Variable swarming time of an intertidal midge (Pontomyia oceana Tokunaga, 1964) controlled by a circadian clock and temperature

Abstract

Emergence of the intertidal midge, Pontomyia oceana Tokunaga, 1964, occurred semilunarly in southern Taiwan. The mean emergence time, or capture time, was concentrated within 1–2 h each evening, but varied by 4–6 h relative to sunset and low tide on different evenings. We used light treatments to investigate whether an endogenous clock was involved. Three light-dark (LD) cycles were necessary for entrainment of the population. Once entrained, the midges retained the rhythm, with a period of 24.5–25.0 h, for several days without LD cycles. These results support the clock hypothesis. Additional laboratory experiments indicated that a temperature change (±5°C) on the day of emergence altered the emergence time. Thus, the swarming time was controlled by both endogenous rhythms and temperature, but was not tightly linked to sunset. The resulting concentrated but variable emergence time makes midge swarming hard to anticipate and may lower their mortality rates from predation.

Keywords:

• swarming
• insect
• intertidal midge
• Pontomyia oceana
• emergence
• synchronization
• behavior

Acknowledgments

We thank many volunteers for helping in field and laboratory work, three anonymous reviewers and Dr J. Purcell for many helpful comments. Keryea Soong was supported by grants for this study from the National Science Council and the Ministry of Education of Taiwan, ROC.