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 defense. 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). When it bites, the TTX-laden saliva immobilizes large prey and has caused lethal envenomation in a few incidents involving humans. Female blue-lined octopuses are about twice the size of males, which bears the risk of males being cannibalized during reproduction. 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.