A mind in each arm
The nervous system of an octopus is one of the most unusual on Earth. An octopus possesses around 500 million neurons, a number comparable to that of a dog. Unlike vertebrates, however, most of these neurons are not in a central brain. The central brain contains only about one-third of the total neurons; the other two-thirds are distributed throughout the eight arms. This arrangement results in more than 350 million neurons reside in the arms, with each arm containing a neural cluster of 40 million or more neurons that can act with considerable autonomy.
This decentralized network allows for a high degree of parallel processing. The arms can independently manage tactile and chemical information from their suckers without constant direction from the central brain. Researchers have observed that a severed octopus arm can still react to stimuli, showing the independent capabilities of the arm's nervous system. This distributed intelligence helps solve the complex computational problem of controlling eight hyper-flexible limbs, each with hundreds of suckers that can feel and taste.
Vision beyond the eyes
A 2015 study by scientists at the University of California, Santa Barbara, revealed that octopus skin can sense light directly, without any input from the eyes or brain. This ability relies on the same family of light-sensitive proteins, called opsins, that are found in the retinas of their eyes. These opsins are located in sensory neurons on the surface of the skin. When light strikes the skin, the opsins trigger a response in pigmented organs called chromatophores, causing them to expand and change the skin's color and pattern.
This process, named Light-Activated Chromatophore Expansion (LACE), allows for localized, reflexive camouflage changes. Experiments on skin samples of the California two-spot octopus (Octopus bimaculoides) showed that shining a light directly on the tissue caused chromatophores to expand. When the light was turned off, they relaxed. The skin does not form detailed images like an eye; instead, it detects changes in brightness. The skin is most sensitive to blue light, with a peak response around a wavelength of 480 nanometers, closely matching the sensitivity of the opsins found in the octopus's eyes. This dermal light sense may provide local refinements to the animal's masterful camouflage, allowing parts of its body to match the immediate surroundings without the brain's direct oversight.