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When boxer crabs fight, anemones are real winners

January 31, 2017

If you're a pom-pom boxer crab, you'll often need to fight to stay alive. When the bell rings, you'll need your anemone to pack some extra sting. Don't have one? Just steal a bit from a friend and feed it like a pet.

Krabbe mit Anemonen
Image: picture-alliance/dpa/Yisrael Schnytzer

Boxer crabs make bonsai cuttings of pom-pom anemones

The relationship between boxer crabs and the sea anemones that they use as pom-pom boxing gloves is one of the most unusual in nature.

Israeli scientists found that it's possibly unique, with crabs - also known as Lybia or pom-pom crabs - controlling the reproductive activity of the anemone.

The group were following up on a previous study that showed the crabs - unflatteringly classed as "kleptoparasites" - regulate the food intake and, hence, the growth of their "guests." Scientists go so far as to liken this to the careful cultivation of bonsai. 

Sea anemones throw out a protective shield around their crustacean host, which can save it from being gobbled up by a predator. 

They also help the crabs to pack a punch, having tentacles on their claws that are covered in stinging cells. This helps ward off predators, and can come in handy in a duel.

Crabs - which hold onto anemone with hooks - can also use them as food mops, gathering debris from around their home. While the anemone admittedly gets to feed off the scraps, it seems they benefit least from the relationship. After all, who wants to be a mere mop or pom-pom?

Every crab has its mits full

Researchers from Israel's Bar-Ilan University and Volcani Center looked at the relationship a little more closely.

Each crab they had seen in the wild had two anemones. The team wanted to know how the crabs might come by their tentacled fighting tools in the wild.

The crabs are tiny and hard to observe in the wild, so the scientists took a look at how they behaved in the laboratory. They found that when one sea anemone is taken from a crab, it will weave two smaller fragments from the remaining one. Over the next few days this then grows and regenerates.

When a crab with no anemone is placed with a crab boasting the usual two, the creature without one will launch a brave assault on its tank-mate.

Handled with care

They'll do so by holding down the claw of their opponent, and removing all or part of the anemone. Most such encounters would end with a successful theft, and the crabs would grows whatever they were left with.

"Sea anemone splitting appears to be a well-orchestrated behavior, conducted with apparent care for the final outcome, ie, two new viable sea anemones," said the authors of the study, published in the open access journal PeerJ. 

According to the researchers, it's a unique example of one animal inducing the asexual reproduction of another. Perhaps that explains a little more of what's in it for the anemone.

It's also what seems to happen in the wild. Using German DNA amplification techniques, the team looked at genetic fingerprints from beaches near Eilat, on the Red Sea. Each crab was found holding genetically identical anenomes, indicating that they were originally one.

In a previous study, the scientists found that the crabs, which themselves only grow to abount 2 centimeters (less than an inch), appear to limit the amount of food that aneome get, limiting their size. Good things, as far as they're concerned, come in small packages.  


Richard Connor Reporting on stories from around the world, with a particular focus on Europe — especially Germany.