Argonauts: Are these the world’s weirdest octopuses?

If there was a prize for the world’s strangest cephalopods, we’re pretty sure argonauts would win.  

Need convincing?

Well, first they make a weird ‘shell’ that isn’t technically a shell. Second, they ride on top of jellyfish and salps, seemingly using them like a kind of gelatinous taxi service, which is just downright adorable.

But if that wasn’t enough, they’ve even got detachable, autonomous penis arms.

Super strange right!

What is an argonaut?

It’s tempting to tack “Jason and the” onto the front of their name, but the argonauts we’re talking about aren’t the burly heroes that hunted for the Golden Fleece. They’re octopuses.

Their shared name isn’t a coincidence. The octopus argonauts do actually get their name from the famous Argonauts in Greek mythology due to both being very good at sailing. In fact, way back in the fourth century BCE, the philosopher Aristotle suggested in his work The History of Animals that these octopuses used their arms as sails and steering oars for their boat-like shells.

For a long time, possibly also helped along by Aristotle’s inconclusive musings, people thought argonauts were a bit like hermit crabs. The theory was that they stole their ‘shells’ from other animals.

That’s turned out to be nonsense! Argonauts actually make their ‘shells’ all by themselves. It’s these thin, spiralling constructions that were the inspiration for their other name the paper nautilus. Their paper-thin ‘shells’ make them look a lot like the cephalopod the chambered nautilus, which is actually only distantly related.

Despite all the confusion, today we’re confident they’re octopuses – though definitely unusual ones.

Paper nautilus ‘shells’

When it comes to unconventional octopus characteristics, the argonaut’s ‘shell’ is a good place to start.

That’s because despite appearances, it’s not actually a shell. It’s an egg case. But argonauts make it out of calcium carbonate – the same stuff other molluscs, such as snails and bivalves, use to build shells. So, what’s the difference?

“Because it’s not produced in the same gland, it doesn’t have that evolutionary lineage to it,” explains our Senior Curator of Molluscs Jon Ablett. “To be analogous across organisms it has to produce the same object from the same tissue. So, a shell is secreted by the mantle, and this isn’t a shell because it’s secreted from the arms.”

Only female argonauts make egg cases. They start to form them from about 12 days old. When they’re all grown up, they use it as a brooding chamber for their unhatched young. 

This egg case also helps the argonaut with buoyancy, doubling as a kind of lifejacket. Female argonauts are naturally negatively buoyant – in other words, without her egg case she’d sink like a stone. That’s not ideal for an animal that prefers spending time up near the surface of the ocean.

They solve this problem by jetting up to the surface and ‘gulping’ air into their egg cases. They then forcefully jet downwards into deeper water and as the air compresses it counteracts their bodyweight, letting them swim suspended in the water – not floating or sinking too much.

People also use their egg cases. They’ve been valued by communities for a very long time, adorning items as far back as Crete’s Bronze Age, around 3000–1100 BCE.

More recently, scientists have been using them to work out how many species of argonaut there are. They made it to a grand total of 53 species and 11 subspecies before running into an issue – egg case characteristics aren’t a particularly reliable way of telling species apart.

The problem lies in argonauts’ ability to repair their egg cases when they get damaged. This means there can be big variations across a single species. Pioneering marine biologist Jeanne Villepreux-Power discovered this ability in the 1830s by running experiments in which she fractured egg cases and witnessed the animals repairing them. 

Argonauts aren’t permanently attached to their egg case. Instead, they cling onto it with two of their arms. But when Jeanne took the structures away completely, the female argonauts couldn’t grow a whole new egg case from scratch and would die.

To help her more easily study her water-dwelling subjects, Jeanne invented the world’s first glass aquarium. However, despite her hands-on work, when she sent her findings to Paris’s Academy of Science, they were declared to be incorrect, with other scientists still hanging on to the opinion that argonauts didn’t make their own ‘shells’.

Jeanne had more luck sending her work to Sir Richard Owen – the founder of our Museum  – who presented it favourably at the Zoological Society, London, in 1839. A fortuitous move, as many of her other notes and drawings were later lost in a shipwreck.

With variations in egg case characters considered, just four living paper nautilus species are officially recognised today.

• Argonauta argo
• Argonauta hians
• Argonauta nouryi
• Argonauta nodosus

Of these, Argonauta argo, or the greater argonaut, is the largest and lives in oceans all around the world. The three other species have more restricted ranges. Argonauta nouryi makes the smallest egg cases and has the smallest range, only found from California down to Peru and as far west as the Marquesas Islands in French Polynesia. 

Male paper nautiluses

Male argonauts don’t produce egg cases, but they’re by no means any less fascinating. In fact, there being two distinct sexes in octopuses is pretty interesting in its own right, as this isn’t the case for many other molluscs.

Argonauts display extreme sexual dimorphism – this is when the male and female look very different. The largest female argonauts can have a total body length of 43.8 centimetres. Males are a fraction of that. Their tiny, squishy bodies only measure about 1.5 centimetres long.

Extreme size differences between sexes aren’t anything new in nature – just look at trilobite beetles or some deep-sea anglerfishes. But much like anglerfishes, argonauts’ approach to reproduction is frankly bonkers.

So, what about those penis arms we told you about. Well, the technical term is hectocotylus. Most squid and octopuses have one and it’s used to deliver sperm to a female.

“Very few squid and octopus have an external penis,” says Jon. “So, most of them have to find a way of transferring the sperm from inside their body cavity to the female.”

Hectocotyli take a variety of forms. Picture octopus arms that are spoon-like, syringe-like or “like a toast rack” and you’ll be roughly in the right ballpark.

“They reach inside themselves and take a sperm parcel and then pass it to the female. Sometimes it’s handed to the female, sometimes it’s injected into the female and sometimes they kind of scratch a hole into the female with their suckers and deposit it there,” Jon explains.

Argonauts have evolved to do things their own way. For one, their hectocotylus isn’t a proper arm anymore. It develops in a pouch under their eye, though it’s covered in suckers and does still look quite arm-like. They detach it from their body and transfer the whole thing to the female. As far as we know, there’s no other octopus that does that. 

It gets weirder. Their hectocotylus can move around by itself. It even seems to be able to sense danger in some capacity. This was something scientists discovered when they tried to pluck one out of an egg case and it escaped from their tweezers and moved into the deepest part of the egg case’s spiral, holding itself firmly in place with its suckers.

This autonomous behaviour is possibly why scientists once thought that the mysterious wriggly things they found in argonaut egg cases were parasitic worms. In fact, respected naturalist Georges Cuvier even went so far as to give the ‘worm’ the scientific name Hectocotylus argonautae in1829. Around 20 years later though, scientists cottoned on to the fact it was actually part of the male argonaut and hectocotylus eventually became the name we use for all octopus arms used for transferring sperm.

Female argonauts can reproduce many times in their lives and are sometimes found to be storing multiple hectocotyli. But for males, reproduction is likely a one-time deal. We think they might die soon after detaching their hectocotylus.

Life in open water

Argonauts live in open water throughout their lives – scientists often use the term epipelagic to describe this lifestyle. This sets them apart from many of their relatives in the incirrate octopus group, which spend most of their time on the seafloor.

This lifestyle is unusual, but it’s not totally unique to argonauts. Haliphron, or the seven-armed octopus, is one of a few other incirrate species that spends time in open water. Then there’s cirrate octopuses, such as the Dumbo octopus, which have fins, an internal shell and typically live in deep water. These tend to be more associated with living in the open ocean.

Argonauts eat crustaceans and other molluscs, using their beak and radula to drill into their hard shells. They’re also sometimes spotted seemingly catching a lift on fish and gelatinous animals such as jellyfish and salps. It’s not entirely clear what benefit this relationship has. The argonaut might be using their ride for self-defence or might even be snacking on them as they ride. 

Female argonauts are also known to link together, forming long chains near the surface and swimming as a unit. They’ll also cling onto other solid structures. Relatively little is known about this behaviour. Some scientists think that by gathering together the females become more conspicuous and by doing so increase their chances of encountering males and so decrease the effort needed for mating. Then again, they may just prefer holding onto something solid and each other’s egg cases will do. The downside of hanging out as a group though is that their cluster may be more likely to attract predators such as fish.

Another threat female argonauts face is one of our making – ocean acidification. Unlike the calcium carbonate shells that other molluscs make, argonauts’ egg cases don’t have a protective outer layer called a periostracum. This makes them more susceptible to dissolving as our oceans acidify, which is one of the knock-on effects of human-induced climate change. This could make these octopuses even more vulnerable than their relatives. For shell-building animals, the more energy they need to put into repairing their home, the less they have for reproduction and growth.

So, there’s a lot we still don’t completely understand about these weird and wonderful octopuses. But if what we’ve figured out so far is anything to go by, who knows what other totally bizarre things we’ll discover.