It can be complicated enough co-ordinating four limbs, but imagine how bewildering life would become trying to control eight. This is the predicament octopuses contend with on a daily basis. They avoid this potential confusion to some extent by not having proprioception – they are not always aware of where their arms are. Also, each arm can move independently without any input from the brain, courtesy of thousands of neurons taking control on a local level. With each arm being covered in suckers, whose reflex is to stick to everything, it begs the question – how do octopus arms not get tangled up? A recent study by Nir Nesher and his team at the Hebrew University of Jerusalem, Israel, published in Current Biology, reveals how a self-recognition mechanism prevents octopuses from getting in a twist.
The team's first exciting observation was that the suckers on freshly amputated octopus arms never attached to octopus skin. The suckers did attach to any parts of an octopus arm where bare flesh was exposed, and also to inanimate objects such as Petri dishes. Nesher explains, ‘It was unlikely to be vision playing a role here, as amputated arms which display such behaviour don't have eyes’. This led the team to start looking at chemical signals. Sure enough, when the team produced a chemical extract from octopus skin and applied it to the Petri dishes, it reduced the suckers' reflex to attach firmly to the object. Interestingly, when they used chemical extracts from fish skin instead, the octopus suckers attached to the Petri dish with great enthusiasm.
A chemical-sensing mechanism is a method by which octopuses can stop getting their arms in a tangle, with minimal involvement of the brain. However, there will be occasions when being able to attach your suckers to another octopus – they're not opposed to a spot of cannibalism – would be a useful skill to have. To do this, though, octopuses would need to override this ‘default’ option in their suckers of not sticking to octopus skin. It appears that attached octopus arms can do just that; using their brains to override the non-stick reflex, octopuses can choose to attach to octopus skin. Furthermore, it appears that octopuses can even tell whether an amputated arm offered as food is one of their own or comes from another individual; they don't like to hold on to their own amputated arms.
So octopus arms will always default to not grasp octopus skin, but higher circuitry within the nervous system can override this reflex. A self-avoidance mechanism of this sort could have major implications for the design of robots and for use in artificial intelligence. In particular, creating a bio-inspired robot whose limbs can react to changes in terrain, for example, without needing instructions from central processors can have implications for advancing technologies in search and rescue operations. For now, though, octopuses can continue enjoying a non-tangled existence, while keeping their options open should a tasty octopus-related snack present itself.
