Coordinating just two arms and two legs can be challenging. Octopuses, however, have twice as many limbs, and their arms behave as if they have a mind of their own.
Researchers, using California two-spot octopuses (Octopus bimaculoides), studied the reaction of these invertebrates to crabs and shrimp placed in their aquariums. The octopuses hid in their dens, peeking out with one eye. Hundreds of video recordings demonstrated how the octopuses consistently used the second arm, which was closer to the vigilant eye, to capture prey. If necessary, neighboring limbs also joined the process.
“Although all eight arms have the same anatomy and are considered equipotential, their use for specific actions may indicate subtle evolutionary adaptations,” the researchers state in their article.
Crabs and shrimp move differently and at varying speeds, so octopuses employ different attack methods depending on the type of prey. For example, for slowly moving crabs, a jumping, cat-like movement initiated by the second arm was used, while for faster and more evasive shrimp, the octopuses acted more slowly, possibly employing subtle movements that help conceal the actions of the arm.
“The octopus is known for its mimicry during hunting, and we suggest that it uses hand movements near the shrimp to get accustomed to the antennae and sensory hairs of the decapod, thereby reducing the likelihood of escape,” the scientists add.
The sequence of attack with the second arm was unexpected, considering that octopuses often appear not very coordinated. However, researchers suggest that this is related to the peculiarities of their field of vision (as noted in previous works).
“Each eye of the octopus covers about 180 degrees with virtually no overlap and shows limited eye movement and no head movement, so it is quite likely that octopuses prefer to position the target in the center of their field of vision rather than at its edges,” the scientists note.
When it comes to foraging in the wild, timing is crucial, and by simplifying the process of catching and utilizing multiple arms, octopuses can significantly increase their chances of a successful catch.
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