The blue-lined octopus is a creature of paradox. It’s a small yet deadly creature with enough paralyzing venom to kill a man. It’s also got a dazzling colorful display but a voracious appetite. To top it all off, females are twice the size of males, making sex a very risky proposition.

Now, scientists have found that mating for these squids involves a hefty use of venom, and that actually helps keep the males safe.

The sex lives of squids

Mating in the animal kingdom is often fraught with risk. Among cephalopods, the group that includes octopuses, sexual cannibalism is surprisingly common. Some male octopuses, like the argonaut, avoid the issue altogether by detaching their specialized mating arm and sending it off on its own reproductive mission. Others have evolved elongated arms to keep a safe distance from their mates. The blue-lined octopus, however, relies on chemical warfare.

For male blue-lined octopuses (Hapalochlaena fasciata), it can be fatal. Their female counterparts have a notorious appetite — one that sometimes includes their partners. Faced with the threat of becoming a post-coital snack, males have evolved a grim but effective strategy: they inject their mates with venom.

A new study published in Current Biology reveals how these males deliver a precise bite, injecting a neurotoxin called tetrodotoxin (TTX) directly into the female’s aorta. This bite isn’t fatal for the females, who have some natural resistance to it; but it does make them numb. Their skin turns pale, their pupils stop responding to light, and their breathing slows down and ultimately stops. With his partner immobilized, the male is free to complete copulation, which lasts between 40 and 75 minutes, without risk of being eaten.

Dr. Wen-Sung Chung, a biologist at the University of Queensland and the study’s lead author, describes it as an evolutionary arms race. “Because the females became much bigger and stronger … the male eventually needed to have a specific strategy to make sure his genes can be transferred to the next generation,” he told The Guardian.

Eat your mates

Usually, evolutionary arms races are between members of different species, but in this case, it’s a competition between males and females.

For the males, using this venom on their mates is a high-risk, high-reward strategy. They must deliver just the right dose — enough to keep the female incapacitated during copulation, but not so much that she dies. Remarkably, they’re very competent. None of the observed females died, and all resumed normal behavior the next day.

Dr. Chuan-Chin Chiao, an ecologist at National Tsing Hua University in Taiwan, called the discovery a striking example of evolutionary competition between sexes. “This is a great example of a co-evolutionary arms race between sexes, where a cannibalizing large female is counteracted using venom in males,” he told New Scientist. Chiao was not involved in the study.

But there is still a mystery at the heart of this strategy. Male blue-lined octopuses mate only once before they die, so why go to such lengths to avoid being eaten? Let’s rephrase that: from an evolutionary perspective, it’s not clear what the males have to gain by this. The males play no role in raising the offspring, either. One possibility is that the venom not only prevents cannibalism but also forces females to mate when they might otherwise be reluctant.

As for the females, what they have to gain from this, that is even less clear. Just one thing seems apparent: in the murky depths of the ocean, the fight for survival is relentless — even in the “bedroom”.

The study “Blue-lined octopus Hapalochlaena fasciata males envenomate females to facilitate copulation” has been published in Current Biology.