The eight wily arms of an octopus can help the animal catch dinner, open a jar and even complete a convincing disguise. But these arms are not entirely under the control of the octopus's brain. And new research shows just how deep their independence runs—even when they are detached.
The octopus's nervous system is a fascinating one. Some two thirds of its neurons reside not in its central brain but out in its flexible, stretchable arms. This, researchers suspect, lightens the cognitive coordination demands and allows octopuses to let their arms do some of the "thinking"—or at least the coordination, problem-solving and reaction—on their own.
And these arms can continue reacting to stimuli even after they are no longer connected to the main brain; in fact, they remain responsive even after the octopus has been euthanized and the arms severed.
The research is in the special September 2013 issue of the Journal of Experimental Marine Biology and Ecology called "Cephalopod Biology" (we'll check out the other fascinating studies in days and weeks ahead).
The researchers, working at St. George's University of London and the Anton Dohrn Zoological Station in Naples, Italy, examined 10 adult common octopuses (Octopus vulgaris) that had been collected and used for other studies. After the animals were euthanized, their arms were removed and kept in chilled seawater for up to an hour until they were ready for experimentation. Some arms were suspended vertically, and others were laid out horizontally. When pinched, suspended arms recoiled from the unpleasant stimulus by shortening and curling in a corkscrew shape within one second. (After this, the arms slowly relaxed and returned to their previous length.) Tap water and acid applied to the arms evoked a similar response. Horizontal arms also moved away from the undesirable stimuli, many bending in a sort of contrived joint toward the top. "The results demonstrate that the arms are capable of reflex withdrawal to a 'noxious' stimulus without reference to the brain," the researchers noted in their paper.
These post mortem reactions might be cued by nociceptors, neurons that are dedicated to sensing physical danger (in our species, they also are responsible for starting the body's perception of "pain"). This is among the first evidence that octopuses possess these neurons.
Sure, we are also likely to jerk our hand away after, say, touching a hot kettle—a nociceptor-induced reflex. But imagine if human arms still did that after death—and after being cut off. (Our "withdrawal" response is, indeed, dictated by our spinal cord.)
These findings suggest new consideration for research on octopuses, and likely other cephalopods, such as squid and cuttlefish. Just this year, the European Union began operating under a directive (pdf) that cephalopods, like vertebrates, should only be experimented on in ways that minimize pain, suffering and distress.
In the wild, these independent withdrawal responses likely help the octopus in its efforts to keep all of its amazing eight arms intact. "The arms of Octopus vulgaris perform a number of functions (e.g. prey capture, exploration), putting them at risk of damage," the researchers noted in their paper. Octopuses frequently send their arms out of sight (under rocks and into crevices) to search for prey, exposing them to all kinds of potential claws, teeth, chemicals and lots of sharp coral. Although, as we'll see in the next installment, even if they are damaged or lost, octopus arms are incredibly efficient at growing back.
Illustration courtesy of Ivan Phillipsen