How do octopuses see their environment? While it, like many animals, relies on sight, a great deal of its orienting ability relies on its sense of touch, as its eight tentacles probe the sea floor down to the smallest crevices. Observation of these cephalopods revealed that the sense of touch plays an important role in predation strategy, but the underlying mechanism of prey selection remained unknown. Recently, two teams of biologists, led by Nicholas Belluno of Harvard University in the US, revealed that taste-sensitive receptors, found in the suckers that cover the arms of octopuses, are involved.
In the 1960s, several studies showed that these hundreds of suction cups are lined with sensory cells. Moreover, when octopuses touch their prey, they sometimes decide to reject it. Biologists wondered if this behavior was related to these sensory cells. Thanks to the complete sequencing of the octopus genome, carried out in 2015, they identified in the suckers a whole set of receptors that are not similar to those normally found in sensory cells. It is akin to a specific receptor found in the nervous system in many animals, the nicotinic acetylcholine receptor. These are known to detect neurotransmitters — the chemical messengers responsible for transmitting information between neurons — and are involved in controlling muscle contraction. According to the researchers, during the evolution of octopuses, these nicotinic receptors would have undergone structural changes that gave them a function unprecedented for chemosensory receptors.
In order to clarify the nature of these alterations, the biologists produced an image of these receptors by scanning electron microscopy to compare them with their counterparts in the rest of the animal kingdom. Thus they determined the difference to be as subtle as it was significant at the level of the ligand’s binding domain (the region of the receptor to which the corresponding molecule binds). Instead of a cage-like structure seen in conventional receivers, here this area takes the form of a relatively superficial groove. This allows a variety of molecules to bind to it—particularly water-insoluble (known as “hydrophobic”) molecules, such as the fatty residue left by potential prey on the sea floor.
Understanding the molecular mechanism responsible for “tactile taste” helps explain some of the unique behaviors observed in octopuses, such as their strategy of predation by tactile exploration of surfaces. These results also highlight the differences with the predatory approach observed in a related animal, the cuttlefish. Indeed, this ambush hunts, hides and catches its prey in its claws when it passes near it. To do this, it uses receptors that are sensitive to bitter molecules, which are completely soluble in water. This approach is impossible for octopuses, whose receptors are ineffective in ‘open water’, because the molecules they detect are, through their hydrophobic properties, not soluble and found near the bottom.
For now, Nicholas Belluno and his team’s findings indicate that this tactile taste system is governed by aversion stimuli: octopuses are particularly sensitive to “disgust” signals, rejecting prey they arouse and gobbling up others. But now the researchers would like to know if, among these chemoreceptors, some are sensitive to appetite stimuli, corresponding to the prey that octopuses particularly like to eat.
Octopuses are ideal model organisms for understanding how molecular evolution affects an animal’s sensory systems, and in turn, its behaviour. However, in general, their sensory capabilities are still not well understood. For example, some of their receivers seem to be able to detect differences in voltage, but the effect on their perception of the environment is still unknown.