Marine Biologists Discover Two New Species of Porcelain Crabs

A duo of marine biologists from the Smithsonian Tropical Research Institute in Panama and the Justus-Liebig-Universität in Germany has discovered and described two new porcelain crab species.

Polyonyx socialis, female, Mun Island, Nhatrang Bay, south coast of Vietnam, South China Sea; right side of carapace deformed by parasitic isopods. Scale bar – 3 mm. Image credit: A. Hiller B. Werding, doi: 10.3897/zookeys.876.37244.

Polyonyx socialis, female, Mun Island, Nhatrang Bay, south coast of Vietnam, South China Sea; right side of carapace deformed by parasitic isopods. Scale bar – 3 mm. Image credit: A. Hiller B. Werding, doi: 10.3897/zookeys.876.37244.

Porcelain crabs belong to Porcellanidae, a highly diverse family of crustaceans found in the shallow waters of oceans worldwide. They also are known as ‘false crabs,’ because they evolved a crab-like form independently of true crabs.

A relatively large number of porcelain crabs are symbiotic with other organisms, allowing scientists to tell a story of a long-time relationship between species from distantly related species.

“Most porcelain crabs live on the hard substrates of shallow waters like the surface of corals or rocks overgrown by algae, microbes and decaying material,” said Dr. Alexandra Hiller, a researcher at the Smithsonian Tropical Research Institute.

“Others live as symbionts of invertebrates like sponges, anemones, sea urchins, polychaete worms and other crustaceans.”

The two recently described species — Polyonyx socialis from the South China Sea of Vietnam and Petrolisthes virgilius from the Caribbean Sea of Colombia — are examples of these symbiotic porcelain crabs.

Polyonyx socialis was found living with other organisms in the compact tube-like shelters built by the polychaete worm Chaetopterus sp.

Its broad, flat walking legs and claw-bearing extremities appear to have been adapted for living tightly-attached to the worm tube walls and avoid being perceived as an obstacle for the other organisms.

Petrolisthes virgilius, Santa Marta, Colombia. Scale bar - 2.4 mm. Image credit: A. Hiller B. Werding, doi: 10.3897/zookeys.818.30587.

Petrolisthes virgilius, Santa Marta, Colombia. Scale bar – 2.4 mm. Image credit: A. Hiller B. Werding, doi: 10.3897/zookeys.818.30587.

Although initially mistaken for Petrolisthes tonsorius in the 1970s, the uncommon colors and atypical habitat of Petrolisthes virgilius — intertidal vermetid snail formations in the Colombian Caribbean — led the scientists to corroborate through genetic analyses that it was a new species.

As a symbiont, Petrolisthes virgilius has evolved in tight association to its distinctive surroundings: a reef-like microhabitat exposed to wave action and consisting of snail shells cemented to each other and to a hard substrate.

“Symbiotic species are thought to be more vulnerable to environmental challenges than free-living organisms,” said Professor Bernd Werding, from the Institut fur Tierokologie und Spezielle Zoologie der Justus-Liebig-Universität.

“Their fate depends on the fate of their host, which may also be affected by local and global conditions and abrupt changes.”

Polyonyx socialis and Petrolisthes virgilius are described in two papers in the journal ZooKeys.

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A. Hiller B. Werding. 2019. A new species of Petrolisthes (Crustacea, Anomura, Porcellanidae) inhabiting vermetid formations (Mollusca, Gastropoda, Vermetidae) in the southern Caribbean Sea. ZooKeys 876: 143-151; doi: 10.3897/zookeys.876.37244

A. Hiller B. Werding. 2019. A new species of Polyonyx (Crustacea, Anomura, Porcellanidae) inhabiting polychaete-worm tubes (Annelida, Chaetopteridae) in the Indo-West Pacific. ZooKeys 818: 25-34; doi: 10.3897/zookeys.818.30587

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