The Freaky Biology of Nautiluses and Allonautiluses

It’s finally time to talk about cephalopods, that group with squids and octopuses. I’m excited! And, because last week was so gloomy, I will try my very best not to say anything gloomy today. We’ll get to learn cool things about some of my favorite animals, exciting! I’ve got an octopus tattooed on my arm because I love them so much.

The audio version of this script can be found in the podcast. A more loose adaptation of the same content is on the channel.

Very much to my dismay, the octopus is still only a side topic, with a few honorary mentions today. Let me explain: The cephalopods are pretty simple to divide into groups. Yay! And there are no boring blobs to skip over either. Seriously, I’m excited about every single organism we talk about. And that excitement lead to a very necessary realization: I will probably need one episode each for nautilus, squids, cuttles, and octopuses. They are too cool to combine. Which also means, you’ll have to wait a bit longer for my favorite cephalopod: the octopus. I know, I know. I keep pushing it. But I don’t want to skip over all the cool bits to get there quickly. But, I promise, the rest of the cephalopods are super cool, too, and each deserve a detailed look.

As always, let’s orient ourselves on where we are on this confusing Tree of Life: We are in the realm of the eukaryotes, so organisms that aren’t Eubacteria or Archaea, that little crabby stuff I care so little about. From there, we climbed along the branch to the animals, the Animalia, passed the Ctenophora, so sea combs, and the sponges, the Porifera. We ignored the Placozoa blobs, talked about the Cnidarians, so sea jellies, corals, and the like, finally reaching the fun branch of the Bilateria. Bilateria are the animals that have two symmetrical halves. Last time, we talked about the mollusks in general and got into some detail on the aplacophora, so blobs without a shell, the monoplacophora, so blobs with one shell, the polyplacophora, so the chitons with their many plates which I called hedgehog knights, and the scaphopoda, the tusk shells. We also covered the bivalves, so mussels and clams and the like, and the gastropods, the snails and slugs. Phew, I guess we did a lot already. And there are so much more exciting things to come. I can’t wait to talk to you about octopuses, but I’m also excited about things like sharks and other fishes, about sea mammals, reptiles, amphibians, even plants. There is so much more to talk about on this precarious climb along the Tree of Life. But back to our cephalopods.

Cephalopod means head-foot, which kinda makes sense if you look at any of them. They essentially look like walking heads. Ironically, they don’t actually have feet but instead arms or tentacles, but yeah. Naming things is always interesting.

There are more than 800 extant species of cephalopods out there. And we keep finding new ones, so there are probably quite a few more. Cephalopods need marine water, so none of them exist in freshwater, and only one species can tolerate brackish water: the Atlantic brief squid, Lollinguncula brevis. That’s probably also why there are so few cephalopods in the Baltic sea. I was quite disappointed when I found out about this last year. We were planning a one-day dive trip to the Baltic Sea, so I googled what we might see. The trip didn’t turn out as planned, in general, but the lack of octopuses was definitely disappointing. Not that I see octopuses on every dive elsewhere, of course. But there isn’t even a chance to see them where we went diving. Sad! I guess, that’s one benefit of climate change? Studies have predicted that the Baltic will become more suitable for cephalopods. But I promised no negative talk, so I won’t go into any of that.

There are teeny-tiny cephalopods of less than half an inch, a centimeter, that weigh less than 1 gram. One of the smallest known cephalopods is the Northern pygmy squid, Idiosepius paradoxus. If it’s not the smallest one, it’s probably the Thay pygmy squid, Idiosepius thailandicus. One of those. Definitely a pygmy one. On the other end of the scale are the Giant squid and the Collossal squid—what creative names—with up to 40 feet or 12 meters.

Cephalopods are super intelligent. They can use tools, too. Some even solve riddles. We’ll get more into that when we talk about octopuses, but it’s not just octopuses that have these characteristics.

And it’s not just their brains that are impressive. They have advanced vision. Surprisingly, their eyes aren’t unlike those of vertebrates like us. Well, except for the part where most cephalopods are thought to be colorblind. Huh. How do these things camouflage so well without being able to see colors? Weird, right? They do make up for it with a faster response to light changes, and an impressive ability to distinguish different levels of brightness.

We’ll get into more characteristics in the next episodes, as I dont’ want to overwhelm you with too much stuff.

So, as I said, they are pretty simple to divide into their major groups. There are only two groups: cephalopods with and without external shells. The ones with shells are the Nautiloidea, the nautiluses, and the ones without shells are the Coleoidea which is the group containing squids, octopuses, and cuttles. Well, and the Spirulida, but people usually have never heard of those things.

But one step at a time. Let’s cover the nautiluses first, because there’s only two groups in that clade. And in characteristic taxonomy fashion, the naming is excellent. The Nautiloidea contain only one extant (so, living) clade called the Nautilida. In turn, the Nautilida only contain the Nautilidae. To be fair, there are extinct groups in each of these that give this naming weirdness a reason, but as I am not a historian, I am left with this fun stacking of Nauti-somethings.

Oh, and to make more things fun, the next level down is the Nautilus. Yep, another nauti-something. The Nautilidae are divided into the nautiluses and the allonautiluses. So many nauti-somethings. I’m sure you lost track already. So, let’s skip all the nonsense in between. Cephalopods have two clades: Coleoidea, the shell-less ones, and the Nautiloidea, the shelled ones. Within the latter, we only care about the Nautilus and the Allonautilus. There, better, right?

We used to think that there was only the Nautilus in that clade extant today, but in 1984, they found a living N. Scrobilatus, then still thought to be a Nautilus. They did some research and found that they were actually anatomically quite different from the Nautilus. A paper from Ward and Saunders explained it was mostly the way the gills worked and the male reproductive system. So, now they are considered Allonautilus. But enough about taxonomy, let’s talk about these things, shall we?

As it’s much easier to explain how the allonautiluses differ from the nautiluses once we know more, let’s start with those. So, as they are Nautiloidae, they have a shell—and a really beautiful one, too. The shell looks kind of like a snail’s shell from the outside. There’s the swirly part, the whorl, ending in a straight bit that opens up to the outside. They grow these shells very similar to land snails. They are all mollusks, so that kinda make sense.

I didn’t really know how snail shells develop, so I’ve decided to explain it, just in case you didn’t know either or weren’t sure. So, in the beginning, snails have something called a protoshell: a tiny single-curl shell. It’s also soft at that stage, as they need to get calcium, other minerals, and some organic material into it before it can harden. The shell then grows from the lip, so the open end.

For the first few years (until fully grown), they add whorls to the shell. After that, they just add a ring to the lip at the opening each year but don’t actually grow anymore. You can actually age a snail using the shell growth. Kinda like tree rings or fish otolith. If you don’t know what fish otoliths are, I have an entire episode on them.

Unlike the snails, the nautilus actually adds a stabilizing layer each year, so a coil divided by septa into chambers called camerae. Another thing Nautiluses have that snails don’t is something called a siphuncle. Siphuncle? I’m probably the only one thinking about an uncle with syphilis now. Cod, my brain. Anyway, the siphuncle is a tissue strand that goes through the entire shell from the newest chamber to the oldest. Its main function is to empty water from the chambers as the shell grows. To empty the shell of water, they actually change the salinity of their own blood to draw water from the shell into the siphuncle. Seriously, nature?

My friend volunteers at the malacology department of our university. Malacology is the fancy word for studying cephalopods. She sent me a photo of a few nautilus shells when she was there last time, and kindly agreed to let me share it with you. So, if you want to see how cool Nautilus shells look both from the outside and the inside—you can even see the holes the siphuncle went through—check out the transcript of this episode on my website.

Photo: Tanya Toofany

Okay, anyway, shell, right. Nautiluses are no giant squids when it comes to their size, not even remotely. Most of them stay around 8 inches, 20 centimeters. The largest ones can reach up to10 inches, or 25 centimeters.

The shell is open on one end, at the lip. On that end, the tentacles of the nautilus stick out. Though, technically, they aren’t tentacles. Let me explain. During early development, their foot divides into 60-90 cirri, so soft, flexible appendages. To make it more weird, each of the cirri can be retracted into a hardened sheath.

As you might remember from our Mollusk episode, the foot is actually an apomorphy, so a characteristic of the group. But, as we discussed then, that doesn’t mean it still exists or looks like a foot. Here, the foot is a bunch of tentacle-like things. Cool, right?

So, why aren’t they tentacles? I didn’t know the difference either, I have to admit. Nautiluses were never really on my list of obsessions. That has definitely changed now. Cephalopods are damn cool. Cirri, cirri. So, tentacles are elastic and have suckers. The Cirri are flexible, so they can bend, but they can’t extend. And they don’t have suckers like the arms of an octopus. The cirri around the eyes have more cilia, those eyelash-like whip things, which is the reason scientists think they use those to aid with smell. So, plenty of cirri, some fused into hard shell, others turned into penis replacements, and others with extra olfactory powers. And I thought the mere number of them was impressive.

During the early development, they actually display ten arms or tentacles or whatever you want to call them at that stage, before they further split into the many cirri they have. Two of the cirri fuse into the hood, a hard part of the shell above the body. And in males, four of them morph into a modified organ called a spadix. Both of those fused appendages allow you to tell the sex of the nautilus.

By Antonov – Own work, Public Domain,

As I said, males have morphed four of their appendages into a modified organ called a spadix. They use that to transfer their sperm into the female’s mantle—that’s the body—during mating. They made face-to-face, by the way, but considering the rest of them is hidden inside the shell, that kinda makes sense. Still looks weird, though. Later, the nautilus female lays eggs onto rocks. It takes them quite a long time to hatch. How long exactly that is depends on the species. For the Chambered Nautilus, it’s ten to twelve months. Scientists think that the long gestation period actually lead to them surviving the big mass extinction events because they skip the vulnerable larvae stages.

The female regrows her gonads after laying eggs, and is ready to do this all over again the next year—much unlike octopuses, as we’ll see. Though they still don’t get too many chances to produce offspring considering they typically don’t reach maturity until about 15 years old. With a lifespan of about 20 years, they might live unusually long for cephalopods, but that doesn’t leave much time in adulthood, does it?

How did we get from their cirri to nautilus sex, you might ask? I am not sure, so let’s return. They use their cirri to hold on to prey, move it toward their beak. Yes, like octopuses, nautiluses have a beak that looks kinda like that of a parrot but is made from chitin like the exterior skeleton of a bee. Behind that beek are two interlocking jaws. The nautilus chomps down on the food, and then use their nine-toothed radula to do the rest of the chewing process. The first part of the digestive tract is a flexible tube that can extend quite a bit. They pass the food through to the stomach in whatever state it’s in. Whatever is left after gets crushed in a muscular stomach. Omnomnom.

I know I said that cephalopods are very intelligent. And while I’m sure Nautiluses aren’t stupid, they don’t really even have a brain. At least not the way you would envision it. They have a nerve ring around their esophagus with nerves extending into the mouth, tentacles, organs, and things like their eyes. So, while they are much more developed, brain-wise, than other mollusks, they definitely are among the more primitive cephalopods. I guess, they didn’t need to be quite as clever because their shell protects them. Good for them! Living fossils, indeed.

The book I’m reading, Squid Empire by Danna Staaf, makes an excellent point about that brain donut: You have to be careful not to swallow food that is poky or too big, or you might actually injure your brain donut in the process. Huh, weird.

And with that, let’s move on to the second group, the Allonautiluses.

The allonautiluses can only be found in a very small region. Or rather, we’ve only encountered them there. Who knows where else they might be. There are actually only a few known sightings of these things. Ward and Saunders were the first so lay eyes on them in the early 80ies, which prompted that paper I mentioned. Ward saw another one in 2011, thirty-one years after his previous sighting.

There are only two extant species, one found around Bali and Papua New Guinea, the other found around Popua New Guinea and the Solomon Islands. There aren’t many references on these things because they get sighted so rarely, but here’s what I found out, mostly from the paper Ward and Saunders published:

Allonautiluses look a lot like nautiluses, but their shell is different. heir shell isn’t smooth like that of the nautiluses and snails. It has very noticeable grooves or ribs. Even more obviously, it’s covered in a thick, slimy, and hairy layer. Yep, they have hairs on their shell. I know, weird. Well, technically, we don’t know if one of them has the hairy layer because we only have shells and that layer doesn’t get preserved.

By Peter Ward –, CC0,

Okay, enough about these super cool things. While I’m pretty sure I’ll never see an allonautilus, I really hope I get to see a nautilus while diving at some point. Wouldn’t it be cool if I could show you some footage of these fascinating shelled cephalopods? I know I keep hinting at this because I don’t want to tell you until it’s all settles, but there are plans for this channel that would make showing you footage of cool animals and telling you about nature, biodiversity, sustainability, and all that from a much more hands-on perspective much more likely. I will tell you more as soon as I can.

Next time in our Tree of Life series, we’ll talk about the Coleoidea, the shell-less cephalopods. We’ll cover the general things about the clade, and get into details on squids. As I’m reading Squid Empire at the moment, I am super excited about that one, as well. Cod, how will I get through the fish episodes? Or the sea mammals? Clam, I care too much about all things nature. I guess that’s good for you, though, as you get sooo much biodiversity content. I pay to get taught a lot less at university.

If you liked this episode and want to help me make more of these research-intensive episodes, consider supporting me. A special thank you to my loyal patreons, Paul and Robert, who donate to me every month, and to the lovely people who have sent one-time donations. You rock! As always, like, subscribe, rate, and all that fun stuff. Thank you all for your support. I literally would not be able to do this without you!

Until next time!

Weirdly yours
Kate Hildenbrand