How octopuses use and recruit additional arms to find and manipulate visually hidden items.

Shallow-water benthic octopuses are tactile creatures that explore their environment mainly using chemotactile receptors in their suckers. Details of how they coordinate multiple arms to sense and manipulate items remain unknown. We developed a behavioral assay that exploits their natural foraging behavior to quantify how octopuses use and recruit their arms when searching for and investigating items in a visually occluded environment. Fourteen Octopus bimaculoides were presented with an opaque rock dome to 'blindly' explore for one of five items placed inside. During 117 experimental trials, 2327 arm actions and 394 coordinated arm recruitments were video recorded. Octopuses most often recruited the nearest-neighboring arm when manipulating items (44%). Recruitment of more distant arms was collectively observed (56%). The most common recruitment patterns were (1) initial arm→nearest neighbor→nearest neighbor and (2) initial arm→nearest neighbor→second arm over. We discovered that octopuses used all eight arms with similar frequency and most often engaged three, four, or five arms simultaneously. These findings further demonstrate the flexibility and functionality of all eight arms and indicate that octopuses can use all eight arms equally, which may inform research into arm neuroanatomy as well as the design of coordinated soft robotic arms.