Blue-lined octopus Hapalochlaena fasciata males envenomate females to facilitate copulation.

A variety of phylogenetically distant taxa, including flatworms, mollusks, amphibians, and fishes, use the deadly neurotoxin tetrodotoxin (TTX) for predation and defense¹. A well-known example is the blue-lined octopus, Hapalochlaena fasciata (Hoyle, 1886), which uses symbiotic bacteria to sequester TTX in its posterior salivary glands (PSG)². When it bites, the TTX-laden saliva immobilizes large prey and has caused lethal envenomation in a few incidents involving humans³. Female blue-lined octopuses are about twice the size of males, which bears the risk of males being cannibalized during reproduction⁴. Surprisingly, we found that the PSG of males is roughly three times heavier than that of females. Using laboratory mating experiments, we show that males use a high-precision bite that targets the female's aorta to inject TTX at the start of copulation. Envenomating the females renders them immobile, enabling the males to mate successfully.