Male Octopuses Sense Progesterone With Mating Arm, Study Finds

Male octopuses detect the female hormone progesterone using a specialized mating arm. This arm, called the hectocotylus, acts as both a sensory organ and a sperm-delivery tool.

Mate recognition shapes how species maintain boundaries and evolve. When individuals prefer certain partners, gene flow between populations declines. These biases often begin with sensory systems like smell or touch, long before genetic differences accumulate.

Researchers already knew that males use the hectocotylus to transfer sperm into the female’s mantle cavity. Octopuses also sense chemicals through receptors on their arm suckers, which helps them find food and avoid threats. However, scientists had not identified a specific mating signal or tested whether the hectocotylus could detect it directly.

To investigate, researchers placed California two-spot octopuses (Octopus bimaculoides) on opposite sides of a barrier. The barrier allowed only an arm to pass through. Males extended their hectocotylus and attempted mating in both light and darkness. They never tried to mate with other males.

Next, the team replaced females with plastic tubes coated with different chemicals. These included progesterone, similar steroids, bile acids, and terpenes. Males spent more time exploring tubes coated with progesterone than any other substance.

In separate experiments, researchers removed the hectocotylus and recorded nerve activity. Progesterone triggered strong electrical bursts and caused the arm to curl and move independently. Other molecules produced little or no response.

The team then analyzed gene activity using single-nuclei RNA sequencing. They found that a chemotactile receptor, called CRT1, is highly enriched in sensory cells at the hectocotylus tip. Follow-up imaging confirmed its precise location.

When researchers expressed CRT1 in cultured cells, it responded strongly to progesterone. The receptor showed an EC50 of about 5.8 micromolar. Cryo-electron microscopy revealed progesterone bound inside the receptor pocket. Evolutionary changes in key amino acids appear to tune CRT1 to prefer progesterone over related molecules.

Lead author Pablo S. Villar and colleagues concluded that the hectocotylus combines sensing and sperm transfer in one structure. This design allows males to assess a mate at the exact moment of contact. Such precision may reinforce species-specific mating preferences.

The study has limitations. Most experiments focused on a single species under laboratory conditions. Some behavioral tests included only a few mating pairs. In addition, parts of the receptor structure relied on computational modeling due to unclear cryo-EM density. The findings suggest a mechanism, but they do not confirm its role in natural populations or speciation.