When octopuses mate, the male propels his sperm into the female’s body using one of his eight arms. A study now shows that this specialized love arm is both a reproductive and sensory organ. In order to find the female’s fallopian tubes, the tentacle is equipped with sensory cells that can “taste” female sex hormones. In this way, the male octopus probably also receives information about his partner’s age and willingness to reproduce – and only delivers his sperm if the chemistry is really right.
Octopuses usually travel as solitary animals. Mating can only occur if they accidentally meet a member of the opposite sex. This sometimes takes place in a complex love game lasting hours, sometimes within a few minutes and sometimes even at a distance. Because you don’t necessarily need a lot of physical contact: the male octopus transfers its sperm with the help of a specialized love arm, called a hectocotylus. He sticks this into the female’s coat, feels the fallopian tubes inside the body and then transports the package of sperm, called spermatophores, from her testicles through a groove on the tentacle to the desired location.
Mating through a hole in the wall
But how does the male know exactly where to deliver his sperm? A team led by Pablo Villar from Harvard University in Cambridge has now taken a closer look at the octopus’ love life and discovered: The hectocotylus is not only used for reproduction, but is also a sensitive sensory organ. This means that the male octopus can even sense and fertilize partners that he cannot see. For their experiment, the researchers placed a male and a female California two-spotted octopus (Octopus bimaculoides) together in saltwater tanks, with the two animals separated from each other by a partition. They could only feel each other with their arms through small openings in the wall.
But the barrier was not an obstacle to mating: “To our surprise, despite minimal visual information, the male stretched the hectocotylus through the barrier, carefully maneuvered towards the female and then inserted the specialized appendage into her coat,” reports the research team. Both animals remained motionless during spermatophore transfer, sometimes for more than an hour. “They mated through the partition,” says Villar. “For us, this was the simplest and clearest evidence that they can recognize each other using chemosensing alone and mate without full physical contact.”
Sense of taste for sex hormones
Electron microscopic studies revealed that the hectocotylus is equipped with tiny suckers rich in sensory cells. One of the receptors, called CRT1, reacts strongly to the female sex hormone progesterone. This is released in the female’s ovaries – to varying degrees depending on her age and willingness to mate. Similar to the receptors on our tongue, CRT1 can “taste” progesterone when touched. Villar and his colleagues suspect that the male’s love arm moves to where this “taste” becomes most intense. “This integrated feature ensures partner assessment for gender, age, and likely numerous other factors that modulate progesterone levels and receptivity to mating,” the team explains. “All of this happens at the location and time of sperm delivery – a strategy that fits well with the rare encounters of these loners.”
As the research team found, other cephalopod species use the same system. They also use the hectocotylus for searching, tasting and fertilization. But the receptors show small but significant differences from species to species and have apparently undergone accelerated evolution. Although they all react strongly to progesterone, they differ in their affinity for other steroids. It is possible that the different octopus species recognize their own species and avoid mistakenly mating across species boundaries. The combined reproductive and sensory organ could also have contributed to the diversification of octopus species.
Source: Pablo Villar (Harvard University, Cambridge, Massachusetts, USA) et al., Science, doi: 10.1126/science.aec9652