Episode 116 – Argonaut: The Shelled Sailor

“And today we’re talking about a nautilus that isn’t actually a nautilus at all, but it is a Greek mythological hero. More on some of that later.”

You may have heard of Alexander the Great’s famous diving bell, but he was far from the first earthling to explore the depths with trapped air. Putzing around the ocean in a paper-thin shell, the argonaut has a sophisticatedly simple way of staying buoyant. But nothing gets this odd octopus down here in LDT.


  • Argonauts are octopuses that are not nautalisis but may resemble a nautilus to some.
  • Females have shells that seem to protect their mantle while their tentacles flail around outside, giving them that signature nautilus chic.
  • However, these shells are more like buckets of eggs.
  • They often come in pale blue colors with some dawning reddish hues. But they can change their color and camouflage.
  • They are very sexually dimorphic, with a size difference that’s reminiscent of angler fish.
  • Males are as much as 5x smaller than females and have no shell. 

Measure Up

Welcome to the beloved Measure Up segment. The official listener’s favorite part of the show! The part of the show when we present the animal’s size and dimension in relatable terms through a quiz that’s fun for the whole family. It’s also the part of the show that’s introduced by you when you send in audio of yourself saying, singing, or chittering the words measure up into ldtaxonomy at gmail dot com. We have a new measure up intro this week from Doug from Seattle!

Female Length

  • Females grow up to 10 cm (3.9 inches)
  • How many females go into the largest ever shell, as in the largest calibre rifled weapon?
  • Hint: The gun was a German WWII weapon called Schwerer Gustav and it was 1,490 tons. It could fire these shells up to 29 miles. It was used against French fortifications in the Maginot Line, the toughest fortifications at the time. 
  • 36 female argonauts. The ammunition was 3.6 m (11 ft 10 in).

Male Length

  • Males are around 2 cm.
  • How many males go into the largest ever shell, as in the largest seashells that come from giant clams?
  • Hint: These clams weigh 200 kilograms (440 lbs) and cal live for over 100 years.
  • 60 male argonauts. The shells are 120 cm (47 in).

Fast Facts

  • Males have what’s called a hectocotylus, which is a tentacle used for reproduction. 
    • They detach from the male and attach to the female, and deliver the male’s seed.
    • The female forms an egg case that looks like a nautilus shell.
    • Unlike typical shells, the casing is made of calcite instead of aragonite.
    • The shells are formed through secretions that come from the females expanded dorsal tentacles. 
      • Imagine cupping your hands and secreting clay in such a way to make a tiny little hut for yourself.
    • Once the shell is formed, she fills the case with eggs and then gets inside with the male’s tentacle.
    • Though it’s not technically a shell, she can cover most of her body and head inside, leaving her tentacles outside.
  • Argonauts are venomous, and inject venom into their prey with a bite.
  • They can also use their radula (tongue thing) to bore into shells and then inject the venom.
  • They typically eat small fish, crustaceans, and mollusks.
  • They get eaten by tuna, dolphins, and billfish (sailfish, marlin… Billy the Marlin’s name makes so much more sense!)
  • They used to think that the argonaut would kill and eat some other animal and take its shell rather than produce its own.
  • They didn’t even know about the males until the mid 19th century
  • The tiny male reproduces by putting all of its reproductive output into one of its arms and detaching it from its body. The arm then attaches to the female.
  • For a long time, scientists thought that the detached arms were parasitic worms. Then they thought that the arm was the whole male species.

Major Fact: Sophisticated Buoyancy

  • Most octopuses are bottom-dwellers that swim only when they are threatened.
  • But pelagic octopuses spend their whole lives never touching the seafloor.
  • This means that the argonaut must live suspended between the seafloor and the surface
    • It can be easy to get trapped on the surface and unable to dive back down, leading to predation by sea birds or eventual beach strandings, which happens a lot.
  • So how do they constantly remain suspended without any fins and having to constantly grip onto an exterior shell?
  • After literally thousands of years of speculation, we now know the answer: buoyancy
    • Female Jasons have air trapped in their shells that they regulate to maintain neutral buoyancy (where Jason’s weight pulling down and the buoyancy pulling up are the same)
    • Then, like most other cephalopods, it can move horizontally by pushing jets of water through its body.
  • So where does it get the air? And how does it regulate it?
    • So researchers did a controlled observation in the wild to see what a Jason would do if she were completely out of air in her shell
    • The argonauts were held upside down until all the air was gone and then they were released. 
      • The first thing they did was to jet up to the surface in spurts awkwardly like a jellyfish.
      • Once there, they stuck their shells out of the water and rocked them in one motion, effectively “scooping” the maximum amount of air into the shell.
      • Then they brought the shell back underwater and sealed it with two of their arms before diving back down to about 16 feet, like a diving bell
      • Once at depth, they obtained neutral buoyancy and could jet around freely while holding their shells and vacuum sealing them with their tentacles.
      • The more air captured, the further down they can go and still be neutrally buoyant
      • They can use this to dive down about 32 feet. Any further and the water pressure compresses the air too much to be buoyant.
    • The reason why this remained a mystery is because researchers were conducting experiments in shallow aquariums that didn’t give Jason enough depth to compress the air and obtain neutral buoyancy. So she would jet to the surface, scoop some good good air, then try to swim down before being yanked back to the surface.
    • In contrast, the Nautilus has a permanently attached shell with complicated gas chambers that allow it to obtain neutral buoyant. But the nautilus can dive down to almost 2,500 feet.

Hey everyone, Carlos here. The argonaut may use trapped surface air to stay buoyant, but you know what keeps us afloat? Reviews! People may see our show on a list, but how will they know that it’s full of truly interesting animal info unless you tell them? They’re definitely not going to believe us, so we need your help! Just log onto your favorite podcasting app, search Life, Death, and Taxonomy, then scroll until you see the option to leave a review. Tell us what you think, how much you love the show, or what you think we could do to improve. Thanks for listening and we’ll see you next week!