Male Octopus Arm: Dual Sensory and Mating Tool

The specialized arm that male octopuses use for mating is also a sensory organ that can detect the ovarian hormone progesterone, according to new experiments conducted by Pablo Villar and colleagues. The researchers identified chemosensory receptors on the arm, called a hectocotylus, that appear to have evolved from ancestral neurotransmitter receptors. Villar et al. say the dual-purpose organ offers an example of how sensory adaptations may shape reproductive behavior in ways that can contribute to speciation. During mating, male octopuses probe the mantle of female octopuses to find the oviducts for fertilization. Once located, sperm travel the length of the hectocotylus to be deposited. But how does a male octopus know when it's found the oviducts? In an experimental setup where the researchers coated the inside of tubes with different substances, the release of sperm only occurred when small sucker cups at the tip of the hectocotylus contacted progesterone, a hormone produced by the ovaries (Tubes coated in other substances "elicited aversive behavior," the researchers report). "Small molecular changes in sensory systems can bias reproductive interactions and potentially reduce gene flow. By reshaping perception, evolution reshapes reproduction," writes Anna Di Cosmo in a related Perspective. "Sensory systems may therefore act not merely as modifiers of mating, but as molecular gatekeepers influencing the earliest stages of speciation."