Octopuses can taste with their arms and may dream in shifting skin colors

An octopus exploring a dark crevice does not need eyes to know whether it has found a crab or a rock, because its arms can taste what they touch. Researchers studying these animals have also found that a sleeping octopus’s skin can erupt into flashing colors, a pattern some scientists suspect may be the closest thing to a dream that an invertebrate brain can produce.

Suction Cups That Double as Tongues

Roughly two-thirds of an octopus’s neurons sit in its arms rather than its central brain, which lets each arm operate with a striking degree of independence, reaching for and identifying objects even when severed from the body. A team led by Harvard molecular and cellular biologist Nicholas Bellono traced this ability to a previously unknown family of receptors packed into the first layer of cells inside each sucker, described in a 2020 study published in the journal Cell. The researchers called these structures chemotactile receptors because they combine chemical sensing with touch, letting the arm register texture and taste in the same instant of contact.

To isolate how the receptors worked, the Bellono Lab cloned them and inserted them into frog eggs and human cell lines, effectively empty vessels with no competing biology of their own. The receptors ignored water-soluble molecules such as salts, sugars, and amino acids that typically interest land animals, responding instead only to greasy, poorly soluble compounds called terpenoids that tend to coat the surfaces of prey and rocks alike. When the team applied those same compounds to the floor of a lab tank, the octopuses reacted strongly only where the terpenoids had been placed, confirming that the arm was reading the seafloor itself rather than relying on any signal from the brain.

A Nervous System Built for Independent Decisions

Because each arm carries its own dense cluster of neurons, an octopus can let a single limb decide in real time whether to grasp an object or keep searching, without waiting for input from the central brain. Bellono’s team described this as a mechanism that lets the arm ask a fast question, whether what it has touched counts as food, and answer it locally. The lab has since begun searching for other natural compounds that might activate the same receptor family, hoping to map how broadly this touch-taste sense extends across the octopus’s foraging behavior.

Two Distinct Stages of Octopus Sleep

Separately from the arm research, scientists studying how octopuses rest have identified two alternating sleep states that look nothing alike. Cephalopod researcher Sylvia Medeiros and colleagues at the Brain Institute of the Federal University of Rio Grande do Norte in Brazil recorded four wild Octopus insularis in a lab and documented the pattern in a 2021 study in the journal iScience. During quiet sleep, the animals turn pale, go still, and narrow their pupils to slits for extended stretches. Roughly every 30 to 40 minutes, that calm gives way to a brief active-sleep episode lasting under a minute, during which the octopus’s eyes dart, its suckers contract, its skin texture shifts, and vivid colors race across its body.

Study coauthor Sidarta Ribeiro noted that the active stage resembles rapid-eye-movement sleep in mammals, though its brevity places octopus sleep cycles closer to the pattern seen in reptiles and birds than in humans. The active episodes occupy less than one percent of an octopus’s total rest time, yet their intensity, full-body color changes paired with muscle twitches, mirrors the visible signs of dreaming in land animals closely enough that researchers have started asking whether something similar is happening inside the octopus’s distributed nervous system.

Costello and the Question of Nightmares

The clearest hint that an octopus might experience something like a dream came from a single animal at Rockefeller University in New York, a Brazilian reef octopus researchers named Costello. Biophysicist Marcelo Magnasco and colleagues reviewed weeks of camera footage after finding Costello’s tank murky with ink one morning and his arms wrapped tightly around a piece of PVC pipe. According to reporting on the still-unpublished study, the footage showed four occasions when Costello appeared to wake from sleep already thrashing, turning deep red, spinning across the tank floor, and releasing ink in a pattern that closely matched how the species reacts to a real predator attack.

Magnasco’s team speculated that Costello, who arrived at the lab with limb damage likely from a past predator encounter, might have been reliving that trauma, though the researchers stressed the observation came from one animal and remains far from proof of dreaming. Other scientists offered a competing explanation: octopuses live only about a year, and evolutionary biologist Robyn Crook of San Francisco State University pointed to Costello’s loose skin, visible lesions, and disorganized movements as signs consistent with the natural decline that precedes death in the species, rather than a nightmare.

What Would Have to Be True for Octopuses to Dream

Confirming dreaming in any animal without language requires more than watching behavior, since color changes and thrashing could reflect leftover motor reflexes rather than an internal experience. Researchers studying the question say they would need direct recordings of brain activity during active sleep, similar to how human sleep researchers use EEG to link REM sleep to reported dream content, before drawing firmer conclusions about octopuses. Until that evidence exists, the chemotactile discovery and the sleep-stage research stand as separate, well-documented findings, while the dreaming question remains an open and actively studied possibility rather than a settled fact.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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