SEROTONIN AND DOPAMINE HAVE OPPOSITE EFFECTS ON PHOTOTAXIS IN LARVAE OF THE BRYOZOAN BUGULA-NERITINA

Adult colonies of the bryozoan Bugula neritina release short-term anen teric larvae that initially are strongly photopositive. Over the cours e of several hours larvae lose their initial photopositivity and eithe r become photonegative or alternate between positive and negative phot otaxis. We report that newly released photopositive larvae rapidly bec ome photonegative upon exposure to 10(-6)-10(-5) M serotonin or its me tabolic precursor, 5-hydroxytryptophan. This behavior was not observed in two congeners of B. neritina, nor in larvae of three other species of bryozoans and seven species from four additional phyla. Antibodies to serotonin label cells in the region of the equatorial nerve-muscle ring and in two tracts extending from the apical disc to this ring. I n a separate series of experiments, larvae treated with dopamine(10(-7 )-10(-5) M) significantly prolonged their photopositive period. This e ffect was also obtained with the D-2 dopamine receptor agonist, quinpi role (10(-6)-10(-5) M). HPLC analysis determined that newly released p hotopositive larvae contained 0.120 pmol dopamine/mu g protein. These findings implicate serotonin and dopamine as important neurochemical r egulators of phototaxis in larvae of B. neritina.