Arthropods, including insects, spiders, and crustaceans, are covered by a hard exoskeleton that protects their soft innards and helps them get around. Molting, or shedding this shell, is necessary for growth. But molting is a lot more complicated than just discarding your old exoskeleton. The process involves a long cascade of events that precede and follow this step and can take days or weeks to complete.
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Animals are vulnerable during molting and frequently seek safe places to hide for its duration. For some scientists, this raises the question of whether arthropods experience anxiety during molting.
Anxiety can be defined as a behavioral reaction to stress that includes long-lasting apprehension of future events. Previous research by Paul Fossat of the Université de Bordeaux and colleagues demonstrated that crayfish respond to stress by displaying anxiety-like behaviors (This term is used in the scientific literature rather than ‘anxiety’ to keep it as objective as possible and to recognize the fact that we do not know what the crayfish are feeling). These behaviors disappear when the animals are treated with an anxiolytic (anxiety-reducing) drug.
In a new study , Fossat and his colleagues sought to determine whether the stressful events of molts induce anxiety-like behaviors in crayfish. They collected crayfish from swamps near Bordeaux and tested their behaviors in an aquatic cross-shaped maze. Two of the arms were dark, which the crustaceans find reassuring, and two arms were illuminated, which is more stressful. Fossat and his colleagues recorded the exploratory behavior of crayfish in this maze at different stages of their molt cycle to gauge their anxiety levels.
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Between molt cycles and during early pre-molt, crayfish spent similar percentages of time exploring the illuminated arms of the maze. But beginning during late pre-molt and lasting into the post-molt period, they began to show anxiety-like behaviors, displaying more of an aversion to light.Source: Alexander Mrkvicka, via Wikimedia Commons. Distributed under a CC BY-SA 3.0 license.
Next, the research team explored whether steroid hormones known as ecdysteroids could modulate anxiety-like behaviors in crayfish. Injection of a high dose of an ecdysteroid triggered pre-molt in crayfish. Four days later, the animals displayed anxiety-like behaviors in the maze. This delay in action suggests a long-term, possibly indirect hormonal modulation of anxiety-like behaviors.
Finally, Fossat and his colleagues treated ecdysteroid-injected crayfish with an anxiolytic drug. The steroid-induced anxiety-like behaviors were rapidly suppressed after drug treatment, confirming that the changes in behavior induced by ecdysteroids were really related to anxiety.
“We observed anxiety-like behaviors in crayfish during molting and showed that these behaviors are also sensitive to an anxiolytic drug,” says Fossat. “We suggest that this critical period of a crayfish’s life generates an emotional adaptation that forces the animal to stay in protected areas.”
In other words, molts and their hormonal control impose internal stress on crayfish, leading to aversion behavior that looks like anxiety.Source: Luc Hoogenstein, via Wikimedia Commons. Distributed under a CC BY-SA 4.0 license.
Fossat says work like this shows that the neuronal mechanisms related to anxiety likely appeared early in evolution and are highly conserved. In fact, it isn’t far-fetched to imagine that other arthropods also experience anxiety during molts.
“There is growing evidence that invertebrates can feel some emotion, at least in a primitive way,” says Fossat. “I think it’s possible that molting could induce anxiety-like behaviors in other animals, but we need to study it before giving a definitive answer.”
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While some people might balk at the idea of an anxious invertebrate, Fossat says his team used the same criterion to describe anxiety that is used in other animals (mostly mammals).
“So if crayfish fulfill these criteria it means either that crayfish express anxiety-like behavior or that mammal models of anxiety are wrong,” he says.
“Our vision of invertebrates has been too limited for many decades. In recent years, many studies have shown that they are much more than ‘reflex machines.’”