The eight arms of an octopus are right there in its name. But these biomechanical marvels share more in common with appendages found in other animals. Like an elephant’s trunk. Or your tongue.
That’s because octopus arms, like those other fleshy protuberances, are examples of muscular hydrostats, which produce force when different muscle groups relax and contract against one another. Such muscular contractions allow for “almost infinite degrees of freedom to bend, shorten, elongate, twist and turn,” said Chelsea Bennice, a postdoctoral researcher at the Florida Atlantic University Marine Science Laboratory.
In a study published on Thursday in the journal Scientific Reports, Dr. Bennice and her colleagues sought to make that seemingly infinite catalog of motions a bit more finite. They pored over footage of wild octopuses, noting how the cephalopods flexed and twisted their arms as they explored their surroundings, hunted and scrambled over the seafloor. The researchers then broke these behaviors down into their base components, aiming to develop a comprehensive visual dictionary of octopus arm movements.
To build their visual dictionary, the researchers examined two hours of video footage each of 25 wild octopuses in a variety of habitats. Based on this footage, they determined four basic ways that an octopus’s arm can move.
From there, Dr. Bennice and her colleagues identified 12 basic actions that octopuses perform with their arms.
Combinations of those basic actions were grouped into 15 behaviors.
After identifying this hierarchy of arm movements, the scientists compiled 25 video clips, each a minute long, from each octopus subject. They then tallied the different behaviors exhibited by each arm.
“It’s a lot of fun to look at the behaviors, but then it starts to get tedious when you’re analyzing or scoring the video,” Dr. Bennice said.
The scientists had to focus their attention on each individual arm.
“So we’re watching the video eight times, and then we’re also looking at what part of the arm the animal is using for a specific arm movement,” she said.
Overall, the team logged 3,907 individual arm actions in the 25-minute cut, often employed simultaneously by different arms. The combinations of different arm movements hint at the intersection octopuses occupy as both predators and prey.
“Time is very precious, and they have to be efficient. That’s why they’re the ultimate multitasker,” Dr. Bennice said. “When they are out and are possibly in danger of a predator, they’re also looking for food. And as soon as they find food, they’re going to go back to their den.”
The octopuses in this study tended to use their front arms more than their back arms. They also often used their front arms for exploring and their back arms for locomotion. But each arm is capable of the full range of movements and behaviors. Previous laboratory studies had hinted that octopuses showed a preference for their right or left arms. However, the scientists in this study did not observe octopuses being “righties” or “lefties” in the wild.
“The beauty of this system, in many ways, is that you have eight arms, and all of those eight arms can do most of these actions,” said Roger Hanlon, a senior scientist at the Marine Biological Laboratory in Woods Hole, Mass., and an author of the study. That flexibility can come in handy if the octopus loses an arm or two to a predator like an eel.
Jennifer Mather, a professor of psychology at the University of Lethbridge in Alberta, has conducted extensive research on octopus behavior but was not involved in this study. She said that she’s not convinced that the underlying hierarchy of arm movements detailed in this study is correct.
For instance, the “elongate” and “shorten” motions could be counted as one type of deformation rather than two discrete types, she said. The “tiptoe” behavior, in which an arm inches along on individual suckers, is better defined as a behavior of the octopus’s suckers than of its arms, she added.
However, Dr. Mather said that studies involving observations of octopuses in the wild, like this one, are critical because octopuses may behave differently in captivity.
“This is a nice step on the way to getting a much better background for what the octopuses were actually doing with their arms,” Dr. Mather said.
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