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Sex might seem an intimate act, but scientists have shed fresh light on how octopuses manage it at arm’s length.

Male octopuses use a specialised arm called the hectocotylus to place a package of sperm inside the female’s reproductive system.

But how this arm detects a mate, or delivers the sperm to the right location, has remained unclear.

Now scientists have found the arm is a sensory organ, similar to a tongue, that can detect the female hormone progesterone. This allows it to seek out and fertilise a mate, even if the male cannot see its sexual partner.

Prof Nicholas Bellono, the senior author of the paper at Harvard University, is not surprised by the mechanism because octopuses are very solitary.

“It makes sense that the arm is both the sensor and the mating organ because in these chance encounters, the arm has to be able to both localise the female, localise the oviduct and very quickly initiate the mating or move on,” he said.

Writing in the journal Science, Bellono and colleagues – including the first author Pablo Villar – reported how they planned to study how octopuses mate.

“This is difficult to do with octopuses, especially octopuses in a laboratory setting, because they’re solitary creatures. They don’t interact very often. And when they do, if they’re both confined to the same tank, they’ll usually fight and often they’ll kill each other,” said Bellono.

The team separated a pair of California two-spot octopuses in a tank using a black, opaque barrier containing holes large enough for their arms to fit through.

Bellono said the plan was to allow the octopuses to get to know each other, then remove the barrier.

However the team found something unexpected: the male put its specialised arm through one of the holes, found the female, inserted its arm into the female’s mantle – the sac that contains the octopus’s vital organs – located the tubes that transport eggs from the ovary, and started to mate.

The researchers found the same behaviour occurred when other pairs of male and female octopuses were placed in the same setup, and even occurred in the dark – supporting the idea the animals were able to copulate without even setting eyes on each other.

However, attempts to mate did not occur when pairs of males were studied.

The researchers then explored whether the reproductive organs of the female octopuses were releasing a female-specific cue. Among the substances discovered in the ovaries and skin of females was the hormone progesterone.

The team found amputated specialist arms of male octopuses moved when in contact with progesterone – but not when in contact with other, similar hormones.

They then returned to their original set up, separating males and females by a barrier with holes. However before mating took place, the female was removed and the holes fitted with tubes filled with different substances.

The results, said Bellano, were striking: unlike the other tubes, males readily explored – and tried to mate with – the progesterone tube, suggesting the hormone alone is enough to trigger key aspects of mating behaviour.

In further experiments the researchers identified receptors on the tip of the specialised arm of male octopuses that appear to be involved in progesterone sensing, adding they seem to show recent, rapid evolution across cephalopods.

Bellano said that suggests different species may be tuned to distinct chemical signals. “This raises the intriguing possibility that these chemical cues help encode both sex and species identity,” he added.

Indeed while the male specialist arm in other species of octopus, and other cephalopods, was found to be sensitive to progesterone, its sensitivity to other hormones varied.

Bellano said the work offers a window into how sensory systems evolve to maintain reproductive barriers, or allow them to blur to enable crossbreeding and the emergence of new species.

But, he added, it also shows the importance of following observations.

“We didn’t really plan to study that this arm was a sensor,” he said. “It was sort of revealed to us by watching the animals.”