When scientists paired two male octopuses in the same setup, the males interacted by touching arms, but they never attempted to mate. This revealed that a specific, female-derived chemical signal was acting as a biological green light for copulation. This immediately raised some questions.
What sensory device might the hectocotyle of a male octopus possess that enables him to clearly detect the oviduct? And what is this female-derived signal that triggers the search?
chemistry of touch
To figure out how octopus sex lives work at a molecular level, Villar’s team first looked at the female’s reproductive organs. They found that the woman’s oviducts and ovaries contained high levels of biosynthetic enzymes important for the production of sex steroids. In particular, the oviduct was filled with enzymes responsible for the production of progesterone.
To test whether progesterone was the trigger, the researchers removed the females from the barrier tank and in their place inserted conical plastic tubes coated with various chemical stimuli into small holes in the wall dividers. When the male encountered a tube coated with progesterone, he actively explored it, and demonstrated the same mating search behavior that he used on the female’s coverts. In contrast, tubes coated with structurally similar steroids, bile acids, or bitter-tasting molecules failed to elicit the same response.
It seems that evolution solved octopus sex by reusing the mechanisms they normally use for hunting. Octopuses use their regular, non-exposed arms to hunt, relying on a taste-by-touch system to explore the ocean floor for prey. This hunting is driven by a distributed nervous system within the arms, which is equipped with specialized chemotactile receptors. It was discovered that the chemotactile receptor in Hectocotyle, a protein called CRT1, also responds to sex signals.
Scanning electron microscopy revealed that the tip of Hectocotyle is covered in tiny sucker cups that are structurally similar to the sensory suckers on their regular hunting arms. Furthermore, these are densely packed with specialized orgasm sucking nerve clusters. Like the arms the crab uses for detection, Hectocotyle expresses a high concentration of mechanoreceptors as well as chemotactile receptors.
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