Genetic adaptation in emergence time ofClunio populations to different tidal conditions

Abstract

1. The emergence times of intertidalClunio-species (Diptera, Chironomidae) are correlated with special tidal conditions in such a way that the immediately following reproduction of the short-lived imagos can take place on the exposed habitat. 2. If the habitat of aClunio-species is situated in the middle tidal region and exposed twice a day by the tidal cycle (T = 12.4 h), a tidal rhythm of emergence with an average period of 12.4 hours may result (example:Clunio takahashii). 3. If the habitat is located in the lower tidal zone, exposed only at about the time of the spring tides, a semilunar rhythm of emergence is expected (examples:Clunio marinus andClunio mediterraneus). These semilunar rhythms are correlated with certain conditions of low tide which occur at the coastal locations every 15 days at about the same time of day. The semilunar rhythm is therefore exactly characterized by two dates: the lunar emergence time (a few successive days around full and new moon) and the diurnal emergence time. 4. According to experimental investigations on the control of the emergence rhythm, the midges are able to determine both dates in advance. 5. Coastal populations differ in their lunar and diurnal emergence times. These differences correspond to the time of low tide which exists at each location during the emergence days of the semilunar rhythm. 6. Crossbreeding between stocks of different populations showed that the differences in diurnal emergence time are gene-controlled.