When an octopus injures an arm, the appendage can split in two, giving the cephalopod an extra limb. But it’s long been unclear just how functional this ninth arm is, or how octopuses gradually accustom themselves to reusing their injured arm. Now, the first long-term observation of one of the animals in the wild reveals this new arm doesn’t just work—it can adapt to protect itself and can become specialized for complex tasks, even dangerous ones.
Researchers captured 24 videos and nearly 6000 photographs of a young male common octopus (Octopus vulgaris) off the Spanish island of Ibiza that had sustained injuries to five arms, presumably during a run-in with a predator. Most of the animal’s appendages regrew normally, but its front-most right arm split into two smaller branches.
At first, the octopus didn’t use its most injured arm in risky behaviors such as catching prey, opting instead to curl its arm under its body or perform other nonaggressive behaviors. To compensate for this possible post-traumatic response to the memory of pain, another arm nearby undertook more risks.
But the injured arm’s split branches took on more perilous tasks as they grew stronger, the team reports in Animals, including probing objects and pouncing on and enveloping prey.
The adaptability, the researchers say, underscores just how flexible octopuses are: Their limbs can react to sensory inputs without requiring their brains to engage, as if the appendages are making decisions independently. This decentralized behavior appears to extend to split arms, too, as they take on new functions with healing and time.