A few years ago my older brother came across the paper nautilus in one of the news sites he follows on a list of cool animals and immediately sent me the link. Since then, they've always been a particular favorite of mine in the cephalopod clade.

This is from my dad's college zoology course in 1957. It is one of my favorite books of all time, because it has fantastic (and classic) science illustrations. This is figure 23-16, "Class Cephalopoda. Loligo, squid; Sepia, cuttlefish… — This is from my dad's college zoology course in 1957. It is one of my favorite books of all time, because it has fantastic (and classic) science illustrations. This is figure 23-16, "Class Cephalopoda. *Loligo*, squid; *Sepia*, cuttlefish; *Argonauta*, paper nautilus; *Octopus*, octopus. Variously reduced."

And last week for my birthday, I decided to see if I could get myself a paper nautilus shell. I managed to find an Argonauta hians shell on Etsy, actually. It arrived this Friday and it is truly a thing of beauty. It will definitely be a treasured addition to my specimen collection. [Ed. note: I usually don't purchase or collect shells, but I don't live in an area where I could easily comb the beach to try and find one of my own. Lake Michigan foils me again.]

Naturally, I knew one of the CFF blog entries would HAVE to focus on them at some point!

The shell is secreted only by the female octopuses and is used as an egg case. Recently, however, in situ observations of living female Argonauta shows they actively capture air at the surface and then dive deep using the air to make themselves neutrally buoyant at depth. This is especially important when the female has a clutch of developing eggs in her shell and thus more mass. (J.K. Finn and M.D. Norman, 2010. "The argonaut shell: gas-mediated buoyancy control in a pelagic octopus.")
The females of this species have been known since ancient times, but male argonauts were only described in the 19th century. They do not secrete a shell and can be as little as 1/600th of the female's weight. (J.K. Finn and M.D. Norman, 2010. "The argonaut shell: gas-mediated buoyancy control in a pelagic octopus.")
The argonauts are a classic example of convergent evolution. Octopuses and chambered nautiluses evolved from a common shelled ancestor. Over the course of its evolution the octopus lineage has lost the ancestral shell. Over its evolution, the paper nautilus has secondarily evolved a secreted external shell. And it is convergent because the shape of the shell is superficially very similar to the shell of a chambered nautilus and serves the same function as the chambered nautilus shell. This shows how evolutionary pressures can lead two creatures to develop a trait that appears superficially similar. (Helen Scales, 2015. Spirals in Time: The Secret Life and Curious Afterlife of Seashells)

And finally, the art! My watercolored version of my new, absolutely beautiful shell!