Hermit crabs show social aggression for survival, study finds

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While the word “hermit” may conjure up images of peaceful solitude and avoidance of conflict, the terrestrial hermit crab has proven that it doesn’t quite adhere to this vision.

In fact, a recent study shows that the land-dwelling hermit crab often acts in the opposite way.

An October study by Mark Laidre, a Miller postdoctoral fellow and biologist at the UC Berkeley Miller Institute for Basic Research in Science, discovered that the tiny crustaceans congregate to evict hermit crabs living in larger shells — and subsequently move in. By investigating how hermit crabs communicate and interact with one another, Laidre was able to understand the social behaviors of the crustaceans.

“This creates a strong level of competitive sociality that leads crabs to evict their neighbors and take their remodeled shells,” Laidre said. “While most animals socialize for mutually beneficial purposes, like protecting themselves from predators, hermit crabs socialize for more self-serving reasons.”

The study, published in this month’s issue of Current Biology and conducted in both labs and natural environments in Costa Rica, tested the survival rates of terrestrial hermit crabs living in shells taken directly from snails versus hermit crabs living in shells that were taken from other hermit crabs that once had belonged to snails.

Because terrestrial hermit crabs have limited access to abandoned snail shells — a common dwelling most often found in the ocean — they often remodel the internal architecture of shells they inherit from long-gone gastropods such as the common snail.

The remodeling is an example of “niche construction,” a process of organism-driven environmental modification that can alter natural selection pressures over time on the organisms, according to the study.

But left unmodeled, the shells do not provide adequate protection for the crabs — in fact, they leave much of the naked hermit body exposed and vulnerable. As a result, the hermits in those unmodeled gastropod-derived shells showed “starkly lower survival” rates, according to the study.

These extreme shell makeovers create a situation where adult hermit crabs can only live in shells that have previously been remodeled by another crab. Unmodeled shells, the study shows, weren’t sufficient enough to protect hermits from outside predators.

The study also found that because there are so few unoccupied remodeled shells in terrestrial hermit crab habitats, crabs who outgrow their shells and need new ones often have no other option than to evict another.

The aggregation of these terrestrial crabs makes it possible for most crabs seeking a better shell to forcibly acquire adequate housing in rapid succession, according to a commentary on the study by UC Davis evolutionary biologist Geerat J. Vermeij.

And while the study demonstrates the evolutionary capabilities and activities of a common crustacean with a knack for survival, it also touches upon motives that perhaps some humans can relate to.

“Broadly, this highlights a key evolutionary reason that organisms socialize: to get something that others have,” Leidre said.

Contact Geena Cova at [email protected]