Male octopus has ‘sex arm’ that can mate in the dark

The California two-spot octopus is a solitary creature. How exactly they manage to find suitable mates has been one of the ocean’s best-kept secrets.

Now scientists have discovered that male octopuses have a unique way of sensing a female’s presence: they use special sensors in the arm they use for sex. Receptors in the suckers on this arm taste female sex hormones, and directly guide the arm to where it needs to go to deliver sperm, researchers report in the journal Science.

“What’s really cool about this is that the reproductive organ happens to also be the sensory organ,” says Nicholas Bellono, study co-author and biologist at Harvard University. “We think the system evolved to allow these rare encounters to be more successful.” 

Octopuses have a sophisticated toolbox in their suckers, which helps them make sense of the world around them. Bellono’s lab discovered in 2020 that they use a particular family of receptors to taste chemicals when their other seven arms touch prey or the seafloor. Microbes living on those surfaces, members of his team reported in the journal Cell in 2025, release chemicals that activate the suckers’ receptors and tell the octopus whether a fish or a crab is safe to eat or rotten, for example. This mechanism works similarly to how our nose detects the chemicals produced by bacteria when we sniff a carton of spoiled milk.

The researchers had no idea the same receptors were also in the tip of the eighth arm octopuses use just for sex: the hectocotylus. Their discovery of how the octopus’ sensory receptors are involved in mating came about almost by chance.

(How scientists learn from the masters of invisibility: octopuses)

A remarkable blind date

When Pablo Villar, lead of the study and postdoctoral researcher in Bellono's lab at Harvard, first found sensory receptors in the tissue of the male’s sex arm, he was surprised, because he knew octopuses keep this arm curled and close to their body when they are out exploring. He wondered: Could the receptors have something to do with mating? 

The octopuses in the lab had been caught in the wild, and Villar worried that they would harm each other when placed together in a single tank. So he separated a male and female of the Southern Californian species Octopus bimaculoides using a black barrier with small holes, just to get the two animals accustomed to being near each other. If those interactions went smoothly, Villar planned to remove the barrier to see if they would mate.

Instead, the coincidental blind date led to crucial observations. The male octopus first put his regular arms through the barrier holes and felt around the water. Soon, he pulled them back and slid his hectocotylus into the female’s mantle. He found the exact path to her reproductive organs, even though he couldn’t see her. Both froze in place, and the female turned white, during the hours-long process that transfers sperm to the female oviduct.

The octopuses did not seem to mind that several members of the lab team gathered around to watch their copulation. They also surprised the researchers by repeating the mating behavior later that day. Villar also observed mating of other octopus couples. Importantly, he demonstrated that the octopuses did not need any visual information to establish that there was a potential mate on the other side of the barrier, and go forward with their insemination.  

“One of the things that, in my opinion, is so special about these studies is that we let the octopuses and nature guide our study,” Villar says. “It was very easy to start from the behavior, and then from there, to the system and to proteins and structures and single molecules.”

(How scientists learn from the masters of invisibility: octopuses)

The molecules of mate attraction

Knowing that the male’s hectocotylus is lined with receptors that can taste chemicals, Villar wondered whether the female releases a specific chemical cue that reveals her whereabouts, or whether the male octopus would attempt to mate with any other octopus.

Anatomically, males are “very similar to females” since they also have an opening in their mantle. Could the barrier trick a male octopus into blindly searching for reproductive organs in another male? Villar found that, with two males in the tank, the sex arm of each animal stayed put.

“To us [this] was a very strong indication that there is something the females produce that the males don’t,” says Villar. It appeared the mystery substance was needed to trigger mating.

To identify it, Villar and his team gathered tissue samples from the hectocotylus and the female mantle. Using a sequencing technique, they found genes that encode the enzymes responsible for producing sex steroids like progesterone. The discovery matched previous studies that hinted octopuses produce more of these hormones during the reproductive season. 

“We thought: this sounds like a good candidate molecule,” says Villar.

But octopuses are hard to fool. To get proof that the molecule had an effect, the researchers had to present it in a way that made sense to the octopuses in the tank. They removed the female and replaced her with plastic tubes smeared with different sex steroids. Over and over again, the male reached for the progesterone-coated tube. This was the final clue.

Why progesterone?

Progesterone is a steroid hormone found in many animals including humans. In females, ovaries produce progesterone to help regulate reproductive functions and maintain pregnancy.

Heather Rhodes, a biologist at Denison University who was not involved in the study, says the new sensory twist in the sex arm means female octopuses might not need to advertise they’re ready to mate with “showy” behaviors.

“He’s just eavesdropping on her natural hormone profile,” says Rhodes.

Progesterone or a related chemical could also be involved in guiding the male Patagonian octopus to the female, says Mercedes Berrueta, a biologist at the Instituto Nacional de Investigación y Desarrollo Pesquero in Argentina, who was not involved in the study. In this species, which she studies, the male gently dances around the female. The pair then touch arms, and Berrueta says the first contact seems very important.  

Progesterone is common in other animals, too. Rhodes thinks octopuses must have a way to determine whether they are sensing progesterone from a female of the same species or from a shark, for instance.

“It could be that there are other signals, other molecules, other receptors that are part of the story as well. And we just haven’t found them yet,” says Rhodes.

We don’t yet know whether changes in steroids could lead an octopus to mate with an individual of a different species, creating a new hybrid species. Understanding this complex puzzle of chemical mating cues could answer questions about how new species are, or aren’t, created.

“This is a really cool example of how sensation is critical for that, and it’s especially cool because [Villar] didn’t mean to do this,” says Bellono. “A lot of the most important science that we can think of comes from unexpected places.”