Comparative studies of circadian pacemaker coupling in opisthobranch molluscs

The eyes of several marine molluscs contain circadian pacemakers. The ocular pacemakers of one mollusc, Bulla gouldiana, are mutually coupled, and this coupling can be demonstrated in vitro. Induced phase separations between the two ocular pacemakers are reduced if the pacemakers are allowed to interact. The interaction between the pacemakers is mediated by the exchange of optic nerve impulses from eye to eye. In contrast, in vivo studies with another mollusc, Aplysia californica, have revealed that this snail's pacemakers are not strongly coupled. We have now determined in Bursatella leachi plei, an Opisthobranch closely related to A. californica, that the ocular pacemakers are mutually coupled. By virtue of the interaction between the pacemakers of B. leachi plei, the period of the ocular rhythm is increased by 1.5 h. In addition, eyes that remain attached to the nervous system show an increase in the sustainability of the free-running rhythm compared with isolated eyes. The increase in sustainability however, is due to the effects of attachment to the cerebral ganglion, and not the contralateral eye, since most single eyes attached to the brain show sustained, short-period rhythms. Thus, two properties of a circadian system, the pacemaker period and pacemaker sustainability, may be influenced by separate physiological mechanisms. We also confirm, using our in vitro techniques, the lack of strong coupling between the ocular pacemakers of A. californica.