A giant Pacific octopus called Mavis has helped researchers to prove that the one thing everyone knows about the creatures is wrong.

The name octopus is derived from the Ancient Greek for eight feet. Mavis, who lives in a tank at Weymouth Sea Life Centre, actually has six arms and two legs.

Researchers who were studying octopuses’ behaviour were taken aback to discover that some of the most basic assumptions about them were wrong.

Until now it had been believed that the tentacles were deployed in two equal sets, one set of four for propulsion and the other for manipulation.

The research, conducted at 20 centres across Europe, was originally intended to establish whether octopuses favoured one side over the other, as people do, or were multidextrous.

Toys including a Rubik’s Cube were placed in the octopus tanks and a careful watch was made of which limbs the animal used to play with them.

Claire Little, who led the research at Weymouth, where the project was devised, said: “We’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.”

Mavis, whose head is the size of a dinner plate and whose limbs are about half a metre (20in) long, is a member of one of the largest species of octopus. Helped by the suckers on her tentacles she can easily prise open clams and mussels that would defeat most people.

The giant Pacific octopus can weigh up to 71kg (more than 11 stone) in the wild, although Mavis has some way to go. Ms Little said: “We haven’t tried to take her out of the tank to weigh her because she’d find that quite traumatic.”

Octopuses are among the most intelligent of marine creatures and can learn to open jam jars and manipulate small objects such as the Rubik’s Cube – although, so far as is known, none has yet succeeded in solving the puzzle.

During the study, researchers discovered that when octopuses get in a tangle they use their third pair of arms to help. Ms Little said: “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.

“More 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.”

Some previous studies had found that octupuses favoured one side over the other, and the explanation for this was that some were shortsighted or had visual impairments.

Ms Little said: “We identified seven octopuses that genuinely do prefer one side over the other, possibly because of some weakness in the other eye.

“If any of those animals should fall sick, we can now care for them that little bit more efficiently by delivering food and medication from the direction they prefer. As with any sickly animal, any measure that reduces stress, however slightly, can make a crucial difference,” she said.

Researchers gathered the data from more than 2,000 separate observations, assisted by members of the public who were invited to take part in the study. The octopuses were mainly the common octopus, Octopus vulgaris, which also inhabits British waters.

A larger-scale study is now planned so that the findings can be published in a scientific journal.

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