In A Nutshell
While they seem harmless enough, every known species of octopus and cuttlefish, and some species of squid, are venomous.
The Whole Bushel
If you have ever picked an octopus up from the water’s edge and felt a small nip, you were right in thinking it bit you, but you may not have realized it was also giving you a shot of venom. The discovery that all octopuses and cuttlefish are venomous is very recent. In 2009, Australian scientists investigating how several species of octopus killed shellfish realized they were using venom. They spread their inquiries further and soon found every species they looked at was the same.
The venom itself is very similar to fugu and some snake neurotoxins, though obviously much milder. Also, snakes inject their venom, octopuses don’t. They use their beaks to drill holes into crustacean shells then deposit (“spit” is the less scientific word) a dose. It may not take much to knock out a sedentary mollusk but some carry their own arsenal of self-defense weaponry so a bit of venom helps.
The only species known to be dangerous to humans is the blue-ringed octopus found in the southern Indian and Pacific oceans, particularly around Australia. With its arms extended, a large one is about the size of a tea saucer and it lives on coastal reefs where its mottled appearance disguises it perfectly. Only when it is challenged does it show off its pulsating, bright blue rings. There is no antivenom but deaths are rare: just three have been recorded in the last century. The most common symptom is severe paralysis.
The relative mildness of other octopus venom explains why it remained unknown for so long. The bite from even fairly large octopuses is innocuous and usually has no visible effect, so the notion it could be injecting venom would have sounded far-fetched. Octopuses also have a reputation as one of the gentlest creatures on the seashore, bothering no one as they lie about in the shallows.
There are about 300 species of octopus known to us, ranging in size from the Wolfi, barely a centimeter long, to the giant Pacific, which can have a span of four meters (13 ft). New species of octopus are being discovered every year, which is excellent news. Medical scientists are especially happy. Animal venom is being used in drugs to treat cancer, diabetes, hypertension, and a range of other diseases, so discovering the octopus venom is like walking into a liquor store and finding a wall of beers you’d never heard of. Of course it also means that one day we may have to reclassify the blue-ringed octopus as the second most dangerous cephalopod.