The American novelist S. E. Hinton once said, “If you have two friends in your lifetime, you’re lucky. If you have one good friend, you’re more than lucky.” By that logic, boxer crabs are the luckiest creatures alive because they can turn one good friend into two by tearing it in half.

These tiny, inch-long crabs carry sea anemones, holding them in place with special hooks on the inner edges of their claws. With their crowns of wavy tentacles, the anemones look like pom-poms, and the crabs like cheerleaders. But those tentacles also pack powerful stings, and a quick jab from them is often enough to ward off an attacking fish. Hence the name: boxer crabs.

Most crabs gather food with their powerful claws, but boxer crabs have adapted so thoroughly to holding anemones that their claws are now feeble, delicate tweezers rather than powerful, crushing pincers. Instead, they rely on their anemones. Some species use the anemones like cutlery, dabbing them onto morsels of food and then bringing them over to their mouths. Others wait for the anemones to passively ensnare food, which they then scrape into their mouths with their front legs. If you remove the anemones, as Yisrael Schnytzer and his colleagues from Bar Ilan University have repeatedly done, the crabs struggle to gather enough to eat.

The anemones, however, flourish apart from the crabs. When Schnytzer freed them from the crabs’ grasp, their colors got brighter, their tentacles became longer, and they more than doubled in size. Left to their own devices, they can grow far bigger than the crabs that once held them. In the words of Schnytzer’s colleague Ilan Karplus, the crabs cultivate “Bonsai anemones,” deliberately stunting their growth to keep them at a manageable size.

On the left: a boxer crab, facing left and holding two sea anemones. On the right: an anemone that grew without any influence from the crabs.
(Courtesy of Yisrael Schnytzer)

But how do the crabs get their anemones in the first place? In 1905, zoologist James Edwin Duerden, in what remains the most thorough account of boxer crab habits, noticed a clue. He wrote that “there appeared to be evidence” that these crabs will tear a single anemone in two to provide one for each claw. Karplus saw similar signs a few decades ago. He noticed that if he took away one of a crab’s two anemones, and came back a few days later, it would once again have two anemones—albeit smaller ones.

He and Schnytzer have now caught several crabs in the act of dividing their partners. It takes around 20 minutes, and the technique is simple: The crab grabs the anemone in both claws, stretches it outwards, and uses its legs to slice through the middle. And since anemones can regenerate their bodies, each half eventually became a complete animal in its own right. The crab, by bisecting its partner, also clones it.

This explains why wild boxer crabs, even very young ones, almost always have two anemones. As long as a crab can get is claws on one, it can easily make a second. And if it has none at all, as Schnytzer found, it can steal a fragment from another crab. “It’s remarkable that these anemones are such a crucial commodity that small, juvenile crabs will actually initiate fights with larger crabs to steal their anemones—and will often win,” says Kristin Hultgren from Seattle University.  

These kinds of fights must happen a lot in the wild. The particular species of boxer crab that Schnytzer studied carries a species of sea anemone that has never been seen on its own in the wild. And yet the crabs always have them, so maybe they all steal them from one another.

Is this how the anemones reproduce? Perhaps partly. But they seem to fall into at least three distinct genetic lineages, and if they only reproduced by crab-cloning, the entire population would be genetically identical. “That suggests the anemones do reproduce on their own,” Schnytzer says. “You can imagine that they could release sperm and eggs into the water, and still breed [while] being held by the crabs.”

It seems that the anemones—their food stolen, their growth stunted, and their bodies regularly torn in two—get very little out of their co-existence with the crabs. “Then again, we’ve never found them free-living,” says Schnytzer. “If they can’t manage on their own, presumably they need the crabs for something.”

Randy Brooks from Florida Atlantic University, who has studied the relationships between sea anemones and other animals, says that some species are only found on the shells of hermit crabs. Those anemones, Brooks found, are capable of reproducing by splitting themselves in half, so perhaps the boxers are only accelerating a process that their anemone partners would naturally undergo. “I've always wished I could work with the boxer crabs,” Brooks says.