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Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world.

Species: Lybia leptochelis (adorned by Alicia)
Habitat: under loose rocks between the coral reefs in the shallow tidal zones of the Red Sea

Keep your friends close but your anemones closer. Boxer crabs have this lesson down pat.

In each claw, these miniscule crabs hold even smaller anemones. It’s these accessories that give the crustaceans their other popular name – pom-pom crabs.

At just a few millimetres wide and heavily camouflaged to mimic fuzzy algae, these itty-bitty crabs are tricky to find in the shallow waters of tidal zones on the Red Sea.

“The crabs are quite camouflaged in their environment, but their anemones are what stick out,” says  Yisrael Schnytzer, a marine biologist now at the Marine Biological Laboratory in Woods Hole, Massachusetts. “When you turn over a rock, you don’t see the crab, but you see these two little flashlights that are the anemones.”

He placed pairs of crabs in aquariums, one holding anemones and one without, and found that they fight and steal one another’s anemones. “No one is ever hurt in these fights as far as we can tell,” he says.

In most cases, the crab that started without anemones comes away with one, and in about half the cases it’s actually the crab holding anemones that started the sparring.

“That’s strange,” Schnytzer says. “You’ve got some valuable resource you may be giving away. That leads us to think there’s something ritualistic here. Maybe they have to fight even if they don’t want to.”

When they’re born, boxer crabs don’t have anemones. They don’t even have claws. They float around in their larval stage as they develop, but when they end up on the sea floor they acquire the anemones soon after they land, Schnytzer says.

Pruning the pom-poms

They have delicate claws that they rarely use for anything but holding onto their anemones. Their claws have a series of small hooks that once embedded in the anemones keep them from getting away.

The crabs tend the anemones a bit like a bonsai tree, limiting their food and even sometimes nibbling at them to keep them small enough to hold easily.

“It’s controlling the fate of the anemone,” says Antonio Baeza, an evolutionary biologist at Clemson University. “Maybe the anemone has no say here and it’s totally at the mercy of the crab.”

Baeza says this is not your typical anemone-crab relationship. In most other partnerships between crustaceans and anemones, a large anemone acts as a host for the crab and they both benefit in obvious ways.

“For the anemone, transportation could be beneficial,” Baeza says. “A more important benefit for the anemone here is maybe profiting from the leftovers of the crabs while eating. Crabs are messy eaters.”

The only time they let them go is while grooming in private moments, but Schnytzer and his team caught these unguarded moments by building the crabs shelters in their lab aquaria that allowed them to peek in.

“They’d let go of one of their anemones and hold it with one of their feet, and with the vacant claw clean their antennae and face. This is common grooming behavior known in other crabs. They’d only do this hiding away in secrecy,” he says.

Split the difference

When a pom-pom crab loses an anemone in head-to-head battle, they simply grab the remaining anemone and stretch it apart, tearing it into two equal parts. The remaining pieces regenerate back to a size similar to the original.

This forced asexual reproduction may offer some benefit to the anemones – the crabs do help them multiply after all – but it results in lower diversity for the anemone population.

“One of the benefits of asexual reproduction, even if it’s forced, is that you have clones of the same genome in the environment. That’s a good way to propagate your genome in an environment that doesn’t change too much,” says Baeza.

“Every single crab we’ve ever found was found holding an anemone, but we’ve never found a free-living anemone,” Schnytzer says. “We started making these crazy theories, like maybe there’s a secret anemone garden.”

The coast where we these crabs live is an area of no more than a few square kilometres and there are dozens of researchers out there, he says, and no one has reported seeing any lone anemones like these.

“The anemones are being carried around, and the very fact that we haven’t found them, we have to say maybe they are dependent on the crabs,” Schnytzer says.