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 anenteric larvae that initially are strongly photopositive. Over the course of several hours larvae lose their initial photopositivity and either become photonegative or alternate between positive and negative phototaxis. We report that newly released photopositive larvae rapidly become photonegative upon exposure to 10-6-10-5 M serotonin or its metabolic 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. In a separate series of experiments, larvae treated with dopamine (10-7-10-5 M) significantly prolonged their photopositive period. This effect was also obtained with the D2 dopamine receptor agonist, quinpirole (10-6-10-5 M). HPLC analysis determined that newly released photopositive larvae contained 0.120 pmol dopamine/ {mu}g protein. These findings implicate serotonin and dopamine as important neurochemical regulators of phototaxis in larvae of B. neritina.