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> It seems the hermit crab has evolved a large penis to use to save his shell home:
> 
> https://www.the-scientist.com/news-opinion/larger-hermit-crab-penises-may-prevent-shell-theft-65325
> 
> "Because the special shells take effort to create and are tough to grip onto, C. compressus often steal them from each other. Male crabs are especially vulnerable to losing their homes during mating, when they must emerge partway to ejaculate into a female’s shell.
> 
> “'If they lose that shell and they don’t have it for twenty-four hours, they’re basically doomed to dessicate and die,” Laidre explains.
> 
> While examining preserved specimens in museums, Laidre was struck by the diversity of penis-like structures, also called sexual tubes, in hermit crabs. In particular, he noticed that C. compressus sexual tubes are “gigantic relative to its body size.” He hypothesized that larger penises may have evolved in this species, which uses extensively remodeled shells and for which sex is particularly dangerous, to minimize the distance they need to come out of their shells in order to mate.
> 
> ***
> 
> "While Coenobita penis size doesn’t come close to matching barnacles, which can have penises eight or nine times their body length, it’s a far larger penis-to-body-size ratio than sported by gorillas, which have tiny penises only about an inch and a half long.
> 
> "Another important part of the story, according to Laidre, is the coconut crab, Birgus latro. These gigantic hermit crabs—up to nine pounds and three feet across—give up their shells as juveniles and recalcify their own abdomens, rather than finding a shell for protection. These big guys have a relatively miniscule penis that’s less than 20 percent of their body size. 
> “What it suggests is that these [crabs] that have no danger of losing any property whatsoever have basically put virtually zero investment into this structure,” he says.
> 
> “'I love his hypothesis and his data does support it,” says Christopher Tudge, a reproductive biologist at American University in Washington, DC, who did not participate in the study. He adds that one open question is whether the idea extends to other groups of crustaceans, particularly the aquatic hermit crabs, some of which also have sexual tube structures, but whose reproductive behavior is less well understood.
> 
> ***
> 
> "Kelly says that it was Charles Darwin who first recognized that “many traits that improved access to mates would actually probably decrease survival in nature, so there’s this inherent conflict between the actions of sexual selection for access to mates versus natural selection for improved survival, and this is a really great example of that'”
> 
> Comment:   Acts as a huge anchor and a safe sex tool to stay in shell while copulating. A great natural  wonder.