The Tiny Blue-ringed Octopus Is the Ocean's Deadliest

By: Stephanie Parker  | 
blue-ringed octopus
When the tiny blue-ringed octopus is threatened, its faint blue rings become bright and vivid to warn predators to back off. Torsten Velden/Getty Images

Don't let the beauty or size of the blue-ringed octopus fool you. The body of the tiny octopus doesn't get much bigger than a golf ball and can be as small as a penny. While the blue-ringed octopus is quite stunning, with bright yellow skin patterned with small blue rings that intensify when it is threatened or under stress, this octopus is extremely venomous.

They're native to the Pacific Ocean, from Australia and Indonesia to the Philippines, Japan and South Korea. They live in coral reefs and tide pools, and when they're not out hunting for food or looking for a mate, they hide in crevices or shells. They generally eat small crustaceans, such as crabs and shrimp, and can live for about two years.


Not Just One Species

The blue-ringed octopus is in fact not one single species. There are two species that are known as blue-ringed octopuses: Hapalochlaena lunulata, the greater blue-ringed octopus, and Hapalochlaena maculosa, the southern blue-ringed octopus, also known as the lesser blue-ringed octopus. In addition, there are two other confirmed species that are members of the same genus, Hapalochlaena.

Peter Morse, a marine zoologist at James Cook University in North Queensland, Australia, who studies these octopuses, in particular their mating habits, says that one of the main differences between the two species of blue-ringed octopus is how they reproduce.


The greater blue-ringed octopuses, along with living longer and generally growing a little larger, have a planktonic larval phase, meaning that they start off as plankton before growing into octopuses we know. This helps them disperse much farther, making their range wider, and their population higher. The southern blue-ringed octopus, on the other hand, hatch as mini-adults.

"They can only get as far as they can crawl in a lifetime," Morse says of the southern species, which means that there is more inbreeding among this species.


They Pack a Venomous Punch

Aside from its striking coloring, what the blue-ringed octopus is most famous for is its highly toxic venom. Its venom is 1,000 times more powerful than cyanide and each octopus has enough venom to kill more than 20 humans within minutes.

The deadly venom is a powerful neurotoxin called tetrodotoxin — the same venom found in pufferfish. While their bite may be very toxic, blue-ringed octopuses are generally not a danger to humans; they usually won't bite unless provoked.


So what happens if a person does get bitten? The venom lasts between 12 and 48 hours, depending on the size of the person and how much venom they get from the bite. The venom is a post-synaptic blocker, which means it blocks neurotransmitters, or nerve signals, in the body. That means the person bitten will go limp in what is known as 'flaccid paralysis.' This only affects smooth muscles, so while it doesn't affect the heart, it does hit the diaphragm, so the person will stop breathing. This happens within minutes of being bitten.

Other signs of flaccid paralysis could be nausea, blurred vision or difficultly swallowing. And the bad news is there is no antivenom available, so emergency care would be required immediately.

"Because they're nocturnal and they're very shy and they give plenty of warning as well, you really would have to be very stubborn to get bitten," Morse says. "The venom is very potent and there isn't an antivenom. But the venom does wear off, so if the [bitten] person could get life-saving techniques during that time, they could be OK."

The good news is there are only a few bites to humans every year, and there have been only three known deaths from blue-ringed octopus bites.


Hunting and Eating Prey

That's likely because blue-ringed octopuses mostly use their venom to hunt and eat. When they're young, they eat very small shrimps, and as they get older and bigger, they take down crabs and small prawns. To feed, they use their venom in a couple of ways:

  • by jumping on the back of their prey, and cracking the shell with their beak and then injecting the venom directly into the wound, or
  • by releasing a cloud of venom into the water near the prey so they will take it in through their gills.

Crabs, for example, have an open circulatory system, so the venom goes through their body pretty quickly and they go limp. Blue-ringed octopuses generally feed on crustaceans that are equal to or smaller than the size of their own heads.


"Blue-ringed octopuses, even though they have venom, usually don't take too many risks because a large crab can still do some damage," Morse says. "Anything bigger is probably not worth it for them."

blue-ringed octopus
The greater blue-ringed octopus, with its iridescent blue rings, is the most venomous octopus in the ocean.
Hal Beral/Getty Images


Venom of Unknown Origins

One mystery of the blue-ringed octopus, according to Morse, is exactly how it gets its venom and when. We do know that the octopuses don't produce the venom themselves; instead it's produced by bacteria in their salivary glands. However, what is still not clear is where these bacteria come from, or how the venom is passed from parent to child, as even larvae in eggs produce the venom.

"We really don't know if you were to keep the octopuses in captivity, if they would need to be exposed to something or eat something to maintain their venom," Morse says.


But as long as they are producing venom, they will remain one of the deadliest animals in the ocean.