‘Just when you thought it was safe to get back into the water’ seems a somewhat trite opening. But whatever the preamble, there remains another reason to watch your step when traipsing through the shallows of north Australia and Southeast Asia, albeit one that is a good deal more aesthetically pleasing than the ugly stonefish: the blue-ringed octopus.
Beautiful and delicate as the coral itself, the four known species of blue-ringed octopus are largely nocturnal, spending their days nestled like land mines in makeshift nests – usually sea shells, but since the advent of man, these flexible invertebrates have made fruitful use of bottles and also cans. Unlike many sea creatures its size (the largest grow to about 57cm), the blue-ringed octopus has been known to become quite aggressive when its seclusion is disturbed – say, by the foot of a clumsy human. It will retract its arms to make its sharp beak protrude in an attempt to discharge one of nature’s most deadly poisons – the deadly toxin tetradodoxin.
Tetradodoxin is the same deadly ingredient found in the flesh of the pufferfish. In the blue-ringed octopus, it is generated by bacteria that live symbiotically within the host creature’s salivary gland. Following a bite, the toxin will usually kill a human – a being many times the size of the octopus – in a matter of minutes. No effective antidote is yet known. The octopus uses its beak to crack the carapace of crustaceans and the shells of mollusks, afterwards injecting its deadly saliva into the prey’s body. It will suck the soft flesh through the hole it has made, leaving behind an empty shell.
Even amidst the endless variation of the animal kingdom, rarely has death been packaged so attractively. Like many of its relatives in the group Cephalapoda (literally meaning ‘head foot’, due to the apparent body structure of octopodes, squid and others), the blue-ringed octopus boasts a flamboyant colour scheme that changes according to its mood. The meanings of cephalapod colouring are complex and not always well understood, though scientists are slowly coming to the conclusion that they are part of a sophisticated communication system that operates between these intelligent animals.
At first glance, the various species of blue-ringed octopus can be difficult to tell apart in situ. But by enraging one, perhaps by interrupting the solitude of a female who is guarding her eggs, a canny (and foolhardy) investigator will cause the octopus to reveal characteristic colours and patterns. The greater blue-ringed octopus is thus named for the size of its rings, not the size of its body. Its rings grow larger and will pulsate when it feels threatened. And for anyone who wishes to get close enough, this species can also be recognized by a characteristic iridescent blue ring that runs through its eye. The lesser blue-ringed octopus is decorated with brown patches, and its rings are barely visible until it is provoked. The blue-lined octopus – a third species – can be grey, beige or brown depending on its mood.