Patagurus rex hermit crab
The dorsal view of Patagurus rex, a new species of hermit crab found off the coast of Moorea in French Polynesia, shows a broad, hardened body—similar to the hardened bodies of true crabs.

Photo courtesy Arthur Anker

There are countless known species on the planet, but a new hermit crab found by a University of Florida researcher proves some interesting creatures remain to be discovered.

Dredged from the deep sea near the island of Moorea in French Polynesia in 2009, the species exemplifies the rarely documented process of carcinization, in which hermit crabs move out of their shells and harden their bodies to resemble true crabs. The new species, Patagurus rex, has a broad, armored body with pointy spines and long legs connected to large claws—making it one of the most distinctive hermit crabs discovered in decades, said Gustav Paulay, invertebrate curator at the Florida Museum of Natural History on the UF campus. Paulay and Florida Museum postdoctoral researcher Arthur Anker describe the species in the journal Zootaxa in October 2013.

“Out of all the animals we collected in Moorea, probably the most interesting for me was this species of hermit crab,” Paulay said. “While dredging at 400 meters off the coast, up came a tiny little thing that at first looked like a crab, but it wasn’t quite. This shows that in the lesser-known parts of the ocean, there are still plenty of cool discoveries to be made.”

four photographs of Patagurus rex
Patagurus rex carried a small shell (Figure A), which is shown detached from the body in Figure B.

Photo by Gustav Paulay (A), Arthur Anker (B-D)

Hermit crabs are a species of decapod crustaceans, which includes shrimp, true crabs and lobster. They have soft, vulnerable abdomens and draw their bodies into unattached shells for protection, unlike true crabs that have broad, armored bodies and small, protected abdomens. Hermit crabs have only once or twice independently given rise to animals that resemble true crabs, with the most celebrated case being king crabs, including the Alaska king crab, Paulay said. Patagurus is extremely close to a true crab-like form and appears to be another attempt by hermit crabs to adopt a crab-like habit, he said.

“This animal looks almost like a missing link between hermit crabs and crabs, although it doesn’t lead to anything and is an end in itself,” Paulay said. “So that’s what makes it really interesting. We know it’s not related to king crabs. It’s an independent line sort of exploring crab space, if you will. All we know about it comes from this one specimen collected in Moorea.”

An international team of researchers discovered the hermit crab during an expedition organized by the French Tropical Deep Sea Benthos program. The expedition helped expand coverage of the Moorea Biocode Project, an international effort to document all animals living on the South Pacific Island, including the surrounding marine life.

The study describes pathways hermit crabs take to become truely crab-like, including developing strategies for protection that appear to rely on armor or speed. Patagurus took the latter route, developing long legs for speed as well as a hardened shield.

“There are two paths hermit crabs take to deal with danger,” Paulay said. “One is to rely on large, heavy shells to pull into with an armored front that plugs the opening when they withdraw, so that predators cannot reach them—the other is to be light and run. If something is coming to get you, you just run like crazy. The new species has very long legs and is fast and agile. So, it evolved from this idea of not hunkering down, but rather running from danger.”

The new species carries only a tiny clam over a reduced abdomen. The rest of the body has hardened as it is no longer protected by a shell. Paulay said DNA sequencing will help researchers determine the origin of the species. One of Biocode’s goals is to sequence every species on Moorea.

“If you have a species’ DNA sequence, you can identify them even if they’re in the belly of a fish, or you can match larvae to adults – like caterpillars to butterflies,” Paulay said. “So when you pull something interesting up from the sea, there is so much to learn from it, beyond its novelty.”

Learn more about the Invertebrate Zoology at the Florida Museum.

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