Entrainment of a semilunar rhythm by simulated tidal cycles of mechanical disturbance

Abstract

The semilunar rhythm of pupation and succeeding emergence in populations of the intertidal midge Clunio marinus Hal. can be entrained by the combination of a 24-h light-dark cycle and a 12.4-h cycle of simulated tides, both of which recur in characteristic phase relationships every two weeks (collectively designated as ‘semi-monthly zeitgeber cycle’). The effective tidal factor in simulated tidal conditions was the mechanical disturbance of the water which is correlated with turbulence, underwater sound, and bottom vibrations. The experiments induced underwater sound of 50–200 cps, 20–30 dB above background noise level, and of 6 h duration per tidal cycle, which corresponded well with the conditions in nature. It is supposed that tidal turbulence and vibration result in mechanical stimulation of the larvae. Tidal changes of hydrostatic pressure and light intensity were ineffective as zeitgeber. The phase relationship between the semilunar rhythm and the semi-monthly zeitgeber cycle is a population-specific property as demonstrated by stocks from two locations with different tidal schedules (Helgoland and Normandy). Comparing experimental data and field data, this phase relationship represents a successful genetic adaptation to geographic differences in the phase of tides.