CIRCADIAN AND LUNAR GAMETE RELEASE IN FUCUS-VESICULOSUS IN THE ATIDALBALTIC SEA

Released eggs from Fucus vesiculosus were counted every 12 or 4 h in t he field and every 24 h in laboratory experiments at constant temperat ure and a 17 h light:7 h dark cycle, mimicking natural light condition s. A fortnightly rhythm was confirmed both in the field and in control led laboratory studies, with a main egg release 2 d before full and ne w moon, but with indication of a minor weekly period of unequal amplit ude. 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 sug gested and different entraining factors presented and discussed. We fu rther suggest that the observed rhythmicity is of great adaptive value for coordination of gamete release in a dioecious species like F. ves iculosus in the atidal environment of the Baltic Sea, where this speci es occurs almost always submerged. Ecologically, the cyclic gamete rel ease means that reproduction and recruitment are reduced to a few occa sions 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 w hich has been severely reduced in abundance in recent decades due to p ollution.