The octopuses in the genus Hapalochlaena, known as blue-ringed octopuses, wield one of the deadliest substances known to science. The tetrodotoxin in their venom is such a potent paralytic that a single bite from the palm-size cephalopods can kill a person. But for the octopuses, it’s just a sedative—one that males use to subdue their much larger mates, researchers report this week in Current Biology.

Sexual cannibalism is common in cephalopods. Females stop eating after laying their eggs and dedicate the rest of their lives to caring for and protecting their developing young. Until then, males are on the menu: Males are much smaller than females so must get up close to insert their specialized sperm-delivering arms, leaving them vulnerable to being eaten midmating.

However, when researchers at the University of Queensland observed six pairs of blue-lined octopuses (Hapalochlaena fasciata, above) as they mated, the males always completed sperm transfer and left unscathed—thanks to tetrodotoxin.