Octopus: sensing the world at an arm’s reach.
With their flexible, color and shape changing body, and eight suckered arms which allow them to move, sense the environment and catch prey, the octopus is an amazing creature. This animal also displays sophisticated behaviors, including learning abilities and the use of tools, demonstrating a degree of intelligence comparable to some mammals. All these sophisticated features and how different octopi look from us humans, have created a popular myth on the alien origin of these creatures. However, more than aliens, octopi represent a different path of evolution that in response to their marine environment and specific requirements, led to a set of adaptations that made them highly successful. Some of these adaptations are evident on the organization of the nervous system and the way the octopus senses the environment.
Vertebrate quadrupeds (four limbed animals), such as humans, have a central nervous system. In this system, sensory information from different parts of the body is processed in a central brain in a way that different body parts are represented in specific parts of the brain. Octopus have a different arrangement: while they also have a central brain, the most prominent part of their nervous system is located in the periphery such that two-thirds of their ≈500 million neurons are located in their arms. This is likely due to the inherent challenge of representing eight flexible arms into a centralized brain. While limb movement in vertebrates is limited by the mobility of the joints, the unlimited range of motion of the octopus’ eight arms, each one requiring high degree of precision, makes a centralized arm representation a big challenge. Instead, a peripheral nervous system in each arm, able to process local sensory information and move accordingly with some degree of independence from the central brain is a solution to the body plan specific to octopus.
The level of autonomy of each of the octopus’ arms, provided by the local nervous system, is evidenced by multiple observations. If an arm is cut off from the body, the severed arm will move and respond to environmental cues for up to an hour. Moreover, each arm has an array of specialized sensory neurons that allow each limb to survey their surroundings and respond accordingly. In response to light, specialized cells in the animal’s skin called chromatophores will expand, resulting in a change of skin color used for camouflage with the local environment. These light-sensitive cells also mediate an avoidance reflex to local illumination of an arm. Without a direct stimulation of the eyes and even when the animal can’t see its own arm, shining light into an arm causes its retraction away from the light. These adaptations provide each individual arm a rapid response to changes of light to either change color and blend with the environment or move away from danger.
When we think about the sense of taste, our immediate thought is the flavors we can perceive with our tongue. This sense is mediated by specialized neurons (called chemoreceptors) that respond to chemicals present in food, translating these chemical components into the electrical activity of the nervous system, which is subsequently processed by the brain. Octopi also have a sense of taste; however, their chemoreceptors are not located in their tongue-like organ, but in their arms. These chemoreceptors respond to prey-derived chemicals and mediate touch-taste based behaviors, adjusting the way they extend their arms and touch their surroundings in the presence of chemical signals. These characteristics allow them to blindly hunt, sticking their limbs into holes and crevices to find prey.
All of the octopus’ adaptations demonstrate how different life forms use diverse strategies to be able to survive and thrive, given their specific environment and particular needs. It also highlights how the human experience of the world is just one of myriads that have occurred over the course of millions of years of evolution and how fascinating other life forms can be.
References
van Giesen L, Kilian PB, Allard CAH, Bellono NW. Molecular Basis of Chemotactile Sensation in Octopus. Cell. 2020 Oct 29;183(3):594–604.e14. doi: 10.1016/j.cell.2020.09.008.
Katz I, Shomrat T, Nesher N. Feel the light: sight-independent negative phototactic response in octopus arms. J Exp Biol. 2021 Mar 5;224(Pt 5):jeb237529. doi: 10.1242/jeb.237529.
Hochner B. An embodied view of octopus neurobiology. Curr Biol. 2012 Oct 23;22(20):R887–92. doi: 10.1016/j.cub.2012.09.001.
