I am definitely not the first person to write about the Blue-ringed Octopus, but once you’ve seen one for yourself it is quite understandable that divers get excited about them. Blue-ringed Octopus are one of the few invertebrates you can call “cute”. Small size (Check!), adorable way of crawling around (Check!), innocent looking (Check!) and iridescent blue rings they look like something out of a cartoon (Check!). With the added level of spice that these are also one of the world’s most venomous animals, it’s only normal that people are interested in these critters.
Blue-ringed Octopuses are several species in the genus “Hapalochlaena“, depending on who you ask, there are anything between 3 to 10 species. They are all small animals, the biggest one (Hapalochlaena maculosa) grows to only 15cm (body + arms). You can find them in the centre of the Indian Ocean to the west of the Pacific Ocean. While their colours might fool you to assume they belong in similarly colourful tropical reefs, they are actually more common in the temperate waters of southern Australia.
Something that is repeated very often is how venomous these little guys are, but just how bad is it? The short version: if you get bitten, you’d better hope to have someone nearby who is highly skilled in CPR. One of the more fascinating effects that occur when bitten is the “locked in syndrome“, where you appear to be dead, but are actually still aware of what is going on. If that and (near-)certain death doesn’t stop you from harassing them to get a pretty picture, I don’t know what will.
The most conspicuous features of the Blue-ringed Octopus, its blue rings, are actually hardly visible for most of the time. When you find one while diving and you don’t bother it too much, they look like any other well camouflaged octopus. The blue rings are a warning signal they only show when spooked or threatened. The mechanism of how they show those rings is a really neat one. The rings are coloured cells that are usually covered by muscles laying on top of them. It is only when the octopus relaxes these muscles that the blue rings become visible. Like a blanket that’s pulled away when unveiling a work of art.
Another curious thing about them is how they mate since male blue-ringed Octopuses can’t tell the difference between males and females! Males will try to mate with any other Blue-ringed Octopus they meet, pouncing on their potential partner and inserting their hectocotylus (=arm modified for mating) into the mantle cavity of the other octopus. They can only tell if their partner is female or not until after they insert this “mating arm” into the other octopus. If the partner turns out to be another male, they amicably part ways, no harm done. In case they get lucky and their partner is a female, the male clings on for a long time: usually more than 90 minutes. As a matter of fact, it seems the male tries to hang on as long as the female allows it, only breaking contact when forcefully removed by the female.
Considering that this animal is one of the most popular critters in muck dive tourism, it is surprising how little we really know about them. So have a good look at them next time you see one and enjoy the antics of these little guys from a distance safe enough not to get bitten or have a mating arm thrust your way.
