Hermit crabs are a common sight scuttling across warm beaches, but many of these charismatic crustaceans wield something surprising and hidden—very large penises, sometimes half the length of their shell-anchored bodies. New research suggests the crabs evolved such outsized organs to help them mate without straying far from home.

Some of these hermits expend a lot of energy “remodeling” the inside of their shells, which unlike most crabs they of course cannot grow themselves, says Mark Laidre, a biologist at Dartmouth College and a National Geographic Explorer.

These animals can whittle their shells and secrete erosive chemicals, allowing them to create a smooth and expansive interior. These refurbished homes give them space to grow and may even provide room for egg storage—making the shells extremely valuable. They are, in other words, not something that you’d like to leave—even temporarily—which many hermit crabs have to do to mate.

But what if you could have a large enough sexual organ to reproduce without exiting your shell, which rivals may steal?

Varying sizes

While studying land-dwelling hermit crabs at Smithsonian’s National Museum of Natural History, Laidre noticed that the animals’ penises, also known as sexual tubes, varied widely in size. But the species that did the most remodeling—and thus had the most valuable shells—had the largest penises.

Laidre’s new study, published in the journal Royal Society Open Science, suggests that hermit crabs evolved bigger sexual organs to avoid becoming homeless.

To test his “private parts for private property” hypothesis, Laidre took a closer look at the genitalia of land crabs in the genus Coenobita, but also set of related hermit crabs over a wide range of habitats and lifestyles. Some crabs modify their shells to varying degrees, while others do not. Some were tiny, tidepool residents, and others, like Petrochirus diogenes, are massive deep sea-dwelling animals.

View Images This male land crab (Coenobita compressus) has a prominent penis, and it normally carries a valuable, easily stolen shell. Photograph by Mark Laidre

Laidre also included the watermelon-sized coconut crab—the world’s largest land-lubbing invertebrate—which doesn’t need a shell home.

Upon measuring the penis-to-body-size ratio of more than three hundred museum specimens, Laidre found that the more extensively a given species’ shell was carved, the longer its penis was for its body size. Laidre even ruled out alternative hypotheses that could explain the pattern, like the possibility that penis length went up with body size, or that particular habitat types were responsible.

This makes sense because terrestrial hermit crabs’ lives revolve around their protective shells. The cunning creatures are ruthless about spotting and stealing superior shell property from their neighbors.

Get a grip

“They get into quite a lot of shell shenanigans, where individuals are constantly at risk of being evicted,” says Laidre, noting that remodeled shells are even more apt to being snatched away, since their smooth interiors are even harder for the crab to grip.

Enlarged genitalia are “really a very sensible evolutionary solution to what is one of the most dangerous things [the crabs] can get involved in,” he says.

Losing a remodeled shell makes fatal desiccation all but inevitable, and the crabs have become so specialized, they can’t even fit into unimproved shells as a temporary fix. “If these guys lose that shell, within 24 hours, they're doomed."

Because of this, Laidre envisions the enlarged penises as an insurance policy for protecting a crucial investment, one that is bolstered by other behaviors that make sex as safe as possible.

This Sea Creature Does an Awesome Hermit Crab Impression Watch these pharaoh cuttlefish change their appearance and behavior to mimic hermit crabs.

Shell to shell

When mating, the two crabs face their shell openings towards each other and get as close as possible to allow the male to deposit sperm without either crab leaving the shell. (There is not penetration involved.) Laidre notes that the process is fairly speedy in remodeled shell species compared to other crabs, and executed in secretive locations, likely minimizing risk further.

This is in stark contrast to coconut crabs, which have one of the smallest penis-to-body-size ratios among crabs in the study. Coconut crabs use remodeled shells as young but become too large to need them before maturity. With no shell to protect as adults, the danger in copulation erodes, as does the need for enlarged genitalia.

“I thought this study was extremely clever,” says Justa Heinen-Kay, an evolutionary ecologist at the University of Minnesota who was not involved in this study. “It is quite remarkable that these animals have actually evolved longer penises in order to stay close to home while copulating.”