Crayfish feel emotional stress and calm down when given a drug used to treat anxiety, research has shown.
The findings suggest that even invertebrates experience rudimentary emotions - something not suspected before.
In the experiment, scientists exploited the fact that the crustaceans are more comfortable in darker water.
After giving crayfish a series of mild electric shocks, they placed the creatures in an aquarium "maze" containing both well lit and darkened arms.
Although non-stressed crayfish preferred the dark arms, they also explored the lit regions. But crayfish given the electric shocks hardly entered the lit arms of the aquarium at all.
The stressed animals' light avoidance was linked to boosted levels of the brain chemical serotonin, which also has a big effect on mood in humans.
Merely injecting crayfish with the neurotransmitter was enough to make them anxious, the researchers discovered.
They also found that the creatures responded positively to chlordiazepoxide (CDZ), a drug used to treat anxiety in humans.
Treated crustaceans lost their nerves and were happier to explore the bright parts of the aquarium.
The team, led by Dr Daniel Cattaert, from the University of Bordeaux, wrote in the journal Science: "Analyses of this ancestral behaviour in a simple model reveal a new route to understanding anxiety and may alter our conceptions of the emotional status of invertebrates .
"Our results... emphasise the ability of an invertebrate to exhibit a state that is similar to a mammalian emotion."