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Researchers Solve the Mystery of How Pom-Pom Crabs Acquire Their Anemone Weapons

Joanna Lawrence
31st January, 2017

A team of researchers has found that an odd species of crab called the pom-pom crab will steal anemones, which the animal needs for survival, from other crabs. The crabs will also split a single anemone into two, cloning it. The findings are in a paper that was just published in the journal PeerJ. The pom-pom crab (Lybia leptochelis), also known as the boxing crab, is a strange crustacean that forms partnerships with small sea anemones. The crabs hold a tiny anemone in each claw, brandishing them at any threats (hence the name “pom-pom” or “boxing” crab). The anemones are capable of stinging any predatory animals the crab runs across. The anemones can also help the crabs stun prey. Researchers are divided on whether or not the anemones benefit from this relationship. One theory is that the partnership provides the anemones with better access to food. The constant movement would also help oxygenate the anemones, which would otherwise be mostly stationary. Although the benefits to the anemones aren’t completely clear, they’re necessary for the crabs’ survival. Researchers from the Bar-Ilan University in Israel collaborated with a scientist from the Agricultural Research Organization, Volcani Center to study the behavior of pom-pom crabs. The researchers had noticed that most pom-pom crabs held Alicia anemones that are not normally found living in the wild without crab hosts. To understand how the crabs were acquiring the anemones, the team observed over 100 pom-pom crabs in the Red Sea. All of the crabs were holding the same Alicia anemones yet years of observation didn’t reveal a single free living anemone of that species. The research team collected wild pom-pom crabs to observe their behavior in the laboratory. When they took one anemone from a crab, they noticed that the crab would then tear its remaining anemone in half. Since anemones can clone themselves, this resulted in two anemones after a few days—restoring the one that had been taken. In a second experiment, the researchers placed two pom-pom crabs in the same aquarium. One had no anemones while the other had a complete pair. The two crabs immediately began to fight, with the anemone-less crab stealing all or part of one of its opponent’s anemones. This helped disarm its opponent while giving the anemone-less crab a new anemone to split into two. The loser then broke his remaining anemone in half. In the end, both crabs had two anemones again. This behavior had never been recorded before and may explain why free-standing Alicia anemones are absent in the wild. The pom-pom crab experiments provided new insights into how the crabs acquire and clone their anemone weapons. The findings have given researchers a possible explanation for the lack of free-living Alicia anemones in the wild. REFERENCE Schnytzer et al. Boxer crabs induce asexual reproduction of their associated sea anemones by splitting and intraspecific theft. PeerJ (2017).