The Mysteries of the Pom-Pom Crab

Greetings, arthropod enthusiasts! It’s been a while, huh? Rest assured that new posts about animals with too many legs will continue to appear in the future, but they won’t be coming out as frequently as before. Nevertheless, this is ArthroBlogger entry number 88, so feel free to browse through previous posts if you’re feeling nostalgic or need to scratch that arthropod-fact itch.

Now that that’s out of the way, let’s talk about pom-poms crabs! Also known as mosaic boxer crabs for their beautiful exoskeletons, these little crustaceans live in shallow waters from the Red Sea to Indonesia and never go anywhere without their trusty twin anemones. Those anemone mittens earn the crabs the simplified nickname “boxer crabs” for their role as cnidocyte-packed boxing gloves.

A beautiful pom-pom crab by Rebecca Tse on Flickr

Yeah, pom-pom crabs don’t carry little cnidarians around just to look fancy: those venomous mittens are imperative to the crab’s very survival. Most crabs would ward off attackers with an impressive pair of pincers, but pom-pom crab claws aren’t big enough to threaten predators. Fortunately, they’re just the right size and shape for carrying stinging anemones! The boxer crab lives up to its nickname by jabbing foes with its anemone-gloved hands, and also uses the anemones as a means of acquiring sustenance, usually decaying organic matter. In return, the anemones feed on the scraps of food the crab leaves behind, and get to see (…feel? Eat) much more of the world than your typically anemone plastered to a rock or other substrate.

Food and stinging fists in exchange for food and mobility; this, folks, is mutualism at its finest.

So, how does the pom-pom crab obtain its pom-poms in the first place? Weirdly enough, nobody really knows. Even weirder, the only way a crab has ever been observed getting a new pom-pom is by cloning its remaining anemone.

I’m so serious.

In a study published in 2017, biologists kidnapped some crabs off the coast of Israel and stole their pom-poms for science. Well, they only stole one pom-pom from each crab, but still; science is weird sometimes. Anyway, they watched to see what the crabs would do when left with a single anemone. As the biologists predicted (because, as scientists, they’re aware that science is weird), the crabs tore their anemones in half.

Don’t worry, the anemones were fine; many other sea anemone species actually tear themselves in half on purpose to clone themselves as a means of asexual reproduction. If anything, being split apart by someone else just sped up the process for these particular cnidarians, and it left the crabs with two pom-poms once again.

This same study also pitted crabs with pom-poms against crabs without pom-poms to see what would happen. The two crabs would fight (they really care about those pom-poms, understandably), and the anemone-less crab would attempt to steal one of the other crab’s anemones. The crabs only ever stole one anemone from their opponent, and then both crabs would retreat and split their anemones as before.

The mystery of the pom-pom crab’s pom-poms is on par with the chicken or the egg dilemma. All the crabs in the 2017 study, whether big or small, old or young, were already holding anemones when they were caught, and DNA analyses showed that most crabs were holding genetically identical anemones. Do most crabs get their anemones by cloning one? How do baby crabs get their anemones? How did the first pom-pom crab get its anemones? Are all pom-poms the descendants of one initial anemone? I need answers!

Well, that was quite the long post to return with, but what can I say? People need to know how weird and awesome pom-pom crabs are. And now you know! Spread the word; there aren’t nearly enough people concerned with the origin of crab pom-poms as there should be.

3 Comments on “The Mysteries of the Pom-Pom Crab

  1. Fascinating!!! Thise Pom-Pom crabs and their anemones – are beautiful. The “unseen” world has vibrant color, personality, and “depth.” Thanks for sharing.


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