Dividing the Spoils

Wild-caught boxer crabs are typically found holding a sea anemone in each claw.

Yisrael Schnytzer

Boxer crabs (Lybia leptochelis) are always found armed with a pair of anemones of the genus Alicia, which protect the crabs’ small, delicate claws, and may provide defense. These anemones have not been found free-living, raising the question of how they are acquired by the boxer crabs.

Some ecologists have reported anecdotal observations of Lybia crabs stealing anemones from each other and splitting them in half, yielding one anemone in each claw. The split anemones then fully regenerate in a form of asexual reproduction known as fission. To test this splitting behavior experimentally, a team led by biologist Yair Achituv and his doctoral students Yisrael Schnytzer and Yaniv Giman at Bar-Ilan University in Israel designed two experiments using crabs collected from two sites on the Gulf of Aqaba in the Red Sea.

In the first experiment, the researchers removed one anemone from each of 22 crabs and observed the crabs for six days. Within this time, 17 of the crabs split their remaining anemones, each of which regenerated into two complete, identical anemones. Such behavior represents the first systematically recorded instance of one marine organism inducing asexual reproduction in another.

In a second experiment, the researchers removed both anemones from 22 crabs, which were then paired with crabs who still held anemones. Of the disarmed crabs, 16 fought with the other crab and successfully stole a whole or partial anemone. Each crab, now with one anemone, then split its anemone into two. Only a single crab that fought with its partner failed to steal an anemone. The remaining five crab pairs refrained from fighting.

During the fights, which lasted up to 40 minutes, all crabs avoided using their claws, wrestling instead with their front legs. The empty-handed crab pinned the other’s claw in its front legs to grab the anemone held there. Once in possession of all or part of an anemone, the crab stopped fighting.

Genetic analysis of 16 anemones from eight crabs found a high degree of similarity both between and within anemone pairs, suggesting that anemone theft and splitting are common methods of anemone acquisition in the wild. (PeerJ)