The Squishy Biology of Squids

Most of you will know today’s animal group much better as calamari than as squid. But I promise, squid are way cooler alive. Between bobtail squids that are so cute that I just can’t, colossal squid that are, well, colossal, and squid that aren’t even squid, there’s a lot of excitingly cool stuff in this group. And, because I’m a lucky human, I get to tell you all about them. Squid are cool!

I’ll abbreviate the orientation of the Tree of Life a lot this time. If you get lost, I recommend looking at the previous episodes. I know, I know, the first six were audio-only. I’m working on re-doing those for YouTube. I’ll get them recorded, edited, and uploaded as quickly as possible so that the entire Climbing the Tree of Life series is available in video, audio, and text format. Yay!

We are on the branch of the eukaryotes, so the stuff that isn’t bacteria (technically archaea and eubacteria, but yeah). We passed a bunch of little stuff, as well as the plants and fungi, to get to the animals. Don’t worry, we’ll return to the plants and fungi at some point. We passed a few more groups—including cool stuff like corals and sea jellies—to get to the bilateral animals, so animals with two symmetrical halves like us. We covered a lot of the mollusks already including clams and mussels and slugs and snails. And last time, we talked about the freaky-cool nautiluses and their more recent cousins, the allonautiluses.

That leaves three (well, technically more like four and a half?) more groups in the cephalopods, the head-footed mollusks: squid, cuttles, and—squeal!—octopuses.

Photo by Farhad Ibrahimzade / Unsplash

This time, I’ll do my best to convince you that squid are ducking cool—a lot cooler alive than as calamari. Though I am less against eating squid than I am against eating octopuses. Seriously, my friends and I saw an octopus, turned upside down on ice, at a grocery store the other day. We were all torn between wanting to buy it to take it apart and learn all about every bit of it, and being appalled by it being sold there. I just don’t know how anyone can eat them. Once you’ve seen an octopus chill out in the sun or their cool dens, it’s very hard to want to eat them. But octopuses are not our topic today, so let’s return to squid.

I mentioned this last time, but the cephalopods only have two extant groups: the shelled cephalopods, the Nautiloidea, and the shell-less cephalopods, the Coleoidea. We covered the freaky cool shelled cephalopods last time, so today is all about the shell-less species.

Within the Coleoidea, organisms are divided by the number of their appendages: eight or ten. The octopodiformes, the eight-armed groups, include the octopuses and the weirdly-named vampire squid that isn’t a squid, of course. Why would it be? Yeah, I know. The decapodiformes, the ten-armed groups, actually have eight arms and two tentacles, but yeah, ten limbs total. They include the squids, the cuttles, and something called a Spirulida, a squid-like cephalopod that also isn’t a squid, but usually gets called a ramshorn squid anyway. Well, and then there’s the pygmy squids where no one can agree where to place them, as they are some cute weird hybrid between all the other coleoids.

By Fritz Geller-Grimm – Own work, CC BY-SA 2.5,

I know, it’s a bit of a stretch to include the Ram’s Horn squid in an episode on squid, but there’s not enough for an entire episode, so let’s just go ahead. There’s only one living, extant, species—the one we call Ram’s Horn squid. Spirula spirula is a squid-like animal with an internal shell that looks like a ram’s horn. You can’t see the shell until the rest of the organism dissolves, and they are rather small, but you can find their pretty cool-looking internal shells washed up on beaches, for example in New Zealand. Duck, I wish I’d known about that while I lived there. I would have spent even more time at the beach.

They are mesopelagic organisms. We’ll get more into the zones of the ocean in another episode soon, so for now, we’ll just translate that as the middle of the water column. The spirula prefer the darker depths of 500 to a 1,000 meters, so you won’t encounter them on a dive. At night—when the entire ocean is darker—they come a bit higher to about 300 meters, but that’s still way too deep to dive for most normal humans. I can’t find much about these cute little things, except for a 33-second Vimeo video that shows the shells and a preserved specimen. So, if you are curious, you could watch that.

By Rachel Caauwe – Own work, CC BY-SA 3.0, (technically, this is drawn upside down, as the light shines downward)

Remember that malacology is the study of mollusks. That word sounds a lot like melancholy and is already weird. Well, teuthology is the study of cephalopods, though it sounds a lot more like dental work.

With that said, let’s return to the Coleoidea, and make our way to the actual squids. As mollusks, the coleoids share some of the common characteristics like a foot—though it’s only found during development and then turns into the eight or ten limbs—the radula, that raspy tongue, and as cephalopods, they also have a beak.

Coleoids are the shell-less ones, so they don’t have a shell, as you’ve probably put together. But that’s not quite right. Some of them do actually still have a reduced internal shell. We’ll get to that. Other very cool things they share are ink sacs, camouflage, gill hearts, jet propulsion, and different kinds of buoyancy control, to name a few. We won’t get into all of those today, but they are shared by the coleoids.

We’ll talk about the camouflage when we get to the cuttles because they are just ducking cool in their ability to camouflage and that nature magic mindduck deserves more attention. If you think chameleons are impressive, these things will blow your mind. And I’ve watched octopuses with their impressive camouflage—still not as impressive as cuttles. It definitely blew my mind.

Like most mollusks, the cephalopods are blue-blooded. They use hemocyanin instead of hemoglobin. Well, technically, the blood is colorless unless it is oxygenates, but as oxygen is pretty important for organisms, it’s usually blue. And it’s definitely blue when you see it because it’s exposed to oxygen then, for certain.

Except for the nautiloids, so the nautiluses and allonautiluses we talked about last time, all the cephalopods also have ink. Did you know you can actually write with that ink? Dana Staaf, the author of Squid Empire, does this with her students. They use the pens—the reduced shell of squid; we’ll get to that—to write with the ink. It’s weird, but pretty fascinating. Did I mention we’ll talk about harming organisms in the name of science another time? Yeah, we’ll do that.

But back to our blue-blooded friends: the coleoids have an ink sac from which they can squish their ink. It’s almost pure melanin, so black pigment, that gets mixed with mucus on the way out and acts as a smoke screen to confuse predators. Unfortunately, I’ve seen an octopus ink because an idiot dive guide thought it would be cool to force an octopus out of their den. Humans!

Another really cool thing about cephalopods is that they are the only mollusks with a closed circulatory system. And in the coleoids, part of that system is two extra hearts. Coleoids have two gill hearts or branchial hearts, that move blood through the gills and oxygenate the blood. The regular, systemic, heart then pumps that blood through the rest of the body. So, they kinda have three hearts. I told you they are cool!

When we talked about the nautiloids, I told you that they have gas-filled chambers in their shells, and they use their siphuncle to draw out water from those chambers. That’s a very slow process of controlling buoyancy, especially since the only way to add water is to wait for it to seep back in. Some coleoids are more active in their buoyancy control. In case you don’t know, buoyancy is how floaty you are, so if you float up or sink down.

Some squids use ammonia—the stuff that makes cat pee smell—to control their buoyancy. They have special tissues where they store an ammonium-rich solution to make them essentially the same density as seawater. At least half of the known squid families share this ammonium-controlled buoyancy mechanism. Fancy!

Before I tell you more about squid, we should probably establish what squid even are:

Squid, the Teuthida, contain two major groups: the Myopsida, the covered-eyed squids, and the Oegopsida, the open-eyed squids. You may forget those names right away. I won’t mention them again because I’ve already forgotten them again. What were we talking about?

Squid are mollusks, so they have squishy-soft bodies. Their soft body, called a mantle, makes up most of their size. On either side, squids have fins, though those are usually not their main mode of movement. Sure, they use those swimming fins for slow movement and staying in place, but if they really need to move, they use jet propulsion. They inhale water through a valve behind their funnel, then contract the muscles in their mantle wall to jet water out of the funnel. They have a lot of control over the force, and the direction by pushing harder for more force or moving the funnel to change direction. It’s pretty effective. Some squid like the Caribbean reef squid can even use this propulsion to propel themselves into the air to avoid predators—or boats.

The funnel is located on the head end, alongside the eight arms and two tentacles.

Squid have a very reduced shell that is essentially just a rod made of chitin (remember, that’s the stuff a bee’s exoskeleton is made of) called a pen or a gladius. This pen thing is inside their body, so internal. It’s essentially just a bendy rod that is kinda like their spine.

As decapodiformes, so ten-armed things, squid have ten appendages attached to their heads. Last time, I explained the difference between the cirri of nautiloids and arms or tentacles to you. So, this time, I guess we need to talk about arms and tentacles, and what the difference between those two is.

It’s pretty simple: tentacles have suckers only at the ends of the limb, while arms usually have suckers along the entire length. The tentacles can also extend to a much longer length than the arms. They shoot them out spider-man style toward their prey, grab what they want to eat, and then retract the tentacles back in. They then use their arms to control and maneuver their prey, as the arms have a lot more dexterity and control.

When I say that arms are covered in suckers from top to bottom, I should specify that the males of especially shallow-water species actually have arms where some of the sucker-covered area is replaced by a pad which they use to deposit the sperm inside the female’s mantle cavity.

To make things weirder, some squid have hooks along some of their arms and tentacles. Scientists think they probably just make it easier to hold on and immobilize prey. You might remember, from last time, that the Colossal squid is one of the two largest species of squid. They have these weird hooks, and the hooks on their tentacles even rotate!

Magister Armhook Squid‘s (Berryteuthis magister) hooks, which are modified suckers. By NOAA.

In addition to the arms and tentacles, the head-side of things also holds the gills, the funnel, and the excretion openings. And, finally, the head also holds the eyes, but that’s probably surprising no one. In between those tentacles and arms is the beak and the mouth opening.

We already established that cephalopods are among the most intelligent of the invertebrates, and squid are no exception. They have a highly developed nervous system, though their brain also looks nothing like ours. Just like nautiluses, they have a brain donut, a ring around the esophagus. But, they do enclose that ring into a cranium, a cartilaginous skull, so it’s not unprotected.

Their eyes are paired, so one on each side of the head, and housed inside this skull. These eyes are a lot like ours in functionality. They have a lens that focuses the image onto a retina, just like ours.

Where fish have otoliths, squids have statocysts, but they work essentially the same when it comes to their functionality as balance-detectors. They also have sensory organs along their heads, tentacles, and arms that respond to weak local water movements, much like the lateral lines in fish. In general, squid are a lot like fish. They are invertebrates, but they fill a similar niche in ecosystems, and share a lot of the characteristics. I know we haven’t talked about fish—we’ll get there.

So, squid are a lot more like fish than most people seem to think. Just because they are squishy, doesn’t mean they are stupid. Not at all. Though most people seem to think that fish are stupid. Well, some people still hold on to the notion that fish don’t have feelings, so that’s not really a surprise. But there are even fish who use tools! But we are still a few episodes away from fish, especially if I keep getting excited about all the cool, fun things along the way.

Another factor that shows how not-stupid squid are is actually how diverse their sexual behavior is. Camouflage comes into play here. We’ll talk more about camouflage next time, but the really cool thing about squid is that they communicate a lot through camouflage. They can actually hold multiple conversations at once with just their camouflage! Impressive! They can show dominance and aggression to another male while courting the female with their other side. Speaking of courtship:

Some squid have the special pads for sperm transfer, which they attach to special glands on their chosen females. So, the female produces eggs in the ovary, then transports them to special glands near the gills. The male produces sperm internally, and then takes them from his penis—yes, squid have penises—rolls them up into a spermatophore bundle, and transfers them to the female’s glands. This is a very precise process.

Way cooler, in my opinion, is the preferred method for most deep-sea species, though I’m very glad I don’t have to do any deep-sea dating. These species have pretty large penises. A paper, published in the Journal of Molluscan Studies in 2010, actually sheds light on just how large these penises are. They were opening the mantle of a male squid to study it, and the penis swelled to almost twice the mantle length of the squid, 67 centimeters long. Several spermatophores were then ejaculated from the tip of the penis. So yeah, that’s definitely weird. But, these giant penises allow the males to reach essentially anywhere, including inside the female’s mantles, so precision is a lot less important. The ologies podcast about teuthology actually mentioned that the males of the Octopoteuthis species down deep see other squid so rarely that they actually just stick their penises into anything that kinda looks like a fellow squid, male or female. And it doesn’t sound like a comfortable process. But yeah, apparently deep-sea squid are bisexual, in a manner of speaking. Though, a lot of species are a lot less heterosexual than we humans like to pretend. And even species with dedicated partners like albatrosses are dedicated but will still go around and occasionally duck another partner if they want to. But I guess that doesn’t fit the story of homophobic people. That ologies podcast is very worth listening to, by the way.

Dana Staaf explained in her book that researchers initially thought that they had found a parasitic worm when they first found male organs inside female squid bodies. Well, yeah, no, those are penises, people. Can’t blame them for not getting there as the first guess, though.

Squid lay eggs with a lot of yolk, allowing the embryos to develop directly past the larval stage. So, like nautiluses, they like to give their young an advantage. They do live a lot less long than nautiloids, though. Many squid have annual life cycles, so they grow quickly, spawn, then die shortly after spawning. They often prioritize spawning so much that they shed their tentacles in preparation and then just become lifeless, weak blobs afterward until someone eats them, or they perish from lack of nutrition. So, many squid only life a year or a year and a half, some life 3-5 years. But nothing compared to the 20 years of a nautilus. If I think about the 10-12 months before nautilus eggs even hatch, this seems even shorter.

One of the items on my bucket list is to dive a squid run. In Redondo Beach, near where we lived in California, there was an event called the Redondo Beach Squid Run. For a few nights every few years, you can watch thousands of squid come up from the deep canyon off the shore there to fulfill their final task and mate in the shallows. And their predators are gathering to eat the weakened couples afterward. I definitely need to see that!

Despite their short life spans, squid play hugely important roles in many ecosystems. What they eat depends largely on the species and ranges from planktonic little stuff and krill to fish and even other squid. They also get eaten by a lot of other organisms, as they are apparently yummy protein snacks. Seals, dolphins, tuna, depending on the squid, you’ll find predators from just about any group. The only thing left once digested is usually the indigestible beaks. Yes, like nautiluses and octopuses, squid also have beaks. And people who know their stuff can identify them based on the beak alone.

But, one thing that is pretty bad for squid populations worldwide is how much variability there is in their population size. This means quota are often too slow to react—and humans really like their calamari.

But, as this is the Climbing the Tree of Life series, we’ll keep the doom and gloom to a minimum, so let’s talk about cool examples of squid species instead, shall we?

We already established that either the Colossal squid or the Giant squid is considered the largest cephalopod. The Colossal squid, Mesonychoteuthis hamiltoni, is thought to be the most massive with specimens known to weigh half a tonne, though scientists assume there might be even heavier ones out there. It’s only found in the antarctic waters, so unless you go swim in icy water, you don’t really run into them. And most squid aren’t aggressive towards humans in any case.

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

The Giant squid is thought to be the largest by length, with the largest measured squid reaching 59 feet, so almost 20 meters. While they live at depths and are rarely encountered, their bodies have washed up all over the world, so their range is likely pretty wide.

By fir0002flagstaffotos [at] gmail.comCanon 20D + Tamron 28-75mm f/2.8 – Own work, GFDL 1.2,

The Giant and Colossal squid share another size-related record: the largest eyes among all the animals. Their eyes can reach 10 inches in diameter, so a good-sized dinner plate. That’s 25 centimeters for my non-American friends. It’s massive!

By Smithsonian Institution –, Public Domain,

The smallest squid I mentioned last time, the Idiosepius, is actually not really a squid. Their place in the decapodiformes is still pretty much up for debate. They might be their own group, even more separate than the already confusing bobtail squids. The NCBI currently lists them as Decapodiformes incertae sedis, so unknown. But, as they are just ducking cute, and we don’t know better yet, let’s pretend they are squid, just so I can talk to you about them again.

By Rickard Zerpe – Tropical Pygmy Squid (Idiosepius pygmaeus) eating shrimp, CC BY-SA 2.0,

As I said, it’s either the Thai pygmy squid, Idiosepius thailandicus, or the Northern pygmy squid, Idiosepius paradoxus, both of which are just teeny tiny. They can fit on most human thumbnails. And this really cool photographer, Tony Wu, managed to capture an image of one of these tiny things fertilizing her eggs. The images are copyrighted, so I can’t just show you, but you should definitely check them out because they are so ducking cute!

The most common squid, so the most abundant species, is probably the European squid, Loligo vulgaris, the kind humans typically eat.

Photo by John Cameron / Unsplash

So, now we’ve got smallest—well, maybe—and biggest, as well as the most common. There was one other squid I wanted to mention today because it’s ducking cool, but it’s unfortunately no longer considered a squid. This whole squid-cuttle division is pretty confusing, so I guess we’ll get to talk about the Hawaiian bobtail “squid” next time, when we cover the cuttles. Though they aren’t cuttles either, as they don’t have cuttlebones. But yeah, we’ll figure that out next time. The same holds true for half of the really cute “squid” I wanted to talk about, like the Striped Pyjama Squid, Sepioloidea lineolata, which is also not a true squid. Name things properly, biologists—well, and normal people.

But, I did find one really weird squid that is actually a squid: The genus of the Heliochranchia, the piglet squid. Yep, piglet squid. So, not only are we calling a bunch of non-squid squid, but now we are calling squid piglets. I know, I know.

But, these things are ducking cute! And I totally get why they are called piglet squid. Though, they look more like a pig in a wig. Dudley Dursley squids, then?

By Taniab – Own work, CC0,

So, the weirdest squids are yet to come—and not true squids. We’ll cover the cuttles next time. They are even cooler than nautiluses and squid put together. I’m not kidding you. They can do the weirdest things, but mostly they are really cute as hell. Cuttles are almost as cool as octopuses—well, they are probably cooler, but octopuses are still my favorites. I don’t even know how I’ll make it through those two episodes without squealing excitedly the entire time. If you don’t want doom and gloom, those episodes will probably be your new favorites.

Have I mentioned that I’m a little obsessed with octopuses? Just a little? Maybe? Yeah. Okay.

Anyway! 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!