Porcelain Crabs Drop a Little Leg When Necessary

porcelain crabs sweep for plankton
Colorful and anemone-based, Neopetrolisthes maculatus is a popular Indo-Pacific porcelain crab to photograph. I like the photo because its feathery plankton-netting extensions are clearly shown. 

IF IT LOOKS LIKE A CRAB AND IT WALKS LIKE A CRAB, THEN IT MIGHT BE A PORCELAIN CRAB, which is a different thing entirely. Which leaves the question: What’s the difference between true crabs and porcelain crabs?

They’re both decapods – crustaceans that resemble each other with hard outer shells and 10 appendages, including walking legs extending on their sides and large claws out in front. They both walk sideways, or “crab-like.” But the differences between true crabs and porcelain crabs are both subtle and significant.

IT’S ALL IN THE LEGS. OR CLAWS.

The least obvious, perhaps, is that porcelain crabs walk on three of their four pairs of legs, as opposed to true crabs’ use of all four pairs. Porcelains’ fourth pair are reduced in size, folded up under their abdomen, generally not visible and used for cleaning. But then, since porcelain crabs are only a half-inch or so in body size, it may be challenging to count.

The most dramatic is porcelain crabs’ penchant for completely discarding their claws or legs to deter predators. It’s not just that they drop a leg and escape. The discarded appendage keeps moving for a bit, distracting the evil predator while they take it on the lam. Happily, the missing parts grow back in the course of several molts.

difference between true crabs and porcelain crabs
This porcelain crab specimen shows up in several sources, although its species is undetermined. Well camouflaged, it lives on a red soft coral in the Philippines.

SIMILAR ARCHITECTURE, DIFFERING DESIGNS

Then, there’s shape: The carapaces, or dorsal shells, or true crabs are generally somewhat dome-shaped. The bodies of porcelain crabs are significantly flattened. And porcelain crabs are universally much smaller – generally about one-half inch wide at most. These small, flattened bodies aid the porcelain crab in its lifestyle of hiding out among rocks, corals, sponges and sea anemones.

For another, unlike true crabs, porcelain crabs have long antennas extending forward from either side of the eye stalks.

PLANKTON FEEDERS

Like true crabs, porcelain crabs are equipped with a pair of large claws, which may actually be bigger than the rest of the crab. Their claws tend to be prominent and formidable-looking but they’re mostly show. They’re used for territorial fights, not to hunt for food. They are used to keep the area in front of the crab clean.

While porcelain crabs will dine out on a variety of foods, including copepods and tiny shrimps, they mainly feed on plankton. Front appendages are modified to filter microscopic zooplankton from the water current. Over-sized and equipped with long, hair-like setae, they’re essentially feathery nets they sweep through to water to bring as in many planktonic edibles as possible.

difference between true crabs and porcelain crabs
Well disguised with muted colors, this guy hangs out on a sea pen.

HIDING OUT

There are some 300 species of porcelain crabs found worldwide in every ocean except the Arctic and Antarctic seas. In temperate waters, rocks and boulders provide hiding spaces for their small, flattened torsos.

On tropical reefs,  you may find them hiding out on sponges, branching corals and soft corals. Some species nestle among the tentacles of sea anemones, in a mutualistic arrangement. The anemones provide them with shelter from predators. The little crabs keep the tentacles clean by eating algae, particulate matter and, possibly, mucus that accumulates on them.

difference between true crabs and porcelain crabs
They’re not all colorfully decorated.

TO SHARE AND SHARE NOT

For porcelain crabs, living with anemones has a downside – so do anemonefishes, and they don’t like to share. When a confrontation occurs, the porcelain crab usually ends up going somewhere else.

One group of animals that does share are large hermit crabs – some are nice enough to share their home shells with the porcelain species Porcellana sayana

Whether they’re noticed or not, porcelain crabs can be abundant. More than 800 were counted living in a square-meter mussel bed on the California coast.

Porcelain crabs live in pairs. In mating, male places sperm within the females,  who carry reddish brown eggs in a brooding flap in the abdomen. A female is said to carry as many as 1,600 on her body, although the typical load is more in the range of 600.

Another anemone-loving porcelain, plankton-sweeping nets on alert.

SHAPE AND CONECTIONSIf porcelain crabs with their flattened bodies have a squat look to them, it’s perhaps because they are more closely related to squat lobsters than to true crabs. For those of you who are technical, both animals are members of infraorder Anomura – true crabs are infraorder Brachyura.

A number of sources like to point out that their crabby resemblance is an example of carcinisation, which sounds significant except that it turns out that carcinisation simply means, looking like a crab.

Several sources also attribute the porcelain part of the term porcelain crab to these animals ability to easily drop their claws and legs in a pinch. The assumption appears to be that porcelain means delicate, when actually it means the material that a plate or pitcher is made of. Perhaps porcelain dinnerware had a reputation for delicateness. The dictionary doesn’t mention that.

PRINCIPAL SOURCES: Reef Creature Identification, Florida, Caribbean, Bahamas, Reef Creature Identification, Tropical Pacific, Paul Humann, Ned DeLoach; Coral Reef Animals of the Indo-Pacific, Terrence Gosliner, David Behrens, Gary Williams; “Porcelain Crabs,” Smithsonian Ocean;  “Wild Fact #188 – Dropping Claws – Porcelain Crab,” Wild Facts; “Really Riveting Facts About Porcelain Crabs,”; Animal Sake;  “Porcelain Crab,” Monterrey Bay Aquarium; “Porcelain Crab,” Maidenhead Aquatics; “Porcelain crab,” “Crab,” “Decapoda,” Wikipedia.com.

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