An octopus has eight arms...
|Gloomy octopus. P/C John Turnbull.
... and, even though it's a mollusk, it doesn't have a shell. They do have a hard beak, but there's no shell in the ordinary sense of a mollusk.
As I've mentioned before, there are exceptions to just about every generalization (even this one). A friend mentioned an octopus that DOES have a shell. How can we find out about this? Is that true? Here are the Challenge questions for the week:
1. Is my friend right? Is there an octopus that has a shell? Really?
Let's start with the obvious:
[ octopus has shell ]
and we quickly learn that it's complicated... The short answer is "yes..but..."
It's not hard to find that there are actually two octopus creatures (that is, they have 8-legs, in the Order of Octopoda). One is the Cirrina sub-order of Octopoda--they have a small, internal shell and two fins on their head, and NO ink sack. Here's a deep-water Cirrinothauma, often called the "Dumbo Octopus" for its resemblance to the Disney character.
|This dumbo octopus (Cirrothauma murrayi) is often called the blind octopod due to the lack of a lens and reduced retina in its eyes. Its eyes can really only detect light and cannot form images P/C NOAA
Then there's the other one--the Argonaut (Argonauta argo), a fabulous creature that also creates a very thin, papery shell that it uses to move up and down in the water column... but only the females do so. They are not attached to the body of the female. But, oddly, they make the shell by extruding it from their body and then holding onto it for the rest of their lives.
So the shell is of the Argonaut, but not part of the Argonaut. They make shells for use as egg-cases. By searching for [ argonaut shell ] I was able to find this lovely image of just the shell.
|Argonauta argonaut eggcase / shell. P/C James St. John (Flickr) CC by attribution 2.0
For our purposes, if the Argonaut creates it, we'll call it her shell. This is what one looks like in the wild:
|The white shell is on the left, with the octopus eye and
tentacles sticking out on the right.
But then there's the more delicate issue of whether or not an Argonaut is an octopus or not. It behaves a lot like a regular nautilus (it's got that nautilus-looking shell that it hangs onto), but the Argonauta argo is a genuine octopus that just seems like a nautiloid. By comparison, a regular nautilus (e.g., Nautilus belauensis) has a very different body plan. In this image you can see the outside and the inside of an "ordinary" nautilus shell.
|Exterior and interior of a nautilus shell showing the chambered interior construction
As we've talked about before, these distinctions are important when you're doing your online research.
|Early illustrations of the Argonauta. P/C Wikimedia. Original from Natural History: Mollusca (1854), p. 22 - "Paper nautilus" [Argonauta]
For a wonderful paper about the Argonauta (in particular, with great details about its shell), I highly recommend the paper Recognising variability in the shells of argonauts (Cephalopoda: Argonautidae): the key to resolving the taxonomy of the family Memoirs of Museum Victoria 77: 63–104 (2018).
2. As I read more, I learned a couple of fascinating details about the life of this particular octopus. Can you find two really unexpected things about this animal?
There are many remarkable things to notice about the Argonauta argo, but perhaps the two most remarkable things I picked up just by reading.
A. The Argonauta argo controls its buoyancy by scooping air into its shell while on the surface. With this little bubble of air, it can hover easily in the water column. By contrast, other octopuses linger at the bottom of the sea. They can swim around, but they prefer to hide out on the bottom. The Argonauta defies this with its mid-water behaviors.
B. The little octopus can repair its shell! The shell is large, but has thin walls with just one chamber (not like the chambered nautiluses above). The material of the shell is high in magnesium, but is primarily a kind of calcium carbonate. The biggest surprise to me was learning that if the shell of the paper nautilus is damaged, a female can repair it or can completely rebuild it as needed.
C. Males have a modified sex arm called the hectocotylus. When mating, the hectocotylus detaches from the male and is left inside the body of the female. Sometimes, mature female Argonauts are often found with multiple male hectocotyli (each from a different male) wrapped around the gills inside their mantle cavities. When this was first observed by naturalist Georges Cuvier, it was thought that the hectocotyli were actually a kind of worm. Seems like a strange mistake to make, but the hectocotyli are small and, lets face it, kind of worm-like. If you see several of them wiggling around inside of a female Argonaut's mantle cavity, you too might think they were worms.
I could go on... but I'll let discover more about these strange and wonderful creatures on your own.
3. Who was the woman who first did serious research on this octopus? What essential piece of research gear did she invent?
Since the argonaut octopus is such a wonderful animal, I was curious about who did the first research on the topic. My query was:
[ first researcher argonaut octopus ]
Note that I added the term "octopus" in order to get better, more focused results. But once I did that, the results were great.
Who was Jeanne Villepreux-Power?
|Jeanne Villepreux-Power P/C Wikimedia
She has a remarkable story. Orphaned at 11, began working as a seamstress in Paris, married an English merchant and moved to Sicily where she began an intensive study of geology, archaeology, and natural history. While walking on the beaches near Messina she came across the washed-up shell of the Argonauta and started studying them. In the process, she invented glass-sided aquariums for research purposes, devising clever ways to work with the difficult Argonauts.
Just as importantly, she published her work as a member of the Zoological Society of London. Unfortunately, much of her work was lost in a shipwreck.But she was the first to show that the Argonaut can create its shell. At the time, some thought that they stole the shells from other mollusks. But Villepreux-Power showed that the paper nautilus actually secretes its own shell material. The ability to generate the shell also lets the creature add onto its shell to make it ever larger, and then repair the shell if it breaks (or if a malicious scientist comes along and breaks off a bit).
1. You may find more than you bargained for! When I started this research, I thought there was only one such octopus. I was very surprised to learn of the Cirrina sub-order of Octopoda. I then found myself in a rathole making sure the results I was finding were about the Argonauta argo, and not the "classic" nautilus. Be careful when you find a lot of results--be sure the thing you're reading is actually about the target of your search.
2. Reading matters. One thing I notice about young searchers is a remarkable ability to NOT read the articles they find. Reading in detail (or syntopical reading) is a real skill that you should practice. That's how you find the most remarkable observations. (Such as learning about hectocotyli that look like worms...)