The blue-lined octopus (Hapalochlaena fasciata) is a fascinating species, known not only for its striking blue markings but also its venomous hunting and self-defence strategies. The neurotoxin it produces is called tetrodotoxin (TTX)  and it is incredibly potent—far more toxic than cyanide. 

Use of TTX

The octopus does not produce TTX on its own. Instead, it forms a symbiotic relationship with specific bacteria in its salivary glands that synthesise the toxin. This means the octopus cannot make TTX without these bacteria, which are passed on from one generation to the next. The toxin is stored in the posterior salivary glands and the animal delivers its venomous bite when threatened, and to immobilise its prey. So this neurotoxin plays a key role in the octopus’s survival. 

And now a recent study has discovered that during mating, males target the female's aorta with a precise bite that injects her with the neurotoxin at the beginning of copulation. This makes her temporarily immobile, allowing him to mate with her without the threat of being eaten. Female blue-lined octopuses are significantly larger than males, growing up to about 20 cm (8 inches) in length. This puts the males at a disadvantage during mating. But the salivary glands of male octopuses are three times heavier than those of females, and his bite effectively tranquillises her.

Life history

The blue-lined octopus is native to the coastal waters of Australia and parts of Southeast Asia. It prefers shallow waters, often inhabiting rocky reefs, crevices, or tide pools, where it can hide and stalk prey. These octopuses are generally nocturnal, hunting during the night when their prey is more active and less alert.

The blue lines on its body are a form of warning colouration, signalling danger to predators. When threatened, these blue lines become even more vivid, acting as a deterrent to would-be attackers.

Like other cephalopods, blue-lined octopuses have a relatively short lifespan, typically living only 1 to 2 years. They have a semelparous reproductive strategy, meaning they reproduce only once before dying. After mating, females lay eggs and guard them until they hatch, after which they die.

Defence and predation

When threatened by predators (including humans), the blue-lined octopus will often display its bright blue markings as a warning. If the predator persists, the octopus may bite, injecting TTX to defend itself. Though small, it can deliver a potent bite that can incapacitate or even kill larger predators. Yet, though the toxin has caused fatal accidents with humans, such cases are extremely rare.

The blue-lined octopus primarily hunts small crustaceans and fish and uses its venom to paralyse them, making it easier to capture and consume them. The TTX quickly takes effect when the octopus bites, shutting down the nervous system and immobilising the victim. It acts by blocking sodium channels in the nervous system, which prevents the transmission of nerve impulses and results in paralysis.

Nature will try anything

The combination of the venomous bite, impressive hunting strategies, and unique reproductive behaviours make the blue-lined octopus a truly remarkable and dangerous creature in the underwater world. The discovery of how the males use their toxin while mating highlights the extreme measures male blue-lined octopuses take to ensure their reproductive success and is yet another unexpected way that nature reveals its complexity.