With the sensory cells in their suction cups, octopuses can perceive not only mechanical, but also chemical stimuli, i.e. both touch and taste. A study has now clarified how exactly the responsible receptors work and how they contribute to the fact that the arms of the octopus can partially react autonomously to sensory stimuli from the environment.
Octopuses have an extremely complex nervous system. Unlike vertebrates, only a small part of it is concentrated in the brain. Most nerve cells, on the other hand, are distributed over the eight arms. This means that the tentacles can partially act independently of the brain. The sensory cells in the suction cups transmit their signals directly to the nervous system of the respective arm and thus enable quick reactions to environmental stimuli. In their habitat on the seabed, where other sensory organs can only be used to a limited extent, this special feature brings the animals advantages when hunting, for example.
Suckers as a sense organ
Researchers led by Lena van Giesen from Harvard University have now investigated exactly how the sensory cells in the suction cups of octopuses function. To do this, they first observed the behavior of the cephalopods: if they offered an octopus in the aquarium a crab that it could not see but could only feel through a hole, it would explore with one arm until it had found the prey, then snatch it and pulled it through the hole to be eaten. If, on the other hand, he felt an object that resembled a crab in the form of a crab, but was not one, he kept looking without pulling the object through the hole. Apparently, the octopus did not rely solely on the tactile shape, but was also able to identify the prey based on its taste.
To find out which mechanisms underlie this ability, the researchers isolated different types of sensory cells from the octopus’s suction cups and stimulated them individually with mechanical or chemical stimuli. One of the isolated receptors turned out to be a mechanoreceptor: It responded to touch stimuli, but not to substances that carry taste. Another type of receptor – a previously unknown type of chemoreceptor – was insensitive to touch stimuli, but responded clearly to compounds released by prey. “The octopus suction cups therefore contain different types of sensory cells, each with their own functions. This forms the cellular basis for the suction cup as a multimodal sensory organ, ”the researchers write in their publication.
“Thinking” arms
But how exactly are the chemical and mechanical signals calculated in order to trigger a reaction adapted to the situation? The researchers were able to answer this question by analyzing the electrical signals that the respective receptors send out when they are activated. As it turned out, mechano and chemoreceptors show a slightly different activation pattern and are ready for renewed stimulation after different periods of time. Your signals are processed directly in the nervous system of the respective arm and can trigger quick reactions without going through the brain. “With the semi-autonomous nervous system, the octopus arm can quickly make the decision: Do I pull myself together and grab the crab or do I keep looking?” Explains van Giesen’s colleague Nicholas Bellono.
In addition, the scientists investigated which natural substances in the octopus’ habitat the chemoreceptors react to. The receptors they examined reacted to different substances to different degrees depending on the subgroup. According to the researchers, when combined, in particular, they can perceive a variety of smells. Among other things, they were activated by crab and fish extracts, but inhibited by octopus ink. It was already known from previous studies that the ink released in case of danger numbs the sense of smell of attackers.
Taste by touch
The olfactory sense of most marine animals is designed to perceive substances dissolved in the water that are spread over long distances. The examined receptors from the octopus suction cups, on the other hand, responded in particular to water-insoluble substances. Since these cannot be distributed in the water, they can only be perceived by touch. “Such molecules could be found on the surface of octopus prey, for example,” says Bellono.
In order to prove that octopuses actually taste water-insoluble molecules when they touch them, the researchers coated part of the aquarium floor with terpenoids, a substance that some animals emit as a warning, for example to signal that they are poisonous. “The octopus reacted very much to the part of the soil in which the molecule was placed,” reports Bellono. While the animal was scanning the rest of the ground evenly, it only touched the surface prepared with terpenoids briefly and quickly withdrew its arm.
The researchers are now planning to explore other receptors from the newly discovered family of receptors, both in octopuses and in other cephalopods. “We are now trying to identify other natural molecules that the animals could perceive,” says Bellono. Among other things, the team wants to find out whether and how the receptors have adapted to the stimuli from different habitats and how they support specific behaviors.
Source: Lena van Giesen (Harvard University) et al., Cell, doi: 10.1016 / j.cell.2020.09.008