CIRCADIAN AND LUNAR GAMETE RELEASE IN FUCUS-VESICULOSUS IN THE ATIDAL BALTIC SEA

Released eggs from Fucus vesiculosus were counted every 12 or 4 h in the field and every 24 h in laboratory experiments at constant temperature and a 17 h light:7 h dark cycle, mimicking natural light conditions. A fortnightly rhythm was confirmed both in the field and in controlled laboratory studies, with a main egg release 2 d before full and new moon, but with indication of a minor weekly period of unequal amplitude. The highest daily release of eggs was typically found from 18:00 to 22:00 h. Dehydration was not found to influence the pattern of egg release and in the field no correlation was found with temperature or water level fluctuations. An endogenous clock in F. vesiculosus is suggested and different entraining factors presented and discussed. We further suggest that the observed rhythmicity is of great adaptive value for coordination of gamete release in a dioecious species like F. vesiculosus in the atidal environment of the Baltic Sea, where this species occurs almost always submerged. Ecologically, the cyclic gamete release means that reproduction and recruitment are reduced to a few occasions during the reproductive period, which was earlier assumed to be several months long. This information may be crucial in the successful management of this ecologically important species in the Baltic Sea which has been severely reduced in abundance in recent decades due to pollution.