This male octopus poisons the female during mating to avoid being eaten

Blue-lined octopuses have a powerful venom called tetrodotoxin. (Wen-Sung Chung via CNN Newsource)

Male blue-lined octopuses inject a powerful neurotoxin into the hearts of females before mating to avoid being eaten, according to a new study.

The males have evolved to use a venom called tetrodotoxin (TTX) to immobilize females, which are normally around twice their size and commonly eat their sexual partners, study lead author Wen-Sung Chung, an animal neurobiologist at the University of Queensland, Australia, told CNN on Thursday.

Blue-lined octopuses can grow to around 4.5 centimetres (1.77 inches), about the same size as a golf ball.

They are known as some of the most dangerous animals in the sea due to their extremely powerful venom, and several people in Australia have died after being bitten by one, according to the Australian Museum.

This is the first time that evidence has been found of a neurotoxin being used in mating, rather than for hunting or defense, said Chung.

“It’s really a surprise,” he said.

Sexual cannibalism is common in cephalopods such as octopuses, as well as in other animals such as spiders and mantises, he said.

Chung added that eating their partner helps female octopuses to accumulate enough energy to produce eggs and then incubate them, describing the males as a “final snack.”

But while other octopus species have evolved longer mating arms that allow them to maintain a safe distance during copulation and avoid this fate, blue-lined octopuses have a comparatively short mating arm that requires close contact, he said.

Chung, who has worked with many different species of octopus for previous studies, filmed blue-lined octopuses to observe how they copulate.

“They have very strange mating behaviour,” said Chung, with mounting the only way for males to impregnate a female.

Chung told CNN that the males would approach the females from behind and attempt to bite them in a specific area that would deliver TTX to their aorta.

Using a closeup lens, Chung noticed that the female would be immobilized and stop breathing for around an hour, giving the male time to mate safely.

After a while the female would come around, and none of the females observed during the study died as a result of the intoxication, said Chung, indicating some resistance to TTX.

As for why the females don’t then eat the males anyway, he said: “When she wakes up she’s still quite weak.”

Describing the process as “an arms race between the sexes,” Chung said that the mating behavior demonstrates how blue-lined octopuses have found a way to transfer their genes to the next generation.

“It’s a kind of survival skill,” he said.

Chung also used a powerful MRI scanner to produce images of the octopuses’ brains and internal organs, discovering that the males had larger venom glands than females despite their smaller size.

Next up, he plans to investigate whether brains of males and females have evolved differently as a result of this mechanism.

The study was published in the journal Current Biology.

By Jack Guy, CNN