The antidepressants venlafaxine (“Effexor”) and fluoxetine (“Prozac”) produce different effects on locomotion in two species of marine snail, the oyster drill (Urosalpinx cinerea) and the starsnail (Lithopoma americanum)

• Human antidepressants have differing effects on locomotion in two species of marine snail.
• In general, for both species, venlafaxine increased speed, but fluoxetine decreased it.
• The mechanisms of action could involve serotonergic, dopaminergic, or octopaminergic neurons.

Human antidepressants have been previously shown to induce foot detachment from the substrate in aquatic snails. Prior to foot detachment, antidepressants also affect snail crawling speed. We tested two commonly prescribed antidepressants, venlafaxine (“Effexor”) and fluoxetine (“Prozac”) on crawling speed and time to reach the air–water interface in two species of marine snail, the oyster drill Urosalpinx cinerea and the American starsnail Lithopoma americanum. Exposure to venlafaxine increased crawling speed in both species, while fluoxetine slowed them down. Our lowest LOEC (lowest observed effect concentration) was 31.3 μg/L venlafaxine in Urosalpinx. Similarly, snails (L. americanum) exposed to venlafaxine tended to move faster and more often to the air–water interface, but exposure to fluoxetine slowed them down. Our lowest LOEC was 345 μg/L fluoxetine in Lithopoma. These results indicate that venlafaxine boosts locomotion, while fluoxetine reduces it, and both behaviors are preludes to foot detachment. The different effects of these two antidepressants on snail locomotion suggest differing physiological mechanisms of action in marine snails as well as possible ecological consequences.