well as selective breeding experiments. The offspring eclosing in the two peaks do not differ in fecundities, egg diameters, thorax and head lengths of males; this is not compatible with the variable adaptive peaks hypothesis. Both peaks exist under various feeding and photoperiods, although peak ratios differed in the former. The results in the first peak lineage did not support there is genetic component in peak ratio determination. The experiments in the second peak lineage had much lower success rates, although the results seemed to suggest a genetic component. The results in a more extreme selection experiment did not support that there is genetic component either. The present results are more compatible with the bet-hedging hypothesis. Wind velocity may be a factor hard to predict by the midges, and it may cause reproductive failure of them. Whereas high emergence synchronization, a prominent feature of the marine midge, may have advantages in many aspects, it also concentrates the risk of total reproductive failure. Spreading offspring to more than one suitable eclosion peak, the midge may have sacrificed short-term reproductive rate for long-term fitness.