The Magical Biology of Cuttles and Bobtails

Sea cuttles, also wrongly named cuttlefish, are masters of disguise. Chameleons don’t even compete. But that’s not the only cool thing about these cute blobs of awesome. So, let’s talk about cuttles!

First, as always, a quick orientation on the tree of life. If you get lost, I recommend watching the previous episodes of this series. (I am currently re-recording the first six to publish them on Youtube with video. Hold tight, coming soon.)

We covered all the bacteria stuff and a bunch of little or simple stuff, for now skipped the plants and fungi—though we’ll get back to them—to reach the animals. We covered more cool groups like the sponges, corals, sea jellies, and such, to reach the bilateral animals, so animals with symmetrical halves like us.

Most of the mollusks, from clams and oysters to snails and slugs, as well as nautiluses and squid, are covered. Well, I’m sure we’ll return to snails and the ducking cool nudibranchs at some point, but for now, they are covered.

At the moment, we’re talking about the cephalopods, the group that contains one of my favorite animals: the octopus. There are two extant—living—groups of cephalopods: the shelled Nautiloidea and the shell-less Coleoidea. We covered the freaky-coll shelled ones in the first cephalopod episode, and then I got overly excited about squid last time.

That leaves us with two very exciting groups within the coleoids: the cuttles and the octopuses.

Photo by Kostiantyn Li / Unsplash

You might remember that the Coleoidea are divided by the number of their appendages: eight-limbed octopodiformes, and ten-limbed decapodiformes. The squid, the Teuthida, are decapodiformes, and so are the cuttles and bobtail squids, the Sepiida, we talk about today. We’ll get into the octopodiformes next time when I finally get to tell you about octopuses.

We also quickly covered the third group of the coleoids, the Spirulida with the whooping one living member, the Spirula spirula, the Ram’s Horn Squid that isn’t a squid.

I also mentioned the pygmy squids, the Idiosepia, a fun hybrid between the different cephalopods that’s currently hovering unspecified in the decapodiformes, waiting to be classified into either their own branch or another branch. They were too ducking cute to ignore. They are so small!

As promised, we’ll talk about another weird hybrid today, the ducking cute bobtail squids. Might feel like they should’ve been covered last time, but as they aren’t actually squid despite their stellar naming, they don’t belong there. More on that later.

Before we get deeper into cuttles, let’s talk about their name. Most people know them as cuttlefish. But they aren’t fish, so why the duck would you call them fish? It’s like the whole starfish nonsense. Starfish aren’t fish either. We’ll talk about what they are when we reach that branch in this series, but they absolutely aren’t fish. Sea stars. Cuttles. No fish. Okay, cuttles.

Part of me really wants to change every single mention of cuttlefish to cuttle on Wikipedia and add a note how stupid the naming is, but I have a feeling it won’t get approved.

Okay, to make things more confusing, let’s return to our tree of life for a moment. Remember the nautiluses with all the nauti-something branches? Well, the Sepiida aren’t much better. To get to the cuttles, we have the Sepiida, then the Sepiina, then the Sepiidae. It again only makes sense if you give a duck about extinct branches. I really, really don’t.

So, Sepiida are devided into the cuttles, the Sepiidae (well, technically the clade Sepiina that includes the cuttles where only the cuttles are alive), and the Sepiolidae, the bobtail squids.

Each of those have subgroups, but I don’t want to bore you to death, so I’ll only mention them if they matter.

Today, it’s hard to decide which group to start with because cuttles and bobtails are both ducking cute and so ducking cool. Usually, I go from boring to fun, but that won’t work today, so I literally flipped a coin. Well, as literally as telling an AI to flip a coin can be. It was heads. As tails obviously has to represent the bobtails, we’re starting with the cuttles.

So, what the duck are cuttles?

Cuttles are marine mollusks. Just like many other mollusks, they can be found all over the world, though these little friends don’t like it too cold. You can mainly find them in tropical and temperate regions, though there are some, like Sepia officinalis, the common cuttle, that can deal with cold water. Research shows that S. officinalis has adapted to be able to reproduce in the English Channel. Not the Artic, but definitely cold water. Similarly, they tend to stay in shallow waters—it’s warmer there and there’s more food—but they have been found in deeper waters. Wikipedia says 600 meters (2,000 feet) but I can’t find anything reliable to back it up. Most of the articles on cuttles seem to just summarize Wikipedia. Or they mix up squid and cuttles and everything is ducking inaccurate.

Okay, I’ve kinda still not answered the question of what they are, have I? Let’s change that. They look a lot like squid, to be honest. Just like squid, they have eight arms and two tentacles. You probably remember this from the squid episode, but tentacles have suckers only at the ends, while arms have suckers along the more or less entire length. Tentacles are also more stretchy. Most people wouldn’t be able to tell cuttles and squid apart unless they are a species they’ve seen before. But, you can actually tell them apart.

The easiest way to tell them apart is probably by the form of reduced shell they have, but that’s something you can’t do while there’s still the actual animal around, so it’s not a favorite. You might remember from last time that squid have something called a pen or a gladius, a thin, bendy rod that is kinda like their spine.

Cuttles, like squid, no longer have an external shell. Their shell is reduced into the cuttlebone. You might have seen cuttlebones in the aquarium trade and pet bird cages, as they are pretty rich in calcium. This is also why cuttles won’t like ocean acidification, which we are causing with our excessive carbon dioxide emissions. But, this is the Climbing the Tree of Life series, so I won’t say more. No doom and gloom here.

Cuttles can actually use their cuttlebone to control their buoyancy, so how floaty or sinky they are. It’s a porous structure made from aragonite. I think we talked about that stuff when we talked about mussels and the like. Calcium carbonate has two forms, calcite which is more stable, and aragonite which is more soluble, and thus more vulnerable to ocean acidification. The tough stuff in mussels and other mollusks, but also in corals, is made from both. They have different properties, but that’s too much chemistry, and I just can’t make myself care. I explain it in detail in a previous episode. I’ll try to find out which one. It’s either the one on mussels or on the carbon cycle.

Right, so the cuttlebone is made from calcium carbonate. It’s porous, so there are holes all through it. The cuttles can change the gas mixture in that thing to change their density and decide how buoyant they are. Kinda like what the squid did with the ammonia. Remember that siphuncle thing in the nautiluses, that tissue strand that went through the entire shell? Cuttles also have a siphuncle, and it’s how they adjust the gas-to-liquid ratio in the cuttlebone. I told you that all the cephalopods deal with buoyancy differently.

But yeah, if you happen to find a dead one, this is an easy way to check. Luckily, there are ways to tell squid and cuttles apart without getting all up in there.

Like squid, they have swimming fins on each side of their mantle—that’s the body—and they can use jet propulsion to move faster. They have very similar arms and tentacles, so that won’t help you either.

Sure, their mantles are a bit broader than those of squid, but to tell them apart by that feature, you’d need both next to each other to compare. It’s like those stupid articles telling you that you can tell the sex of a species by size because “the females tend to be larger” or something like that. Larger doesn’t help me, especially when you consider there is variability in size.

I think, the easiest way to tell them apart is by their eyes. I already told you that cephalopod eyes work a lot like ours. It’s really weird how close they are. They developed independently of the vertebrate eyes, but they work pretty much the same. The main difference seems to be that their eyes don’t have a dead spot like ours. So, they independently developed eyes that work like ours, just better. It’s called convergent evolution, by the way.

The main difference between the cephalopod eyes seems to be the shape of the pupil. Squid have a round pupil like ours, and cuttles have a W-shaped pupil. Yes, W-shaped.

By wildxplorer – Flickr, CC BY 2.0,
Photo by Will Turner / Unsplash
By Alexander Vasenin – Own work, CC BY-SA 3.0,

So, if they look sleepy as duck, they are cuttles. Sometimes, the W-shape looks more like a U-shape, if you ask me, but yeah, definitely not round.

The photo that adorns the header of my website and Youtube channel is a cuttle, as you now probably are able to tell.

The bobtail squid are closely related to the cuttles, but despite their name, they aren’t squid. But, they also don’t have a cuttlebone, so they obviously aren’t cuttles. So, closely related but not quite it. Either way, the name doesn’t make any sense. They make up their own order within the cephalopods as the Sepiolida.

The sepiolids have a rounder mantle than the cuttles making them look even cuddlier and cuter. They also tend to be smaller, about half the size of the cuttles. And the cuttles already aren’t the biggest creatures. The smallest cuttles are probably the Dwarf cuttles, Sepia bandensis, while the largest are likely the Australian giant cuttle, Sepia apama. A lot of pages seem to copy from each other and list Spirula spirula as the smallest, but we know better, as we talked about the Spirulida last time. They aren’t cuttles…

Giant cuttle swimming (
By Richard Ling from NSW, Australia – Australian Giant Cuttlefish, CC BY-SA 2.0,

So, the smallest are probably S. bandensis, with about 4 inches, so about 10 centimeters, mantle length. Five times as long is the Giant Cuttle, S. apama, which lives around the lower half of Australia.

By Rickard Zerpe – Dwarf cuttlefish (Sepia bandensis), CC BY 2.0,

The bobtail squids are, on average, smaller than the cuttles. The Hawaiian Bobtail Squid, Euprymna scolopes, which we’ll talk about more later when we talk about camouflage, averages a whooping 3 centimeters, so a little over an inch. Most bobtails seem to reach about 2 inches, so about 5 cm, so not much at all. It’s part of why they are so ducking adorable.

For both cuttles and bobtails, the lifespan is rather short, even shorter in the bobtails. Cuttles average one to two years, while the bobtails get to less than a year, usually between three and nine months.

So, absolutely nothing compared to the long lifespans of the nautiluses. Like squids, the cuttles and bobtails mate only once in their lifetime. Whether cuttles mate in their first or second year depends on their environment and also kinda determines the lifespan, as they usually go into senescence after spawning. That means that they essentially start rotting away until they get eaten or die of starvation. Bobtails typically are able to mate after their second month, but considering their short lifespan, this makes sense.

Both cuttles and bobtails grow over their entire lives, and females are rarer than males. So, when exactly they mate tends to depend on how fast they grow, in addition to environmental factors like temperature and how sneaky they are.

And this kinda segues well into the camouflage, as part of their impressive camouflage is the ability to hold two conversations at once, e.g. showing aggression to a rivaling male while courting the female.

But let me start at the beginning. Camouflage. Like other cephalopods, they have ink they can squirt out to create a smokescreen, but considering their ducking incredible camouflage, they rely on this less than other cephalopods.

Like other cephalopods, cuttles are thought to be colorblind, which kinda makes it ironic that they are so ducking good at imitating their surroundings. Within fractions of a second, they can change their looks. The Max-Planck Institute for Brain Research and the Frankfurt Institute for Advanced Studies at the Goethe University are working hard to figure out how exactly the camouflage of cuttles work. They want to be able to copy the mechanism because AI working to emulate similar function currently are super complex, and the fact that cuttles inherit this trait means there must be a rather simple mechanism behind the magic. I mean, sure, cuttles and octopuses have the largest brains of the invertebrates, but that doesn’t sound as impressive if you consider what the competition is. No one would expect mussels or slugs to be the most intelligent or to have the largest brains.

Cuttles can not only change how light or dark, pink or white, their skin is, but also if their skin is smooth, coarse, wrinkled, bumpy, or even has shapes protruding out from the skin—and bend light to make colors. They control all of this magic with their brains. So, they check out their surroundings and somehow extract the essential patterns and structures from them to figure out how to blend in. So, they don’t just blindly copy the exact pattern, but emulate the features intelligently.

How many patterns and shapes the cuttles can make depends on the species, but all of them are impressive as duck. I have linked the three sources in the text below, but there is a lot of overlap between them, so some stuff is technically from multiple of these sources. Good, that I’m not writing a scientific paper.

Let’s take the common cuttle as an example: they have 16 light features, 17 dark features, 6 textural variations, 8 postural components, and 6 locomotive (so movement-related) components that allow them to blend in. And all that is controlled by one brain!

Okay, so, how does it work? The skin has a lot of specialized cells that make up the image, kinda like pixels make up your computer’s picture. They have three types of these specialized cells.

Chromatophores are essentially elastic sacs filled with pigments of different colors, ranging from yellow or orange through red to dark brown. The exact color options depend on the species. But that doesn’t mean they are limited to those colors, as we’ll see in a moment. The sac is closed in the relaxed state, but when the brain sends the signal, they can open up in a fraction of a second to expose the pigment.

In addition to the chromatophores, they have iridophores which allow them to change how they reflect the light, thus changing what color we perceive when we look at them. And with weird filters called biochromes, they can even make stunningly pretty blue and blue-green colors, though even science doesn’t seem to get that part yet. And to make this even weirder, they can polarize the light, too. I have forgotten everything from physics lab, but polarized light is light where the waves are parallel or something like that. I don’t really care. I just know that they produce polarized light signals that can be seen by other underwater organisms. We don’t know if they use the polarized light signals for communication, but it looks like it.

The third type of specializes magic mindduck cells are the leucophores, which I really didn’t understand at all. It’s something to do with diffusing the light and reflecting it differently, but that’s just so chemical (or physical) that I’m out.

Together, these three types of cells create the patterns of the  impressive color-changes. An article I found had them listed as “light-bending iridophores, light-reflecting leucophores, and light-absorbing chromatophores,” so maybe that helps you understand better.

But, what’s more is that cuttles can change the texture of their skin. To achieve this, they use papillae, which are skin structures they can expand to make shapes. Kinda like controllable balloons. So, they can make their skin smooth, rough (which is kinda like our goosebumps) or with protruding bumps. Ducking impressive.

Scientists have worked hard to codify the different patterns for different species, creating lists like the one I told you about before with the different elements, and pattern tables with all the chronic, so longer-lasting, and acute, so short-term patterns. They hope that having a list of “words” will make it easier for them to understand the language.

What fascinates me most is how sneaky these little bastards are when it comes to mating. I already told you that they can keep two conversations going, signaling one thing with one half of their body and another with the other half. Well, they can also pretend to be females. Smaller males often don’t get the females because the rivaling males are too big. So, they get ducking sneaky. They hide the arms that have the sperm-transfer pads and change their outside appearance to that of females. Well, females that are not receptive to be mated with. They even manage to keep this rouse going with one side of their body while courting the female with the other side. This way, they don’t get attacked by the bigger males, and as soon as the big buff dude doesn’t pay attention, they sneak in to mate.

If they are lucky, and the female responds, they use their funnel to blow out the other male’s sperm, and mate with the female. The other, larger male will then continue to guard the female until she lays her eggs, completely unaware that the sneak attack has happened.

Sneaky bastards indeed. But, what I find really interesting is that the larger males win against the other males in what the Germans would call a proverbial Schwanzvergleich, so a competition of who has the largest dick, while the female seems to have other criteria for choosing whom to mate with in the end. Sure, she lets the large, impressive male duck her, but he’s not who she lets father her children.

There are a lot of parallels to humans in here somewhere. I wish we could talk to cuttles and ask them how they choose. I wish I could just have a day to talk to all the animals I want to talk to and ask them questions, learn more about how they think. You know, kinda like a scientist’s version of a children’s book. I’m sure we are completely misunderstanding a lot of things. I mean, we keep finding out completely new things about everything we look at patiently. Did you know trees can communicate, and they can make it rain? Yeah, we’ll get there. But, I really think, there is so much more to the world out there, and if humans would get their heads out of their asses and stop thinking that they are better than everything, we might be able to understand nature better.

And if you think all of that is impressive, let’s revisit the cute little bobtails for a moment. They have something called a light organ, in which they host symbiotic bacteria to ducking glow in the dark. Seriously! They use this to create something called counterillumination where they brighten up their belly with the bioluminescent bacteria to make it harder to be seen from below. Where they would usually be a dark shape against the brighter surface of the ocean, they blend in. They only do this at the bottom, to still be camouflaged against the lower layers when seen from above.

By Ian Alexander – Own work, CC BY-SA 4.0,

An example of a species that does this is the Hawaiian bobtail squid we’ve mentioned before, Euprymna scolopes. They live in the central Pacific Ocean, so unsurprisingly around Hawaii. They are teeny tiny, as we’ve established, seriously smaller than most human thumbs. By using the bioluminescent Aliivibrio fischeri in their light organ, they can avoid being hunted down by Hawaiian monk seals which like to munch on them for some reason. They must be tasty little snacks.

By Margaret McFall-Ngai – Divining the Essence of Symbiosis: Insights from the Squid-Vibrio Model, CC BY 4.0,

Due to their size, they are used as a model organism, getting studied in depth by researchers around the world, just like their symbiotic bacteria. Space X even launched these little cuties into space. Weird, right?

There are many more cute species in the cuttles and bobtails. Honestly, they are kinda all cute, so let me just mention a few more examples:

Last time, I was disappointed that the Striped Pyjama Squid, Sepioloidea lineolata, is not a squid but a bobtail, so I kinda have to mention it now. It’s aptly named with weird black and white stripes, but what’s way more interesting is that they are both venomous and poisonous.

By Scubagirl85 – Own work, CC BY-SA 3.0,

Remember, poisonous is if you try to eat it and get hurt by the toxin, and venomous is if it tries to eat you, and you get hurt from the toxin. Okay, technically, that’s oversimplified. It’s also poisonous if you just touch it because you wanna know what it feels like, and it’s also poisonous if you get bitten in defense, but yeah.

Most animals are either venomous or poisonous, so a cuttle that is both is kinda cool.

An example of a toxic cuttle would be Metasepia pfefferi, the flamboyant cuttle, which is thought to be as lethal as the Blue-Ringed octopus. And it’s ducking pretty, too, with vivid hues of pink and orange and contrasting black and white.

By Christian Gloor from Wakatobi Dive Resort, Indonesia – Flamboyant cuttlefish, CC BY 2.0,

Speaking of colors: Probably the coolest colors of any bobtail are displayed by the hummingbird bobtail, Euprymna berryi, which is metallic blue and purple. Ducking cute, those things!

By Nick Hobgood – Own work, CC BY-SA 3.0,

Honestly, you can just look at any bobtail, and they’ll probably be cute, exciting, and fascinating. And it’s a pretty similar story with the cuttles, too. Just take a look at a photo of the Giant Australian cuttle, Sepia apama, we talked about earlier, and you’ll find a rainbow of colors displayed. Seriously, you should google that one because there are really cool photos of these.

Photo by Will Turner / Unsplash

And the list goes on and on and on, so these are just random examples of cool bobtails and cuttles, at least one of which I mainly chose for its name, though most of them were chosen because they had something else going on for them. I’ll let you guess which ones are which.

And as I could go on forever about how cute and ducking cool these things are, I’ll stop here. It’s as good a place as any, and I’ve talked for a long time already.

I hope you learned new things about this fascinating planet today. These episodes take forever to make, as there is a lot of research and memorizing of weird taxonomic names and facts, but I am loving this series. Nature is so ducking cool! If you want to keep me going, consider supporting me on Patreon for as little as two bucks per month or buying me a coffee. If you don’t have the coin to spare, please like, subscribe, or look for my name in your favorite podcast app and rate the podcast there. Know anyone who might get fascinated by nature? Tell them about my show. All these things help more than you can imagine, so thank you!

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You all are amazing, and I couldn’t do this without you. Thanks, everyone! Until next time.