When an octopus touches a surface, it is also detecting microbial signals on that surface.

As octopuses navigate their surroundings, they extend their arms into crevices, burrows, and coral gaps, exploring environments where visibility is limited. Distributed across the suckers on their arms are dozens of chemotactile receptors (CRs), which allow them to "taste by touch." By contacting the surfaces of prey or other objects, octopuses probe the microbial signals present and use this information to guide subsequent behavioral decisions.

Study organism: the California two-spot octopus (Octopus bimaculoides)（圖片來源：Jeremyse，CC0 1.0 公共領域）

In one study, researchers focused on two ecologically critical targets for octopuses. The first was food, primarily crabs; the second was the octopus's own eggs. Both are essential, yet they require different forms of evaluation. A crab is not simply recognized as food based on its shape—an octopus must distinguish whether it is a live, edible prey item or a decaying, potentially harmful source. Likewise, eggs are not treated uniformly; brooding females must determine which eggs are viable and worth continued care, and which should be removed. Microbial communities on these surfaces provide precisely the kind of signals that reflect such underlying states.

Using scanning electron microscopy, researchers observed that the carapaces of live crabs harbored relatively sparse microbial populations, whereas decaying crabs were densely covered by bacteria of diverse morphologies. A similar pattern was found in octopus eggs: the microbial composition on tended egg casings differed from that on rejected ones. These differences were further confirmed through sequencing of microbial genetic material, demonstrating that distinct surfaces and conditions indeed correspond to distinct microbiome compositions.

The researchers then isolated a large number of culturable bacterial strains from crab and egg surfaces and tested whether the small molecules secreted by these microbes could activate octopus chemotactile receptors. They found that specific bacteria released chemical compounds capable of directly activating these receptors, and that different strains acted selectively on different receptors. One receptor in particular, CRT1, showed strong responses to secretions from certain crab- and egg-associated bacterial strains.

（Three-dimensional structure of the octopus CRT1 receptor, PDB ID: 8EIS）

The key molecules identified from crab-associated bacteria were β-carboline compounds, whereas those from egg-associated bacteria included lumichrome, a flavin molecule. These chemicals are sufficient to be detected by a single octopus receptor and can trigger both neural activity and behavioral responses.

Structure of a β-carboline molecule（圖片來源：Fvasconcellos，CC0 1.0 公共領域）

Structure of lumichrome（圖片來源：MERK，CC0 1.0 公共領域）

Although both classes of molecules bind within the hydrophobic pocket of CRT1, their modes of interaction differ, resulting in distinct binding conformations. Consequently, even though they activate the same receptor, they can produce partially different patterns of neural activity in octopus arms. In all cases, however, they enable the octopus to rapidly evaluate contacted surfaces.

Molecules derived from microbes associated with decaying crabs reduce the tendency of octopus suckers to adhere to surfaces, effectively discouraging the animal from grasping the object after contact. When these microbial compounds were applied to artificial crab models, octopuses still approached and initially explored them, but after contact, they were more likely to avoid the models, mimicking their response to naturally decaying crabs.

A parallel experiment examined maternal behavior. When lumichrome was applied to artificial egg models, brooding octopuses rapidly rejected these eggs. This provides strong causal evidence that microbial-derived signals alone are sufficient to alter behavioral decisions.

These responses can be likened to taste receptors on the human tongue, which immediately evaluate whether a substance is appealing upon contact. This study reshapes our understanding of animal sensory systems: when an octopus touches an object, it is simultaneously assessing—through microbial chemical cues—whether that object is worth keeping, consuming, or rejecting.

Author: Shui-Ye You

Reference:

Sepela RJ et al. (2025). Environmental microbiomes drive chemotactile sensation in octopus. Cell.