Keeping two arms and two legs coordinated is not always easy. Octopuses not only have twice as many limbs to manage, but their arms behave as if they have a mind of their own.
New research suggests that the task of motor control could be simplified by latching on to a preferred arm for catching prey.
Using California two-spot octopuses (Octopus bimaculoides), the researchers tested the response of the invertebrates to crabs and shrimp dropped in their tanks. The octopuses were hidden inside the burrows, with one eye peering out. Hundreds of video clips revealed how octopuses consistently used the second arm from the middle, on the side where their watchful eye was, to catch their prey. When necessary, neighboring weapons were also used.
“Although the eight arms share a gross anatomy and are considered equipotential, this use of arms for specific actions could reflect subtle evolutionary adaptations,” the researchers write in their published paper.
Crabs and shrimp move differently and at different speeds, causing octopuses to use their attack methods for each. For example, crabs were used for cat-like movement, directed by the second arm, which move much more slowly than flying shrimp.
For the faster, more evasive shrimp, the octopuses drove more slowly with the second arm, possibly incorporating subtle movements that help camouflage the arm’s movements.
“The octopus is known for mimicry during feeding, and we speculate that it swings its arms close to the shrimp to habituate the antennae and sensory hairs on the shrimp’s uropod, thereby reducing its likelihood of escaping with the tail,” the researchers write.
Once contact was made, the neighboring arms (numbered one and three) were then used to secure the helpless hold.
The consistency of the second arm attack was somewhat surprising, given that octopuses often seem anything but coordinated, but the researchers say it’s likely to do with their field of vision (as has been suggested in studies previous ones).
“Because each eye of the octopus covers about 180 degrees with virtually no intersection and shows limited eye movement and no head movement, it is likely that octopuses prefer to place the target in the middle rather than at the edge of the their visual field,” they write. the investigators.
Seconds count when it comes to catching food in the wild, of course, and it seems as though by simplifying the catching process and using multiple arms, octopuses can maximize their chances of getting food.
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The next step for biologists would be to analyze how the activity of neurons is related to such precise motor movements. Scientists don’t think the octopus’s central nervous system is necessarily involved in recruiting extra arms to capture prey; instead, it is likely to be more of a reflex action.
Having a better understanding of the mechanics behind this arm coordination could also help in the development of soft robots, the researchers say, especially those that will work underwater.
“Octopus are extremely strong,” says biologist and study author Trevor Wardill of the University of Minnesota. “For them, picking up and opening a door is trivial, given their dexterity.”
“If we can learn from octopuses, we can apply that to make an underwater vehicle or a soft robot application.”
The research has been published in Current Biology.