Fertile hermit crabs turn shy

If you happen to come across a hermit crab, pick it up and watch what happens. The little crustacean will retreat into its shell and stay there for a bit, even after you put it back down on the ground. This action is known as a startle response, and the timing of a crab’s re-emergence varies from crab to crab.

Danielle Bridger of Plymouth University in England and colleagues wanted to know how that startle response might be related to a hermit crab’s life history — specifically fertility and aerobic scope, which is related to stamina. In many species, boldness gives an individual a lot of advantages, helping the organism win fights or win girls. The researchers thought that there would be similar advantages for bold crabs.

The team began by collecting Pagurus bernhardus hermit crabs from two beaches near Plymouth. They brought them back into the lab, took them out of their shells and picked out healthy males. (The crabs live in abandoned periwinkle shells and swap shells as they grow.) The rest the researchers gave new shells and returned to the sea. Then once a day for eight days in a row, a researcher would remove each crab from its tank, invert it for five seconds and then return it to the tank. The time until the crab poked back out of its shell was recorded. And at the end of the experiment, the scientists took a sample of each crab’s hemolymph (to be analyzed for a measure of aerobic scope) and dissected the crustaceans to obtain each one’s packet of sperm, or spermatophore.

Spermatophore size and aerobic scope were related, but sperm amount was the one factor that was related to crab personality, the researchers report February 11 in the Proceedings of the Royal Society B. Spermatophore size was closely associated with the length of a crab’s startle response. The shyer the crab, the bigger his sperm package was. “This result seems counter to the idea that the most productive individuals should also be the boldest,” the scientists write.

Shyness isn’t necessarily a bad strategy for a hermit crab when he’s packing a lot of sperm, the researchers argue, because it lets him avoid a lot of risk. “Until their spermatophores are provided to females, the pay-off from their investment [in sperm] has not been secured,” the researchers note. In that kind of situation, being bold may not make sense, because it won’t help a crab use that sperm. He needs to save it for later.

Being shy might come at a price, though, since it would leave less time for activities other than hiding, the researchers note. And it remains to be seen whether the shy hermit crabs actually get to make use of those large packages.