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Hermit crabs with more sensitive claws may be bolder in their decisions

If you have ever looked closely into a rockpool along the U.K. coastline, you may have been lucky enough to find a tiny hermit crab scuttling about. What you might not realize is that hermit crabs are constantly monitoring their environment for changes and threats.

Often, when they detect danger, they retract into their shell—but after the initial shock of being startled, the crab uses its sensory organs to help determine if it is safe to emerge and start exploring once more.

In one local species of hermit crab, tiny hair-like structures—otherwise known as sensilla—on the claws appear to be important in gathering information about their surroundings, including the presence of potential predators.

A new study, in the Proceedings of the Royal Society B: Biological Sciences, has shown that crabs with more sensory hairs on their claws recover faster from a startle response. Animals that recover faster from a startle are considered bolder and in this study, crabs with more sensilla were consistently bolder individuals. They were also more predictable in the time it took them to recover from their fright.

The study was carried out by researchers from the University of Plymouth, who believe it offers a fascinating insight into the biology of a tiny creature found right along the U.K.'s coastline, and begins to answer a number of questions about the connection between animals' sensory abilities and behavior.

"My research is often inspired by my observations of hermit crabs in the lab and out in their natural habitat, and focuses on the role of information and sensation in crustacean behavior and physiology.

"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. The patterns I observed led me to wonder if these hermit crabs might be using their claws to help assess risk from the environment.

"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," says Ari Drummond.

For the study, researchers first analyzed how individual crabs responded to being startled in the lab, then waited for each crab to shed its skin and collected the molted claw tissue. This shed tissue was examined in detail using images captured with a scanning electron microscope, housed within the Plymouth Electron Microscopy Center (PEMC).

It enabled the researchers to mark all the sensilla on the surface of a claw, without needing to remove the limbs of living crustaceans, as has often been done previously in these types of studies. Finally, they assessed if the number of sensilla was related to the relative boldness of each crab.

The analysis showed that bolder hermit crabs have more sensilla on the claw surface. It also revealed that more sensilla, essentially suggesting that a crab has better access to information, appear to make the crab more able to consistently determine that the surrounding environment lacks risk.

This relationship between relative boldness and sensory ability helped the researchers to suggest a new hypothesis about how sensation and animal behavior are linked, which the researchers have termed "sensory investment syndrome." The research team hopes this will inspire other work that examines how sensory traits might help shape animal personality and decision-making.

"We've known for a long time that individual animals of the same species can show consistent behavioral differences from one another. 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," says Professor Mark Briffa.