Occasional paper: The interesting home life of the Blue-Ringed Octopus

The blue-ringed octopus! An elegant little creature, native to the southwest Pacific, particularly the waters around Australia. Pretty to look at… but mostly famous for being very, very venomous. The blue-ring’s bite is deadly.  A single sharp nip can kill an adult human in minutes.

But why? The blue-ring is a modest little creature that lives in shallow water, preying on small fish and crustaceans. A bite that can paralyze a 10 gram fish or a 20 gram crab, sure. A bite that can kill a 70 kilogram human dead? What’s the point of that?

Well: the good news is, a recent paper has discovered just why the blue-ringed octopus is so deadly. The bad news is… um, it’s kind of disturbing.

Trigger warning for sexual assault, cannibalism, and existential horror. I am not kidding.


A bit of pedantry first: the “blue-ringed octopus” is actually four or five closely related species scattered across the southwest Pacific and Indian Oceans.  The paper refers to “blue-striped octopus” but from a layman’s point of view they’re almost identical.

The Venom

The blue-ring’s venom is a cocktail of several toxins, but the main component is a nasty piece of work called tetrodotoxin.  Tetrodotoxin is a powerful neurotoxin.  Without getting into the biochemical details — receptors, sodium channels, that stuff — the TLDR is that even a tiny dose will basically short-circuit your nervous system.  A very small dose causes paralysis, very quickly.  More than a very small dose, the nerves that tell your heart to beat and your lungs to breathe start shutting down.  Usually the lungs go first, and the victim dies of asphyxiation.


[Never, never do this.]

There’s no antidote.  If you get a serious bite, the only way you survive is if they get you to a hospital in time to put you on a respirator.

Interesting detail #1: tetrodotoxin is a wide-spectrum neurotoxin, affecting pretty much anything with a functioning nervous system.  It’ll paralyze or kill insects, molluscs, earthworms, birds, whatever.  This, it turns out, is very relevant.

Interesting detail #2: the octopus doesn’t actually produce the toxin itself!  Rather, it’s produced by symbiotic bacteria that live in the octopus’ salivary glands.

And this is where it gets mysterious!  Symbiotic bacteria don’t come cheap.  You have a bunch of bacteria living in your gut, yes?  They help you digest your food?  Well, they charge a tax for that service: they take some of the food for themselves.  You end up coming out ahead because you get more *net* nutrition, but the bacteria are taking their cut.  The octopus has to spend calories and nutrients to support a lot of bacteria that don’t do anything but produce a toxin.  So you’d expect natural selection to favor octopuses that produce only just as much toxin as they need. 

Instead the blue-ring produces much, much more toxin than it seems to need.  Why?

And interesting detail #3: while both male and female blue-rings have the bacteria, produce the toxin, and are venomous, the male is more venomous than the female.  That’s because the male’s salivary glands are larger.  They can be up to 1% of the male’s rest weight, which would be like you having salivary glands the size of oranges.  Because they’re big, they contain a lot more bacteria, meaning a lot more toxin, meaning a more deadly bite.  If a female blue-ring bites you, you’re going to have a very bad day, but you might not die.  If a male bites you… it’s not great.

Ah wait, so males and females are different?

The Interesting Home Life

The blue-ring, like most octopuses, is short-lived and “semelparous”.  Semelparous means that once they reproduce, they die.  Salmon swimming upstream to spawn, yes?  Like that.

A female blue-ring only lives a couple of years.  Towards the end of that period, she becomes sexually receptive.  She may mate with more than one male; if so, she stores their sperm, and she may lay eggs fathered by several different males. She’ll find some quiet safe place to lay them — a crevice inside some coral, a space under a rock.  Once she has laid her eggs, she stops eating.  In this final stage of her life, she doesn’t wander or hunt.  Food can swim right past and she’ll ignore it.  All she does is care for the eggs.  She protects them from predators and parasites, gently blows water over them to keep them oxygenated, and occasionally nibbles and licks them.  Eventually, right around the time the eggs hatch, she dies.

(In some octopus species, the female becomes active one last time shortly before the eggs hatch: she leaves the nest and wanders some little distance away without bothering to conceal herself.  This usually results in her being eaten by a predator.  It’s possible this behavior might have evolved so that at least one local predator is distracted and / or well fed when the eggs hatch, giving them a slightly better chance of survival.  We really don’t know.  But if you saw “My Octopus Teacher”?  That may have been what was going on there.)   (And if you haven’t seen “My Octopus Teacher”, check it out — it’s really good, not what you’d expect, and totally worth an hour of your time.)

In order to survive several months without eating, a female has to build up fat reserves.  The more she can eat before laying her eggs, the better her chances of staying strong and surviving until they hatch.  So females are voracious and aggressive predators at all times, but as they enter their mating period they become particularly ravenous.

A male is about one-third the size of a female.  The picture above?  That’s a male.  Males are also short-lived, but they can mate more than once.  In fact, male blue-rings are notorious horndogs.  They’ve been observed trying to mate with other males, with octopuses of different species, and even with vaguely octopus-shaped clumps of seaweed.  Perhaps when you only live a year or two, you just want as much action as possible.  Whatever the reason, the males are always up for it.

Okay so:  when a male approaches a fertile female, he’s putting himself in danger.  That’s because a fertile female is a hungry female.  Once the female has the male’s sperm, she has every incentive to devour him.  And she’s three times bigger, so once she has a firm grip on him, that’s Game Over.

This is formally known as sexual cannibalism, and there are examples scattered across the animal kingdom.  Black widows, right?  And praying mantises.

 
[(c) Gary Larson, The Far Side, 1986]

Different animal species deal with this different ways.  Some male insects just give themselves up, mating only once and done.  Some do courtship displays that distract or soothe the female. Some spiders present the female with a gift of prey, so that she’ll eat that instead of him.

And this is the paper.  It turns out that the male blue-ringed octopus has a different strategy: he jumps on the female and bites her.  His bite injects a massive dose of venom, which leaves her paralyzed.  

(A female friend:  They *roofie* the females?)
(Me:  Yup, they sure do.  Because, to be fair, if they don’t then the female eats them.
(Friend:  They probably deserve it!)

The female blue-ring has resistance to the toxin.  Not immunity, but resistance.  So while it will almost always paralyze her, it won’t kill her.  She’ll stay paralyzed for anywhere from several minutes to an hour, giving the male plenty of time to do the deed and flee.

(Mrs. Muir:  “He rapes her!”)
(Me:  “Because otherwise she would eat him!”)
(Mrs. Muir:  “That does not make this okay!”)

Okay, well this is all very interesting and perhaps a bit disturbing, but… how does it connect to the blue-ring’s lethality?

Putting the pieces together

So at some time in the past, the blue-ring had a mildly toxic bite, just enough to paralyze small prey and make hunting easier.  Females practiced sexual cannibalism and there wasn’t a lot the males could do about it.

Then one day a male bit a female during mating… and it slowed the female down enough for the male to escape.  That male was able to mate again!  His genes spread. 

Soon there were lots of bitey males.  And there was selection for a stronger toxin, one that would render the female safely paralyzed (though without actually killing her, of course). The males evolved bigger salivary glands, and also more powerful bacteria, because males with these attributes could mate more often and spread their genes.

But!  The females had a vote here.  Females that could resist the toxin, or shake off its effect faster?  Those females still got to eat their male attackers.  In terms of natural selection, they would get an extra meal, rich in fat and protein, before embarking on their final fast.  So genes for resisting the toxin would also tend to spread.

And now we have what biologists call an arms race.  Females evolved resistance.  So males had to evolve venom that was ever more, er, potent.  So females evolved more resistance, in a feedback loop… until we reach today, when the male’s bite is capable of killing a horse.  Humans who have died from blue-ring bites?  Collateral damage from that arms race.

Incidentally, this is why the female is venomous, but less venomous than the male.  She has inherited the potent tetrodotoxin-producing bacteria.  But she doesn’t need to grow large salivary glands to support huge colonies of them.  So her venom is as deadly, but there’s less per bite.  Which brings us to the third player in this game:

The Bacteria

You remember I mentioned that the brooding octopus mother nuzzles and nibbles her eggs?  Well, she does that for a reason: she is transferring the bacteria out of her salivary glands.  Before they even hatch, each baby octopus is equipped with a complete set of venom-producing microbes.

— You might think: wait, half those eggs will be males.  The mother is giving them the tools to assault females?  And the answer is, yes, because it’s now in her genetic interest to do so.  She may not have wanted to be bitten and paralyzed.  But if her sons are properly venomous, then it’s more likely she’ll have lots of grandchildren.  

Meanwhile, the bacteria… are doing extremely well out of this whole thing.  They get to ride around inside octopus bodies, safe and protected.   Geographically they’ve spread over thousands of miles of ocean, which is pretty good for a microbe.  The octopus has evolved a bunch of mechanisms to make sure the bacteria are happy and well fed, and the males in particular have grown giant glands to give the bacteria lots of elbow room.  True, they have to pay the rent by producing tetrodotoxin.  But that really seems a small price to pay.  Tilt your head sideways, and you could argue that both male and female blue-rings are simply puppets, dancing to the tune of their bacterial masters.

Anyway, that’s the paper.  Nature is always interesting, and we’re still discovering new stuff!  It’s just not always comfort reading.