This is a touching boy-meets-girl story about the octopus.
A new study by Harvard biologists reveals how octopuses feel their way to potential mates with a "taste by touch" sensory system and even can couple at arm's length without actually seeing each other. In a study featured on the cover of Science, the researchers deciphered how one male appendage serves as a multipurpose organ for seeking, sensing, and seeding—and even continues to respond to female sex hormones after being severed from the body.
"The specialized arm for mating had been documented long ago, but it wasn't known that it's also a sensory organ," said Professor of Molecular and Cellular Biology Nicholas Bellono , senior author of the new paper. "This is the mechanism by which the octopuses recognize their mates and facilitate fertilization."
In the male cephalopods, one of the eight arms known as the hectocotylus is devoted to reproduction. In mating, it snakes its way to the female mantle (a cavity in the main body containing the vital organs), finds the oviduct, and deposits a seed-containing packet called a "spermatophore." The hectocotylus also contains a special groove for passing the sperm packet from the testes in the male mantle to the tip of the extremity.
The role of the hectocotylus has long been known to biologists and the appendage was even mentioned by Aristotle. But little was known about the arm's sensory capabilities—until now.
The Bellono lab has done extensive research on the cephalopod sensory system . Octopus arms explore the seafloor like eight muscular tongues; a single suction cup contains some 10,000 sensory cells. Most of the 500 million neurons in the octopus are distributed in the arms—not the brain—and the appendages can operate autonomously.
The new study, like a lot of events related to reproduction, began by accident.
Pablo Villar , a postdoc in the Bellono lab, was conducting a broad survey of octopus receptors and was intrigued to find the hectocotylus dotted with sensors just like the ones in the other arms.
"That was surprising, because the males generally don't use the hectocotylus for exploring or finding food," said Villar, lead author of the study. "They keep it close to their body, coiled up, and don't really use it for sampling the sea floor."
The scientists decided to let the animals demonstrate how they used the arms. They watched couplings of California two-spot octopuses, the species Octopus bimaculoides native to the Pacific coast of the Americas.
The scientists put male and female octopuses on either side of a black barrier in a saltwater tank. The divider contained small openings just wide enough for the arms.
Even without visual cues, the male could reach into the other compartment, find the female, and insert the hectocotylus tip into her mantle. Females—who had the ability to retreat to an unreachable corner of the tank—often welcomed the outreach. The scientists described the scene: "both the male and female paused all movement, sometimes for over an hour during spermatophore transfer."
"They mated through the divider," said Villar. "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."
Researchers saw similar pairings between different males and females, even in complete darkness. When two males were paired, however, they did not attempt to mate. To the scientists, these clues suggested that females must emit some kind of sex signal.
The researchers analyzed tissue samples from female reproductive organs and found them enriched in precursor molecules for the female steroid progesterone.
Indeed, two experiments showed the power of this sex hormone. Researchers amputated a hectocotylus, exposed the appendage to progesterone, and saw it moved vigorously.
In another experiment, researchers placed male and female octopuses on either side of a barrier but just before their coupling was consummated the female was replaced by tubes coated with progesterone. The males probed the progesterone-laden tubes like a female mantle, but showed no interest in tubes smeared with other chemicals.
Peering through an electron microscope, the researchers saw that the hectocotylus tip was dotted with small sucker cups like the ones on other arms. With staining and single cell sequencing techniques, they revealed that these tissues were densely laced with nerves and sensory cells. These clues indicated that this arm played some vital sensory function.
Next they sought to pinpoint which receptors detected the female sex hormones. Only one responded strongly to progesterone—a receptor called CRT1, which previously been identified for detecting microbes on the surface of prey. It also turned out to play a role in mating.
Progesterone is an ancient hormone that is "conserved"—meaning it has been retained through evolutionary history—but among octopuses its receptors have undergone unique modifications in each species.
The team—a collaboration of 12 coauthors from Harvard, the University of California San Diego, and universities in Okinawa and Sweden—discovered that chemotactile receptors showed evidence of recent rapid evolution, perhaps because these sex steroids helped these animals to recognize potential mates of their own species and distinguished them from other closely-related species.
Octopuses are solitary animals that encounter each other only infrequently to mate. They forage for food by sweeping their arms over the seafloor and craggy rock formations. Sometimes they find each other—and the new study shows how such meetings become affairs of suckers for love.
Bellono said the study underscores how sensory biology can maintain reproductive barriers between lineages and even contribute to the branching of new species—the great problem identified by Charles Darwin. This sex signaling system exemplifies what biologists call "diversifying selection" to sharpen the distinctions between closely-related species.
And it all started with a little curiosity and serendipitous discoveries.
"There's also a philosophical point about how one does science," added Bellono. "Support and emphasis 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, which is like THE biological question."
