Look, but don’t touch

Giovanna Fasanelli | VOLUME 31, ISSUE 5
These blue rings signal a clear warning to the attacker
Of all the colourful animals that can kill, the blue-ringed octopus is possibly one of the most venomous.

Here’s a riddle: what flashes electric blue rings, can kill humans, and can squeeze through a wedding ring?

One of the most famous – and infamous – of cephalopods is the gorgeous, highly lethal and surprisingly diminutive blue-ringed octopus. It can be found within shallow reef waters from southern Australia to Japan, and from the Solomon Islands westward to Sri Lanka.

There are currently four described species of this marine predator, though more are likely to be uncovered as genetic studies continue. All species have in common their most striking feature: pulsating, neon-blue rings or lines that cover the body and are turned on when the octopus perceives threat. These blue rings signal a clear warning to the attacker: caution – highly venomous!

In fact, these tiny molluscs are so venomous that just one adult, weighing a mere 25g, possesses sufficient venom to fatally paralyse 10 humans. Children playing in tidal pools are at risk of overlooking such a deadly creature, for the blue-ringed octopus is no bigger than a hand and, at rest, is supremely camouflaged, often hiding within empty shells and among rocks.

The venom is contained within the salivary glands and is usually delivered via a small, often painless bite that can make it difficult for emergency services to know exactly what has befallen the victim. Within minutes, the venom’s highly powerful tetrodotoxin begins to paralyse the victim’s muscles, including those used to communicate and, most importantly, to breathe. As there is no known antidote for the venom, the key to surviving a blue-ringed octopus bite is to receive rescue breathing until paramedics can place the victim on artificial respiration. With time, the body will metabolise the toxins and victims should recover.

Accidents can happen and, for this reason, blue-ringed octopuses should not be kept as aquarium pets.

The life of this small, yet sophisticated, predator is surprisingly short. From birth, the blue-ringed octopus has around one year to become proficient at hunting small crustaceans and to find a willing mate. The art of crab-killing involves stealthy manoeuvres through rocky reef or sand, using its supreme camouflage to get within pouncing distance of its prey. In a few sudden movements, the unsuspecting crab is enveloped within the octopus’s sticky tentacles, with no chance of escape. A sharp, parrot-like beak then bites a hole through the crab’s carapace, injecting the lethal venom that subdues the struggling crustacean within seconds.

Also of interest is that several cases of cannibalism among juveniles and adults have been observed. The act of mating is met with equal ferocity, as the male impolitely mounts the female and repeatedly inserts his modified mating tentacle, known as the hectocotylus, into her mantle, thereby transferring the all-important sperm packets. In one species, Hapalochlaena Iunulata, the female even has to forcibly remove her over-zealous mate.

The female then lays a single clutch of up to 50 eggs inside a brood chamber, which may be an empty shell, rocky crevice, beer bottle or drink can. The female guards her brood with her life – quite literally, for after they hatch several weeks later, she dies. It’s a real case of Romeo and Juliet, as the male also passes on after keeping his side of the deal. Depending on the species, the hatchlings either enter the plankton or begin life on the reef as miniature adults, ready to attack any crawling crustacean that happens by.


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