Hermit Crabs Are Attracted to the Smell of Their Own Dead, for One Very Morbid Reason

When a rich relative passes away without a will, it can create a mad scramble amongst kin for the resources that have been left behind. It turns out, it's no different for hermit crabs.

The discovery began with a rather morbid experiment by two Dartmouth College biologists. Professor Mark Laidre and undergraduate student Leah Valdes wondered how hermit crabs might react to the death of a fellow crab, so they chopped up some deceased hermit crabs and placed the bits in plastic tubes around a beach. Within five minutes, the tubes were swarmed by crabs—as many as 50 showed up at one tube, reports Science News.

"It’s almost like they were celebrating a funeral," said Laidre.

This was no funeral, however. The crabs were not mourning their fallen comrade; they were looking for opportunities. Researchers guessed that hermit crabs follow the scent of their own dead in a wild frenzy to move into the open shell that was probably left behind.

The most surprising thing about this discovery is not that these critters can smell an opportunity, but that abandoned shells are so valuable that they are sought after with such fervor. Clearly, the scent of dead hermit crabs is something these animals have evolved a special sensitivity for.

Perhaps it's not so surprising, however, when you look at the numbers. Hermit crabs are obsessed with shell-hunting, constantly searching for new adequate dwellings. That's because good shells are hard to come by, and hermit crabs must continually find larger shells in order to grow.

None of the roughly 850 known hermit crab species can produce their own shells, so these creatures rely entirely on other animals, usually snails. Those animals only relinquish their shells after having perished themselves, and many of the causes of snail death can also damage their shells. In other words, good shells are rare and it's not easy to stumble upon an ideal fit that isn't already occupied by another crab.

Researchers also tested the crabs' sensitivity to dead snail flesh, but the snail flesh was not nearly as attractive to the crabs as flesh from another crab. This makes sense if you consider that perfect shells (for hermit crabs) are more likely to have been occupied by other crabs, as opposed to a fresh snail shell that might have defects that aren't ideal for crabs.

All of this is an important reminder for humans who might have a proclivity toward conservation, to remember for their next trip to the beach. "We can tell the public: 'Don't take shells from the beach,' " said ecologist Chia-Hsuan Hsu, who studies hermit crabs at National Taiwan University in Taipei. By taking home a lovely specimen, you could be depriving a hermit crab of its next much-needed dwelling.

Furthermore, the lack of shells could be pushing crabs to settle for plastic bottles and other litter they encounter on beaches. Sometimes crabs get stuck inside bottles and cannot escape; they die, attract yet more crabs, and all of them get stuck. It's best for them to inhabit the shells they evolved to use.