Competition for abandoned shells turns into a lively gathering
The death of a millionaire with no heir draws an opportunistic crowd. So, too, does the demise of a land-dwelling hermit crab. Researchers working in Costa Rica found that the curious crabs are drawn to the smell of flesh torn from one of their own.
Dartmouth College biologist Mark Laidre, along with undergraduate student Leah Valdes, set out 20 plastic tubes on a beach, each holding bits of hermit crab flesh. Within five minutes, almost 50 hermit crabs (Coenobita compressus) swarmed around each sample, the pair reports online February 10 in Ecology and Evolution. “It’s almost like they were celebrating a funeral,” Laidre says.
The reality, however, is more macabre. That scent of flesh is a signal that a fellow land hermit crab has been eaten, and that its empty shell is available for the taking, Laidre says. The crabs “are all in an incredible frenzy to try to move into that leftover shell.”
None of the roughly 850 known hermit crab species, most of which live in the sea and some on land, can grow their own shells. Instead, the crabs occupy shells originally left behind by dead snails. A hermit crab grows to the size of its shell, but to grow further, the creature must find and occupy a larger shell.
For the roughly 20 or so species of land hermit crabs, finding a suitable shell can be especially challenging. Big shells with lots of extra room to grow may be too heavy in the short term for a hermit crab to tote around on land without the buoyancy of water to help lighten the load, and lighter shells may be too small.
FOLLOW THE CROWD Within three minutes on a beach at Osa Peninsula, Costa Rica, land hermit crabs (Coenobita compressus) crowd a tube containing flesh bits of their own kind. Researchers say the smell signals that an empty shell may be available for the taking.
Land hermit crabs can remodel their shells, making them bigger, Laidre reported in 2012. The animals use corrosive secretions and scraping to widen a shell’s opening, remove the internal spiral and reduce wall thickness. Remodeling can double the available space while trimming one-third the weight. But remodeling is taxing and slow. It’s much faster to take over an already remodeled shell of another land hermit crab, alive or dead. Hence the strong attraction of land hermit crabs toward smells that suggest another is dead, Laidre says.
The researchers also found that land hermit crabs approached bits of snail flesh, though the scent appears to be far less alluring than that of their own species. Sea hermit crabs, however, didn’t find the smell of another hermit crab’s corpse more attractive than those of snails.
That makes sense to Laidre. For sea hermit crabs, upsizing to bigger and heavier shells is relatively easy, thanks to water’s buoyancy that helps the crabs support a shell that’s a little too big at first. That, combined with the fact that there are also many more empty shells in the sea than on land, means that sea hermit crabs face less competition when looking for a home, he says.
By highlighting that shell availability is limited for land hermit crabs, the study makes an important point for conservation, says ecologist Chia-Hsuan Hsu, who studies hermit crabs at National Taiwan University in Taipei and wasn’t involved in the research. “We can tell the public: ‘Don’t take shells from the beach,’ ” he says.
L. Valdes and M.E. Laidre. Scent of death: Evolution from sea to land of an extreme collective attraction to conspecific death. Ecology and Evolution. Published online February 10, 2019. doi:10.1002/ece3.4912.