One would think that the answer to this no-brainer is eight (the name octopus comes from the Greek oktos, meaning eight and pous, meaning foot), but a Pacific Giant Octopus (Enteroctopus dofleini) in the Weymouth Sea Life Centre named Mavis is providing researchers with surprising answers to this obvious question.
It was previously assumed that octopuses used four tentacles for propulsion and the other four for manipulating objects, but the research has discovered that they actually use two tentacles as legs and the other six as arms.
This unusual finding is the result of a study conducted at 20 institutions across Europe, whose original aim was to determine if octopuses displayed handedness (a preference for using the limbs on one side of the body over another), or were multi-dextrous.
The study involved 2000 observations (mostly of the Common octopus, Octopus vulgaris) of octopuses manipulating toys such as a Rubik's Cube.
According to Claire Little of the Weymouth Sealife Centre, e've found that octopuses effectively have six arms and two legs. It had been thought they used four tentacles for movement and the other four for feeding and manipulating objects, but observations showed that they use the rearmost two to get around over rocks and the seabed. They also use these two legs to push off when they wish to swim, and then other tentacles are used to propel them.
The study also found that octopuses use their third pair of arms to help them out in a bind.
The real surprise was the frequency with which octopuses employed their third tentacles from the front on both sides. Though it was markedly less than the front two pairs, it was more than we expected, given that earlier studies suggested the four rearmost limbs were reserved mainly for propulsion.
As for the original aims of the study, ore than half of the octopuses studied were found to display no bias at all for either right or left-sided limbs. The rest were split fairly evenly between those preferring the right side and those favouring the left. An octopus's eyes are angled towards the front of its body, so if it used its eyes to determine which tentacles it mobilised, you would expect the choice to favour those more directly in its line of view. That was precisely what we found.
A larger study whose results can be published in a scientific journal is now planned.