What makes a hermit crab more bold? Hairy claws.

If you’ve ever sifted along a tide pool or visited a souvenir stand on a coastal boardwalk, you may have seen a hermit crab. These crustaceans that have been on Earth for roughly 500 million years, are always alert, monitoring their surroundings for any perceived threats. Like turtles, they will retract into their shells if they detect any danger. Once that initial shock passes, these crabs will use sensory organs in order to decide if it is safe to emerge and get back to business. 

To gather data about their surroundings, Pagurus bernhardus, a common hermit crab species, uses small hair-like structures called sensilla. According to a study published July 2 in the journal Proceedings of the Royal Society B, crabs with more of these sensory hairs on their claws appear to recover faster when startled and may make bolder decisions. These more hairy-clawed animals were also more predictable in how much time it took for them to recover from their initial fright.

“For this study, I was especially intrigued by how they used their claws and other sensory appendages, such as their antennae, in their explorations and when re-emerging from their shell,” Ari Drummond, a study co-author and marine biologist at the University of Plymouth in England, said in a statement. “The patterns I observed led me to wonder if these hermit crabs might be using their claws to help assess risk from the environment.”

Drummond and her team initially analyzed how individual Pagurus bernhardus hermit crabs responded when startled in a lab. They then waited for each crab to shed its skin and collected the moulted claw tissue. Next, they put the shed tissue under a scanning electron microscope at the Plymouth Electron Microscopy Centre (PEMC) to examine it in greater detail. The microscopes allowed the team to mark all of the sensilla on the surface of a crab’s claw, without removing the limbs from living crustaceans.

Lastly, they looked to see if the number of sensilla was related to the relative boldness of each individual crab. 

The analysis revealed that the bolder hermit crabs have more sensilla located on the claw’s surface. Additionally, more sensilla appears to make the crab determine that the surrounding environment lacks risk more consistently. This basically suggests that more sensilla equals better access to information. 

This relationship between sensory ability and relative boldness led to a new hypothesis about how sensation and animal behavior could be linked. The team calls this “sensory investment syndrome,” and hopes that it inspires more study into how sensory traits influence both animal personality and decision-making. 

“We’ve known for a long time that individual animals of the same species can show consistent behavioural differences from one another,” study co-author and University of Plymouth marine biologist Mark Briffa said in a statement. “Our new research suggests that in hermit crabs, some of this variation may be linked to how individuals sense the world around them. This possibility has been largely overlooked, but if sensory investment helps explain personality in hermit crabs, it may do so in other animals as well.” 

“In a world where environments and species are increasingly at risk from human impacts on the environment, I believe it is essential that we gain a better understanding of what information animals detect, how they use that information and then respond to stay alive,” concluded Drummond.

 

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