Antidepressants are putting crayfish at increased risk of predators as drugs seeping into ponds and streams cause them to behave more boldly, a new study has suggested.

Crustaceans were found to spend more time searching for food – thereby putting themselves in danger – when water was polluted by medication.

The findings emerged after researchers placed crayfish in a tank to examine their behaviour. Experts discovered the crustaceans were more brazen when water was contaminated.

"Crayfish exposed to the antidepressant came out into the open, emerging from their shelter more quickly than crayfish not exposed to the antidepressant," said Dr Lindsey Reisinger, a co-author of the study from the University of Florida.

"This change in behaviour could put them at greater risk of being eaten by a predator.

"Crayfish eat algae, dead plants and really anything else at the bottom of streams and ponds. They play an important role in these aquatic environments. If they are getting eaten more often, that can have a ripple effect in those ecosystems."

The research team wanted to understand how crayfish responded to low levels of antidepressants in aquatic environments. They achieved that by re-creating their natural environment, controlling the amount of antidepressant in the water and observing their behaviour.

Some crayfish were exposed to environmentally realistic levels of antidepressant in the water for a few weeks, while a control group was left in clean water.

Crayfish exposed to antidepressants spent more time looking for food

To test how antidepressant exposure altered behaviour, the researchers used a Y-shaped maze which has a short entrance and two lanes. At the start of the experiment, the researchers placed each crayfish in a container that acted as a shelter, and that shelter was placed at the entrance to the maze.

When the researchers opened the shelter, they timed how long it took for the crayfish to emerge. When the creatures did so, they then had to choose a direction at the fork in the road. One lane emitted chemical cues for food, while the other emitted cues signalling the presence of another crayfish.

The pair of researchers recorded which direction the crayfish chose and how long they spent out of the shelter.

Compared to the control group, crayfish exposed to antidepressants emerged from their shelters earlier and spent more time in pursuit of food. They tended to avoid the crayfish side of the maze – a sign that the levels of antidepressants used in the study did not increase aggression.

More than half of people with depression benefit from pharmaceuticals, with the most common a class of drugs called selective serotonin reuptake inhibitors, or SSRIs. These work by boosting serotonin levels – a chemical that creates feelings of happiness and lifts one’s mood – in the brain, with treatments approved for use in the UK including Prozac.

Ponds and rivers are known to be polluted with trace levels of SSRIs which reach waterways through improper disposal, such as down a sink or toilet. The drugs can also invade the habitat of fish is via the urine of people taking them, which re-enters the water system despite being filtered and treated.

Dr AJ Reisinger, the lead author of the study and husband of Dr Lindsey Reisinger, said there were steps people could take to reduce the levels of antidepressants and other pharmaceuticals in water bodies.

"The answer is not for people to stop using medications prescribed by their doctor," he said. "One big way consumers can prevent pharmaceuticals from entering our water bodies is to dispose of medications properly."

The research is published in the journal Ecosphere.