How can a crayfish become anxious? Researchers designed an experiment to trigger this feeling in the crustaceans. | AAAS/Carla Schaffer
A new study of crayfish — the freshwater crustaceans that look like little lobsters — shows that the arthropods can get anxious when they are exposed to prolonged stress. But, the arthropods also can be calmed down by a drug known as chlordiazepoxide (CDZ), which is used to treat anxiety in humans.
These findings suggest that complex emotional states like anxiety, which occur even when the triggering stress is absent or unclear, aren't limited to just mammals and other cognitively complex vertebrates, as many researchers have assumed. In fact, this primitive form of crayfish anxiety probably shares some evolutionary origins with the more developed human form, researchers say, although the ways in which this emotion has shaped each species remain unknown.
Pascal Fossat from the Université de Bordeaux in France and a team of French researchers used mild but repetitive electric shocks and a plus-shaped aquarium tank to show that, unlike fear, anxiety remains with a crayfish as it moves from one environment to another.
"There have been previous studies suggesting that fish or lower vertebrates feel some kind of anxiety, but the most evidence we have for invertebrates, I think, is found in this study," suggested Fossat. "This primitive emotion probably appeared in a common ancestor long ago and then became conserved on different branches of the phylogenetic tree."
The researchers' complete report appears in the 20 June issue of the journal Science.
"There have been very few studies of the crayfish brain," explained Fossat. "It's a small structure compared to a mammal's, for example, but our study shows that you can still have access to complex behavior with this small structure."
The researchers stressed some of their crayfish out with electric shocks before moving them to the plus-shaped tank. Two of the tank's "arms" were well-lit and the other two were left in the dark. Fossat and the other researchers knew that crayfish normally prefer darkness, but they guessed that the crustaceans would explore the lighted arms of the tank too — and that's exactly what the non-shocked, non-stressed crayfish did.
But, the stressed-out crayfish behaved anxiously in the plus-shaped tank, rarely swimming into the light at all. Fossat and his team used liquid chromatography to get a view into the crustaceans' brains and observed that their light avoidance was caused by increased levels of serotonin, a neurotransmitter that is likewise associated with anxiety in the human brain.
When the researchers injected non-stressed crayfish with some serotonin, these crustaceans also began acting anxiously. However, when they gave their stressed crayfish some CDZ, the human anti-anxiety drug, the crayfish grew bolder and started exploring the lighted areas of the tank.
As for why anxiety has been conserved for so long across species, the researchers are less clear. "This anxious state might be an adaptation that gave crayfish a chance to survive…to cope with danger and to avoid and protect," Fossat mused. "Our study suggests that anxiety appeared very early on in evolution, but it probably evolved differently in crayfish and mammals because the two are organized so differently."
The implications of an ancient form of anxiety are murky at the moment too. Do the findings mean, for example, that "screaming" lobsters are going through complex emotions when they are thrown into a boil?
"There has been some evidence of pain in crabs," Fossat said. "But we don't have the neural support mapped in arthropods yet, like we do in mammals, so we just don't know.
"We might have to change the way we cook lobster," he added jokingly. "But I don't think its ['screams' are] related to any suffering — and I'm not sure that evolution prepared lobsters to cope with a boil."