The Oregon shore crab, Hemigrapsus oregonensis. Photo Courtesy Jerry Kirkhart

Antidepressants are immensely helpful for millions of people across the globe, but when they find their way into the environment and accumulate, their effect can be decidedly not helpful for wildlife. New research has revealed that environmental contamination with one popular antidepressant, Prozac, can have substantial impacts on the behavior of crabs living on shorelines near human habitation, making them more susceptible to predation and death.

Pharmaceutical pollution is a rapidly emerging reality of the Anthropocene, with waste runoff from population-dense areas causing our drugs to become a fixture of nearby ecosystems. Already, this situation is proving not fun for wildlife: The pain-killer naproxen, for instance, is wreaking havoc on crayfish by mellowing them out to the point where they can’t compete with each other for shelter or resources. Antidepressants can be especially dangerous as a contaminant, because many of these drugs influence brain chemistry that relies on compounds with important functions. The drug featured in the new research on crabs—fluoxetine, aka Prozac®—is a selective serotonin reuptake inhibitor (SSRI), which increases the levels of serotonin outside of cells by limiting the chemical’s reabsorption. If present in high enough concentrations, SSRIs can influence the behavior and physiology of everything from deer to dragonflies.

Most investigations into the impact of pharmaceutical contaminants have focused on single species or groups of similar organisms, but very little attention has been given to how the pollution’s influence can reverberate ecologically. The new research—published by scientists at UC Santa Barbara in the journal Ecology and Evolution—looks not only at how Oregon shore crabs (Hemigrapsus oregonensis) change their behavior under chronic exposure to fluoxetine, but how they treat the risk posed by nearby predators differently. The researchers found that crabs exposed to high (but previously field-detected) levels of fluoxetine experienced troubling alterations to their behavior—ones that would easily get them killed.

To do this, the researchers went to Oregon’s Netarts Bay, and collected dozens of shore crabs, as well as red rock crabs (Cancer productus), which commonly hunt and feed upon the shore crabs in nature. The research team then housed the shore crabs in the lab, placing them in aquaria outfitted with water composition, rocks, and lighting all to mimic the crabs’ normal estuary habitat. They then divided up the crabs into three treatments: no fluoxetine in the water, low concentrations of fluoxetine, and high fluoxetine levels. Over the course of nine weeks, the scientists repeatedly observed the shore crabs’ behavior, both with and without predators around, and in the day and nighttime hours.

The control shore crabs did as shore crabs are wont to do: they chilled out during the day, and moved, fed a bit more, and occasionally fought with other shore crabs during the night. This nocturnal activity pattern is largely to avoid predators that are active during the day (like the red rock crab). When the predatory crabs were present in the tank, the control crabs hunkered down even more, not taking the risk of being caught out in the open by the giant, armored terror stalking their enclosure.

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The red rock crab (Cancer productus). Photo Courtesy Kirt L. Onthank

The shore crabs exposed to low levels of fluoxetine behaved similarly to the control crabs, but the crabs living with a high concentration of the drug—ten times the amount of the low level—acted recklessly.

The fluoxetine made the shore crabs far more active, at all times of day, whether or not the predatory crabs were around. They moved around their environment more, seemingly unperturbed by hungry, clawed death scuttling mere inches away. The shore crabs also fought more often than the control crabs, exhibiting more aggression toward each other. This combination of decreased wariness of predators and elevated intraspecies belligerence led to more of the high-exposure shore crabs dying over the course of the study, either by getting eaten, or killed by other shore crabs caught up in a fluoxetine-induced rage. The effect of the fluoextine got more intense as the weeks pressed on too, possibly because the drug sticks to fats, allowing it accumulate within the crabs’ bodies.

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That crabs exposed to fluoxetine made riskier choices is not a great sign for many marine ecosystems. As human populations swell along coastlines and sheltered bays, and more antidepressants like fluoxetine pour into the ocean, this effect—which impacts crucially important predator-prey interactions—is likely to get worse. This isn’t to say that antidepressants and other pharmaceuticals should be vilified, since their benefits to humanity are multitudinous, but much like with the ubiquitousness of human-made plastics in the environment, this is a growing and pervasive conservation issue that isn’t going to go away anytime soon, and will require solutions that halt untreated waste from reaching the ocean.

Jake Buehler is a Seattle area science writer with an adoration for the Tree of Life’s weird, wild, and unsung—follow him on Twitter or at his blog.