Octopus in a tank
When they're alarmed, blue-lined octopuses display iridescent blue rings on their arms to ward off approaching predators. Totti via Wikimedia Commons under CC BY-SA 4.0
Animals have evolved many different ways of protecting themselves, from prickly quills and razor-sharp teeth to clever camouflage and sturdy shells. Now, scientists have discovered a surprising new technique that appears to be a defense mechanism: Some male octopuses use venom to avoid being eaten by their female mates.
Researchers reported their findings this week in a new paper published in the journal Current Biology.
The species in question is the blue-lined octopus (Hapalochlaena fasciata), a small, highly venomous cephalopod that lives in the Pacific Ocean off eastern Australia. Their tentacles are covered in iridescent blue rings, which they display to warn approaching predators to steer clear.
Blue-lined octopuses are tiny, measuring just six inches across, but their venom packs a serious punch. Their salivary glands are full of symbiotic bacteria, which pump out a deadly poison called tetrodotoxin, or TTX for short.
The small cephalopods use the venom to protect themselves and kill their prey. While hunting, they can immobilize a potential meal by injecting the toxin with a quick bite. Conversely, if a predator eats one of these octopuses, the toxin acts as a potentially lethal poison.
In humans, TTX poisoning causes muscle paralysis, dizziness, vomiting, difficulty breathing and, sometimes, death. Scientists have not discovered an antidote to TTX.
Researchers in Australia recently learned that male blue-ringed octopuses also use TTX during reproduction. Females are much larger than males, and after copulation, females often eat their partners.
“Sexual cannibalism is very common in cephalopods,” says study lead author Wen-Sung Chung, a biologist at the University of Queensland in Australia, to the Guardian’s Donna Lu. “When female blue-lined octopuses lay eggs, they spend roughly six weeks without feeding, just looking after the eggs. They really need a lot of energy to get them through that brooding process.”
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But males have evolved a clever defense against this cannibalism. During laboratory experiments, scientists watched male blue-ringed octopuses inject a dose of TTX into their mate’s aorta on the back of her head.
These bites did not kill the females, which are naturally resistant to tetrodotoxin, but they did immobilize them. Their breathing slowed—and, after about eight minutes, stopped altogether—and their bodies went pale. Their pupils did not respond to light.
Once the toxin took effect, males mounted the females and copulated for between 40 and 75 minutes, without the risk of being gobbled up. The mating sessions stopped when the females regained control of their bodies and pushed the males away.
Afterward, the females went about the rest of the day eating and behaving normally—with the addition of one or two swollen lumps near their aorta, the scientists noted. And for roughly three days after copulation, females had an open wound on their aorta from the males’ bites. All the females proceeded to lay eggs 3 to 29 days after mating.
Males have to get the dosage just right: They need to inject enough venom to give themselves time to copulate, but not so much that they end up killing their sexual partner—a scenario the team did not observe but that could theoretically happen, given their small sample size, reports IFLScience’s Stephen Luntz. The male also needs to accurately bite the female’s aorta—otherwise, she’s liable to wake up much sooner. During the experiments, one female woke up after just 35 minutes, because the male missed her aorta, the researchers write in the paper.
“Transferring the spermatophore takes time and would be almost impossible to complete without sedating the females,” Chung tells IFLScience. “Because [females] have the resistance, a lot of TTX is needed.”
Scientists also discovered that male blue-lined octopuses had much larger venom glands than females—the male’s is roughly three times heavier. This size difference is likely linked to the males’ reproductive defense mechanism.
“This is a great example of a co-evolutionary arms race between sexes, where a cannibalizing large female is counteracted using venom in males,” says Chuan-Chin Chiao, an ecologist at National Tsing Hua University in Taiwan who was not involved with the research, to New Scientist’s Martin Lührmann.
Since male blue-lined octopuses mate just once in their lifetimes, it’s not entirely clear what they stand to gain, evolutionarily speaking, by using the toxin to avoid being eaten. Another possibility is that they use the venom to subdue unwilling females, who have lots of different males to choose from or may not want to mate at that time.
In the lab, the males “repeatedly approach the females, and the females become so angry,” Chung tells IFLScience.
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Sarah Kuta | READ MORE
Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.
Filed Under: Animals, Biology, cephalopod, Death, New Research, Oceans, Reproduction, Sex, Wildlife