Crayfish experience anxiety much like humans do, say scientists (+video)

After being exposed to stressors, the nervous systems of crayfish behave much like those of humans, according to researchers in France.

By , Staff writer

How can a crayfish become anxious? Researchers designed a clever experiment to trigger this feeling in the crustaceans.

Before you toss a crayfish in a pot of boiling water, you might wonder how it feels about the impending splash.

A new study suggests that these tasty crustaceans actually experience anxiety. After a team of French researchers put them in stressful situations, crayfish behaved more cautiously and attentively to potential risks. What's more, their nervous systems behaved much like those of anxious humans.

Daniel Cattaert, a neuroscientist at the Université de Bordeaux and a member of the research team, clarifies. "This type of anxiety is not pathological," he says. Instead, the crayfish are reacting to stress much like a human would to the SATs. Except the crayfish would be anxious about predators.

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Crayfish are the first invertebrates known to display such a reaction. Since these animals are so distinct from us – our ancestors diverged from invertebrates at least 525 million years ago – crayfish anxiety tells a tale of common ancient origins of the physiological response, says Dr. Cattaert.

The researchers, led by Université de Bordeaux neuroscientist Pascal Fossat, placed each test subject in a cross-shaped maze. Two arms of the maze were illuminated, while the other two were dark. Crayfish generally prefer the dark, but will explore lighter areas to search for food and to understand their surroundings.

That's exactly what the crayfish did. But when the researchers introduced stressors and returned them to the maze, the crustaceans became too anxious to enter the lighted areas.

Initially, the researchers removed the crayfish from the maze and subjected them to repetitive, low electrical fields. When the frazzled animals were returned to the maze, they were reluctant to explore the lighted areas.

When the researchers noticed elevated levels of serotonin in the stressed crayfishes' brains, they decided to test the influence of this neurotransmitter further. Instead of exposing unstressed crayfish to electricity, they injected serotonin into the invertebrates. (To confirm that the crayfish were responding to the neurotransmitter and not the needle, the researchers injected saline into crayfish in a control group.)

The increased levels of serotonin produced the same results as the electric fields, with the stressed crayfish avoiding the light. This suggests that serotonin is involved in the animals' nervous system's response to stress.

In order to further prove the similarities between anxiety experienced by crayfish and humans, the scientists treated the stressed crayfish with an anti-anxiety medication called chlordiazepoxide. The drug appeared to return the animals to their unstressed state.

Why did they decide to test crayfish for anxiety in the first place? This quest began with experiments about the aggressiveness of crayfish, says Cattaert. When put together in a tank, two crayfish instantly begin fighting. The researchers began this experiment to understand the effect of stress on the animal's behavior and nervous system, but needed a simpler experiment to start with. The next step, says Cattaert, will be to study the effects of anxiety in social interactions.

Crayfish anxiety is important from an evolutionary standpoint, says Cattaert. "This kind of behavior is very adaptive. It is important for an animal to survive" to produce descendants, and anxiety helps them be cautious of danger. "All the keys [of anxiety] are present in the crayfish. The same molecules and the same systems are involved" as in vertebrates. This parallel shows the importance of anxiety in animals for species survival.

Do crayfish experience emotions? Cattaert says people ask him this often when they discover his work. "Yes, the animal is capable of producing behavior that in every point fits anxiety behavior. But the dimension of emotion is, of course, very different in humans than it can be in crayfish," he says, explaining that the animal does not have the same kind of brain structure we have.

Although crayfish have significantly fewer neurons overall than humans and other vertebrates, they have about 2,000 sensory neurons, according to neuroscientist Cattaert, "so sensing is very important. They are capable of sensing and reaction," and likely feel something like pain as their body's way of prompting action in danger.

So, what does the crayfish feel when held over boiling water? Cattaert says you'd likely see an anxious response when the animal felt the steam. But, he says, if you really want to know what the crayfish is feeling "you should ask the crayfish."

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