In a darkened tank divided by a barrier, a male octopus extends one arm through a narrow opening, probing the space beyond. On the other side, a female remains out of view. The two animals never fully meet, and yet the male locates her, positions the tip of his arm, and begins the process of mating.
In a study published in Science, researchers found that octopuses don’t rely on sight to reproduce. Instead, they use what scientists describe as a “taste by touch” system, detecting chemical signals through specialized sensory cells in their arms. Even when separated by barriers, males can still locate females and complete mating using this system alone.
“They mated through the divider,” said Pablo Villar, lead author of the study, in a press release. “For us, that was the simplest and most clear demonstration that they can recognize each other just using chemosensation and mate with no full body contact.”
Octopus Arms That Sense, Decide, and Act
On an octopus’s arm, each suction cup contains roughly 10,000 sensory cells. Most of its approximately 500 million neurons are distributed throughout its arms rather than concentrated in its brain, allowing each arm to explore and respond to its surroundings with a degree of independence.
Male octopuses have one arm specialized for reproduction, known as the hectocotylus. During mating, it transfers packets of sperm, called spermatophores, into the female’s mantle cavity, locating the correct opening and delivering them with precision.
The same arm is also doing the sensing. It detects chemical signals released by the female and uses them to guide the entire process, from locating the mantle to completing fertilization.
“It wasn’t known that it’s also a sensory organ,” said Nicholas Bellono, senior author of the paper. “This is the mechanism by which the octopuses recognize their mates and facilitate fertilization.”
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An Accidental Discovery Leads to a Test
The discovery started with an odd observation. Researchers found that the hectocotylus was covered in sensory receptors similar to those on other arms, even though males rarely use it to explore their environment and instead keep it coiled close to the body.
To understand what the arm was sensing, the team designed a series of experiments. When males were placed near females but separated by barriers, they consistently reached through openings and located the female’s mantle. When the females were replaced with tubes coated with progesterone, a hormone associated with reproduction, the males responded in the same way, probing and attempting to mate. Tubes without the hormone did not trigger the same response.
When researchers severed the hectocotylus from the body, the arm still responded to progesterone by moving vigorously, as if it were searching for a mate. This showed that sensing and response were not controlled solely by the brain. The arm itself could detect the signal and act on it.
A System Shaped by Evolution
The team identified a receptor called CRT1 that responds to progesterone, an ancient hormone conserved across evolution. In octopuses, however, these receptors have diverged between species, likely helping individuals recognize their own kind.
That level of precision matters for animals that encounter each other only rarely and have little room for error when they do.
“There’s also a philosophical point about how one does science,” said Bellono. “To be open-minded and follow what diverse biology shows us is actively being deterred. But this study shows that approach can produce something very fundamental—not only about octopuses‘ mating, but also about the origin of species
In octopuses, sensing and action are tightly linked. A single arm can locate a mate, confirm its identity, and complete reproduction without sight.
Read More: Octopuses Pick Up On Invisible Microbial Cues to Avoid Rotting Food
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