At first glance, the octopus that was found by scuba divers in the waters of an artisanal fishery near the coastal village of Puerto Ángel in Mexico looked like a bizarre nine-armed specimen. But a closer look revealed that it had the usual eight arms after all—only, researchers found, its second arm was bifurcated, which means that it was divided into two branches. The likely cause was a genetic mutation, according to a study published in the current issue of the American Malacological Bulletin. The investigators, who found the specimen in 2012, suspect that the bifurcation was unlikely to be from an arm injury and theorize instead that it was caused by a mutation in one of the Hox genes, which play a crucial role in the formation of limbs and organs. This octopus joins a line of others with similar arm abnormalities: In a particularly extreme case published back in 1965, researchers in Japan described an octopus with multiple branching in seven of its eight arms, which resulted in a total of 90 branches.