The Sucker's Pocket: How the Octopus Evolved an Ion Channel to Taste by Touch

An octopus hunting along the seafloor does not rely on its highly developed eyes. It reaches its arms into dark crevices, under shelves of shale, and deep into the silt. The arm behaves like a semi-autonomous predator. It crawls, probes, and retrieves prey without receiving continuous instruction from the central brain. If an arm touches a stone, it ignores it. If it touches a crab shell or a decaying egg, the suckers immediately adhere and pull the object toward the beak.

The decision to grasp or release occurs at the surface of the sucker itself. The skin lining each sucker is packed with specialized cells that can sense both mechanical pressure and chemical signals. In 2020, physiologist Nicholas Bellono and his colleagues at Harvard University discovered a unique family of proteins called chemotactile receptors (CRs) embedded in these cells (PMC). These receptors allow the animal to "taste by touch" (PMC).

In 2023, Bellono's team partnered with structural biologist Ryan Hibbs of the University of California, San Diego, to resolve the precise three-dimensional structure of the primary octopus receptor, CRT1, using cryo-electron microscopy (Nature). They mapped the protein at a resolution of 2.62 Ångströms, depositing the structure in the Protein Data Bank under the accession ID 8EIS. The structural map reveals how the octopus adapted its ancestral neuroanatomy to detect insoluble chemicals that do not dissolve in marine environments.

The Repurposed Neurotransmitter Receptor

The structural data show that CRT1 is a pentameric ligand-gated ion channel (PMC). It is made of five protein subunits arranged in a ring that traverses the cell membrane (RCSB PDB). This barrel-like architecture is highly homologous to human nicotinic acetylcholine receptors, which operate within internal synapses to facilitate muscle contraction and cognitive processing (PMC).

However, nicotinic acetylcholine receptors are designed to bind small, highly polar, water-soluble neurotransmitters like acetylcholine. These compounds diffuse rapidly across synapses. If an octopus used similar polar receptors to search for prey, the receptors would continuously fire from water-soluble chemicals drifting through the current, flooding the nervous system with noise.

The octopus's evolutionary path bypassed this problem. By sequencing the genes of Octopus bimaculoides (the California two-spot octopus), the researchers demonstrated that chemotactile receptors diverged from ancient nicotinic acetylcholine receptors via gene duplication followed by rapid genetic mutation (PMC). The channel pathway that lets ions pass through the membrane remained largely unchanged (PMC). Instead, the mutations occurred at the exterior ligand-binding pocket, reshaping the physical and chemical interior of the protein's mouth (PMC).

The Hydrophobic Pocket

The Cryo-EM structure of CRT1 reveals why the octopus can sense compounds that do not dissolve in water. While the binding pocket of a human acetylcholine receptor is hydrophilic and attracts polar molecules, the binding pocket of the octopus's CRT1 is lined with non-polar amino acids (The Wire Science). These hydrophobic residues repel water but readily attract greasy, waxy compounds (PMC).

These waxy compounds include terpenes—poorly soluble molecules secreted by marine invertebrates, algae, and benthic microbes as chemical defenses or signals (PMC). Because terpenes are hydrophobic, they do not drift away in the water column; they remain bound to the surface of the organism that produced them (PMC).

When the octopus's sucker skin presses directly against an object, the hydrophobic pocket of CRT1 secures the target molecule, excluding water. This binding changes the shape of the receptor, opening the central pore. Positive ions flow through the open pore, depolarizing the cell membrane and initiating an electrical impulse that travels to the peripheral nervous system within the arm (PMC). Because the receptor only responds to contact-dependent molecules, the octopus can hunt in pitch-black crevices, mapping the chemical landscape of the seafloor centimeter by centimeter.

A Divergent Hunting Strategy

This structural modification is not shared by all cephalopods. While the octopus evolved to crawl and probe, squids adopted an ambush-style predatory strategy. They hunt visually in the water column, throwing out two long tentacles to capture moving prey and bringing it to their arms.

In their comparative analyses, Bellono and Hibbs discovered that squids express far fewer chemotactile receptor genes than octopuses (PMC). Furthermore, the primary chemotactile receptor in squid suckers, CRB1, is structurally distinct from the octopus's CRT1 (PMC).

Rather than responding to insoluble terpenes, the squid's receptors are tuned to water-soluble, bitter-tasting molecules, such as denatonium (PMC). The squid's visual tracking brings the prey in, and its arms use taste receptors to determine whether the prey is toxic before swallowing (Science News). The molecular divergence of the receptor correlates directly with the different predatory strategies of the two organisms.

Beyond hunting, newer research has shown that the octopus has adapted these receptors for reproduction. In 2026, Bellono's lab identified related chemotactile receptors on the specialized mating arm of male octopuses, the hectocotylus, that are highly sensitive to the ovarian steroid hormone progesterone (bioRxiv). This allows the male to identify and coordinate mating through physical contact, even when separated by barriers that block sight. The same basic, pentameric structure that senses a crab in the dark has been modified to detect mates.

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