The octopus sits on maze and prepares to insert an arm into the central tube. Credit:
Credit: Tamar Gutnick
The octopus sits on his den and reaches out towards an onlooker.
Credit: Michael Kuba
LONDON: Octopuses are able to use visual cues to guide a single arm to a location, a complex movement that was not thought possible due to their lack of a rigid body structure, say researchers.
The octopus' arm is made up primarily of muscle with no skeletal support, so octopuses were previously believed to have a low level of body awareness and only limited control over their limbs. However, this study has shown for the first time that they can direct a single arm in a complex movement to a target location.
"Octopuses have a central nervous system that is advanced for an invertebrate, but simple compared to a vertebrate, yet it is capable of controlling a much more 'difficult' arm," said lead study author Tamar Gutnick, a researcher at the Hebrew University of Jerusalem in Israel.
"Because of the unique body plan of the octopus its ability to control a single arm in a complex movement is quite amazing."
Too soft for complex movement?
Octopuses were thought to have no conscious central nervous system-directed (CNS) control over their arms with movement being controlled solely by the activity in the complex array of nerves (PNS) present in the limbs.
However, the visual aspect of the task carried out by the octopuses in this study suggests that there must be an exchange of information between the CNS and the PNS during such behaviours.
In Gutnick and colleagues' experiment, six out of seven octopuses succeeded in using a single arm to select a visually marked compartment containing a food reward in a three-choice, plexiglass maze.
The animals were required to reach the compartment containing the food reward at least five times in a row out of a total number of trials ranging from 61 to 211. The octopuses could only use one arm to complete the task, as the tube leading to each compartment was only wide enough for one limb.
How brains control behaviour
The team observed that the chance of a successful trial improved significantly during the last 20 trials for each animal compared with the preceding trials.
They also noted that the animals seemed to learn that they needed to see the three boxes to improve their chances of getting the reward and were significantly more likely to be in view of the boxes during the last 20 trials than during the earlier tests.
The octopuses also adapted their arm use strategy from mostly 'straight', involving a direct unrolling or pushing upwards of the arm through the tube, to a 'search' strategy, involving probing and crawling in the central tube and above the choice boxes before deciding on a compartment.
