Octopuses don’t think like we do. In fact, most of their neurons aren’t in their brains at all, they’re spread across their eight arms, giving each limb a degree of autonomy that borders on eerie. These semi-independent arms can taste, touch, and even make decisions—without checking in with the brain.

Now, a study published in Cell reveals something even stranger: those arms are reading the environment by detecting the chemicals produced by microbes. The octopus’s unique nervous system isn’t just wired for sensation — it’s tuned to the invisible signals of microbial life coating every surface they touch.

This unusual sensory feat even allows them to distinguish between fresh prey and rotting food or between viable eggs and those no longer worth guarding.

What’s an octopus thinking?

To get a sense of just how bizarre the octopus nervous system is, imagine your arm moving around and touching or grabbing things. You see it, you know it’s your body, but your brain isn’t really in control of what’s happening.

The octopus nervous system is unlike anything in the animal kingdom. Roughly two-thirds of its 500 million neurons are located in its arms, not its central brain. That means each arm can process information, make decisions, and perform complex tasks on its own.

That’s even weirder than it sounds, because how do individual arms sense things? For scientists like Nicholas Bellono, that’s a puzzling question.

Bellono and colleagues previously found that an octopus’ suckers contain a family of receptors that allows them to “taste by touch.” These receptors are sensitive to poorly soluble molecules — compounds that stick to surfaces rather than float freely in the water. That made them ideal for detecting whatever might be coating a rock, a crab shell, or a clutch of eggs.

But that’s not the end of the story. The scientists found that it isn’t the receptors themselves that send the signals, but rather, the microbial communities that live around them.

“We asked how microbial information was being relayed to the octopus sensory system,” says Rebecka Sepela at Harvard, who led the new study. “These receptors lie at the interface between the external environment and the nervous system, so we wondered what kinds of microbes could activate them.”

Microbes are my senses

The team collected nearly 300 microbial strains from the surfaces of prey and eggs in the octopus’s natural habitat. In a painstaking screen, they tested each strain to see whether it could trigger the mysterious chemotactile receptors they had identified.

They found that only a handful of microbes had this power and they were found most often on decaying crabs and unhealthy eggs. These are precisely the kinds of cues that an octopus needs to interpret quickly and correctly.

But these weren’t the only signals the octopus cares about.

The team used a mix of natural product chemistry and structural biology to isolate and characterize the specific molecules that activated the octopus receptors. Collaborating with labs at Harvard Medical School and UC San Diego, they uncovered how each molecule bound to the receptors in slightly different ways.

These subtle molecular differences translated into dramatically different outcomes. One microbial molecule might trigger a neural signal that says “avoid this” — another, “care for this.” The same receptor could respond to multiple microbial compounds, but the ion channels it opened — and the behaviors it led to — depended on the molecule’s shape and chemistry.

“The microbiome is acting almost like a chemical translator,” Sepela says. “It integrates environmental signals — like changes in temperature or nutrient levels — and outputs molecules that inform the octopus how to behave.”

In this way, microbial signatures become a kind of invisible code. The octopus arms read that code and decide whether to eat, reject, or protect what they touch.

It’s probably not just octopuses

Octopuses are the “aliens” of the animal world. They have many peculiarities and are unique in several ways. But they may not be the only ones who use microbes this way.

Microbes shape animal physiology across species, influencing everything from digestion to immunity. They even shape human behavior. What this study adds is a rare, direct link between a microbial signal and a specific behavior; but this link might not be that rare in the animal kingdom.

The implications may run deep into evolutionary history. Even the closest single-celled relatives to animals — choanoflagellates — begin to form multicellular colonies only when triggered by microbial molecules.

That suggests this mode of sensing may have been with animals from the very beginning.

“This might seem like a very specific case — an octopus exploring the seafloor,” Bellono says. “But what we’re seeing is actually a general rule about how organisms sense microbiomes.”

“The octopus gives us a way to study cross-kingdom communication with reduced complexity,” Bellono says. “It’s a system where we can link a microbial signal directly to a behavior — whether that’s predation or parental care.”

This project grew out of a seemingly simple question: How does the octopus use its arms? In the end, the researchers uncovered a new way through which animals sense the world.

The study was published in Cell.