Octopus sex is even weirder than you think

Sex with an octopus is even weirder than you think

By Cody Cottier edited by Claire Cameron

The sex of octopuses is based on a particular anatomical trick. Instead of a penis, the male has a special mating arm called a hectocotylus. He feels it inside the female’s mantle—the bulbous structure behind the eyes that houses all of an octopus’s organs, including the reproductive organs—until he finds her ovaries. He then slides a bag of sperm down his arm and deposits it. But the male can’t actually see what he’s doing. So how does he know when he has found the right place to send the sperm? It turns out the answer lies in the arm itself.

In a new study published today in Scienceresearchers show that the sex arm of the male octopus can detect the female’s sex hormones emanating from the oviduct.

The suction cups on octopuses’ arms are equipped with chemotactile receptors that allow them to “taste” their environment through touch. But octopuses do not generally use hectocotylus to hunt or explore the seabed; males instead hold it close to their body when not mating. Still, this appendix, like the other seven, is loaded with receptors, says Pablo Villar, a postdoctoral researcher at Harvard University and co-senior author of the new paper.

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To understand what these receptors might be used for, Villar and his colleagues persuaded two California two-spotted octopuses to mate in the laboratory. Because octopuses can be aggressive, the researchers installed a divider in the tank with a few small holes so the two could keep warm. This arrangement may seem ill-suited for making love, but surprisingly, the male simply crossed the barrier and got to work. The researchers tested four other couples and obtained the same result, even in complete darkness. “They made it look super, super natural,” Villar says.

Octopuses are very visual creatures that communicate through body language and color changes. But these flourishes do not seem essential for mating. “They were able to do it without visual cues,” Villar says, “just by touching.” According to him and his team, females must emit some sort of chemical signal to guide males.

They discovered that the octopus’ oviduct produces enzymes used to make the sex hormone progesterone. This hormone appears to be what makes the hectocotyl work: When the researchers attached tubes to the tank’s separator holes, each coated with a different chemical, the males were quickly attracted to the one containing progesterone. Even the amputated mated arms behaved the same way, responding to progesterone but not to other molecules.

Many animals rely to some extent on the detection of sex hormones to mate. But the organ that detects these hormones is generally distinct from the one that delivers the sperm; in male octopuses, the hectocotylus does both. That way, says Nicholas Bellono, a molecular biologist at Harvard University and Villar’s postdoctoral advisor, “you make sure at the release site that it’s the exact location.”

Females of different octopus species may have unique chemical signatures, and the males’ receptors may be tuned to respond to only the right mix of hormones. If so, this mating strategy could help separate species and potentially give rise to new ones. “The boundaries between species are shaped not only by the genes that organisms carry, but also by the molecular systems that determine how organisms perceive each other,” wrote Anna Di Cosmo, a zoologist at the Federico II University of Naples, in a commentary accompanying the new study. “By reshaping perception, evolution reshapes reproduction, which reshapes the tree of life.”

Elena Gracheva, a neurophysiologist at Yale University who was not involved in the new study, says it is too early to say whether all octopuses mate this way or what role these sensory systems may play in evolution. She is, however, impressed by the rigor of the research, which began with naturalistic observation and continued through to fine-grained molecular analyses. “You have very striking animal behavior, and then you move on to the single molecule, which I think is beautiful,” she says. “But I would say this is just the beginning of the discovery.”

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