Differential effects of fluoxetine on the phototactic behavior of 3 amphipod species (Crustacea; Amphipoda)

We document phototactic responses in different amphipod populations of Gammarus minus, Stygobromus tenuis, and Crangonyx shoemakeri, each collected at 2–3 sites within the Washington DC area. We then assessed how baseline phototaxis was altered following either short-term (3-week) or long-term (6-week) exposure to 0.05 µg/L or 0.5 µg/L fluoxetine. Our results classify all species as significantly photonegative, a response that depended solely on the presence, not quality, of light. Short-term fluoxetine exposure caused some animals to become photoneutral, regardless of concentration, while others remained photonegative. Long-term exposure to 0.5 µg/L fluoxetine caused photoneutral behaviors in all surviving populations; exposure to 0.05 µg/L had variable effects. These differential effects were due to a significant effect of population/sampling location on photobehavior. Overall, these results identify species-specific effects of chronic fluoxetine exposure and underscore how the response to light in 7 geographically distinct populations is uniquely tuned to requirements for survival.