A team of researchers from France have determined that crayfish are able to experience anxiety, which was previously thought to be too complex for these animals to feel.

The study, which was published in the Science journal, defined anxiety as a behavioral product of stress. The data that the researchers gathered was the first to show that invertebrates can feel anxiety.

"Crayfish are primitive, they have been around for hundreds of millions of years," said research team member and neuroscientist Dr. Daniel Cattaert, from the University of Bordeaux in France.

"The idea that this animal could express some anxiety didn't seem possible, but with our experiments we're more and more convinced that this was the case."

The researchers began the experiment by exposing crayfish to an electric field, which served as a stress-inducing environment for the crustaceans. The crayfish were then transferred to a cross-shaped tank, wherein two of the arms were dark and the other two arms were lighted.

The team found out that the crayfish that were not previously exposed to the electric field, and therefore not stressed, will move to all parts of the tank, though more likely to the dark areas, which crayfish generally prefer. However, for the stressed crayfish, they avoided the lighted areas completely, perceiving these areas as threatening to them.

During this part of the experiment, the stressed crayfish will begin producing increased levels of serotonin, which is a chemical that the brain releases to fight against anxiety.

In addition, when the researchers took the stressed crayfish and injected them with anti-anxiety drug benzodiazepine, they stopped being threatened and began to move to the lighted parts of the tank.

The results show that crayfish anxiety is propagated by the same chemical components that can be found in humans. This is why the crustaceans responded well to the anti-anxiety drug, which is also used by humans to treat anxiety.

"Anxiety is different from fear, which is something that even the simplest animals show. Anxiety is a kind of fear of the fear, and animals who experience it will display adaptive behavior to minimize the threat," said Cattaert.

Bob Elwood, a professor at the Queen's University Belfast, commented on the study, saying that while the crayfish do show behavior related to anxiety, it is impossible to confirm what they are feeling as the animals are not able to respond to researchers the way humans can.

However, Elwood added that the possibility that crustaceans feel anxiety and pain may mean that changes in how they are treated should be considered, especially as governing bodies such as the European Food Safety Authority do not consider them as sentient beings.

The other members of the research team are Pascal Fossat, Julien Bacqué-Cazenave, Philippe De Deurwaerdère and Jean-Paul Delbecque.