Zebra mussel spawning is induced in low concentrations of putative serotonin reuptake inhibitors

Serotonin (5-hydroxytryptamine, 5-HT) and its receptor ligands induce both oocyte maturation and spawning in zebra mussels (Dreissena polymorpha). The selective serotonin reuptake inhibitors (SSRIs) fluvoxamine ("Luvox"), fluoxetine ("Prozac"), and paroxetine ("Paxil") are commonly prescribed drugs for the treatment of depression in humans. They act to increase 5-HT neurotransmission by inhibiting reuptake transport proteins at synapses. I tested the efficacy of these drugs at inducing spawning in zebra mussels. All three compounds induced spawning in both sexes at concentrations lower than that for 5-HT itself. Fluvoxamine was particularly potent, inducing spawning in 100% of both sexes at 10(-5) and 10(-6) M. The concentration that induced a significant percentage of animals to spawn was as low as 10(-9) M for males and and 10(-7) M for females. The lowest concentration of fluvoxamine to induce spawning was 10(-8) M for females (40%) and 10(-10) M for males (20%). Gametes spawned in fluvoxamine (10(-5) M and lower) were viable, and swimming trochophores were formed within 20 hours. Fluoxetine was also an effective spawning inducer, causing 100% of males to spawn at 5 X 10(-6) M. The concentration of fluoxetine required to induce a significant percentage of spawning was as low as 5 X 10(-8) M for males and 5 X 10(-6) M for females. In both fluvoxamine and fluoxetine, more than 60% of the males spawned within the first hour of exposure. In contrast, paroxetine was a weak spawning inducer. At concentrations of 10(-5) and 10(-6) M it induced significant, but low (50% and 40%, respectively) percentages of males to spawn. Paroxetine did not induce significant spawning in females. Thus, fluvoxamine, fluoxetine, and paroxetine can induce spawning at low concentrations, and fluvoxamine is the most powerful spawning inducer in any bivalve. These may be useful agents for stimulating invertebrate serotonergic mechanisms without applying exogenous 5-HT, and they are potentially important in bivalve aquaculture. Moreover, these results suggest, for the first time, the presence of 5-HT reuptake transporters in bivalve molluscs.