Behold, my new favorite creature

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Porcellanopagurus nihonkaiensis wearing a bivalve shell (Source)

Some of you may be aware that I harbor great affection for hermit crabs. I own terrestrial Caribbean hermits. Your mental image of hermits may feature a wardrobe of gastropod (snail) shells, which are by far the most common mollusk contractor they use to construct their homes, but as I’ve discussed, they actually have great flexibility in their choice of abode. It turns out that there is yet another option which hermits take advantage of as a mobile home: the flat shells of bivalves and limpets!

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What a cutie, wearing a limpet like a hat (source)

Porcellanopagurus nihonkaiensis is a species of marine hermit found off the coast of Japan. It uses the relatively flat, unenclosed shells of clams (and also limpets) for protection. Though lacking the 360 degree protection afforded by a snail shell, bivalve shell valves can be more plentiful in the marine environment, and being able to utilize a different shell frees them from competition with other hermit species which are specialized to work with snail shells.

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Without the shell! From Yoshikawa et al, 2018

Hermits typically have a long, soft coiled body which fits in where the snail’s body once was, using “uropodal endopods” (little feet at the end of their bodies) to hold themselves in the shell. Some species like Porcellanopagurus, however hold a bivalve or limpet shell on their backs, which still provides protective cover for their bodies. One recent study talked about their method of acquiring and holding the shell. They actually took a cute little series of pictures showing how the crab picks up a shell it with its front claws, places it on its back and then holds it in place with their fourth pair of legs. So now I’ve found a creature that combines my beloved clams and hermit crabs in one fun package. Gonna have to keep an eye out if I ever dive off of Japan!

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Play by play of how they pick up and carry away their new home (in this case a limpet shell) (source)

The Many Homes of Hermit Crabs

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My boy Harry, a purple pincher (Coenobita clypeatus) inhabiting a tapestry turban snail (Turbo petholatus) shell. These seem to be his favorite kind, even though they do not come from his native Caribbean.

Hermit crabs (superfamily Paguroidea) are most famous for using snail shells as their home, having evolved a soft, spiral abdomen to be able to use them for protection. But they are more flexible about their choice of abode than you might expect.

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This crab was likely preserved buried alive in sediment. Note how it uses its claw as a protective trap door sealing the opening of the ammonite shell. Source: Jagt et al, 2006

Different groups of shelled organisms have risen and fallen in abundance through geological time. During the time of the dinosaurs, ammonites (relatives of modern squid and octopus) were among the most common marine organisms, and hermit crabs were there to recycle their shells when they died.

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Each tiny pore (zooid) in this bryozoan contained a tiny tentacled organism. Together they grew in a shape that made for a nice hermit crab house (image 5 shows a cross section where the crab’s abdomen would fit). Source: Taylor and Schindler 2004

Mollusks aren’t the only contractors for hermit crabs. Some hermits utilize the skeletons of colonial organisms like bryozoans as a home. Bryozoans are filter-feeding colonial animals made up of thousands of tiny tentacled organisms living in the pores of a shared skeleton. The extinct bryozoan Hippoporidra lived in symbiotic partnership with hermit crabs, growing around a gastropod shell to attract a hermit crab partner. This was an example of mutualism: by providing a home for a crab, the bryozoan would be transported to new environments with plentiful food particles to eat, and also would be protected from their arch-enemy, nudibranchs (sea slugs). Some modern day hermits, such as Manucomplanus varians of the Gulf of California, have evolved very similar partnerships with live staghorn corals.

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Manucomplanus varians at Monterey Bay Aquarium

Not all hermit crabs live in hard houses. Some deep sea forms partner with anemones, with the stinging tentacles serving as an effective defense.

Source: Okeanos Explorer

The recently discovered green-eyed hermit crab, which also lives in deep water, lives in a glued-together mass of sand created by tiny anemones, which continue to grow the structure to fit the crab as it increases in size.

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The green-eyed hermit crab was found over 200 m deep off of South Africa. Source: Lannes Landschoff via Eurekalert 

Unfortunately, hermits adapted for gastropod shells are unable to find adequate homes in some areas, due to overharvesting of shells for the tourist trade as well as an excess of plastic trash. These crabs make do with whatever items that they can find. Plastic is not an ideal home material for hermits. Bottlecaps and narrow tubes do not allow the crab to fully retract for protection and leach chemicals which may harm the crab. The crabs also nibble on their shells as a source of calcium, which is obviously not possible with plastic.

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Coenobita purpureus, a land hermit crab on Okinawa. Source: Shawn Miller

But hermits continue to impress me with their flexibility and ingenuity in their search for homes. For a hermit crab, home is where the abdomen is.