I was over the moon about the recent video of the baby colossal squid, captured by the Schmidt Ocean Institute. My first encounter with Mesonychoteuthis hamiltoni occurred several years ago at Te Papa Museum in Wellington, Aotearoa, though that particular specimen was dead and pickled. Every time I think about colossal squids — and honestly, cephalopods in general — my mind is a little bit blown that I share a planet with these incredible creatures.
It’s hard to overstate my fondness for cephalopods. One of my favorite freelance jobs I’ve ever done was writing OctoNation’s Octopedia pages back in the summer of 2020. I’ve been known to spend hours watching cuttlefish at the aquarium. The first (and only) time I saw an octopus in the wild, snorkeling in the Galápagos Islands, I felt like I was floating on air for the rest of the day. One of my D&D (dungeons & dragons) characters gets magical powers from a kraken and is all cephalopod-themed. I love cephalopods. So it was a bit of a shock to realize that, while I’ve mentioned them in passing, I’ve never actually done a full blog post about them.
What exactly is a cephalopod? The word translates approximately to “head-foot,” and it refers to many-limbed marine critters: squids, cuttlefish, nautiluses, and octopuses (and yes, “octopuses” is the correct plural term). Cephs are invertebrates, so they don’t have bones, and they belong to the mollusk phylum, which also includes snails and shellfish.
Diplomoceras maximum is one of my favorite ancient cephs. Image by James McKay, via NewScientist
Mollusks go back a long, long time in the fossil record, with the first cephs appearing in the Cambrian period, over half a billion years ago. Back then, they evolved from snail-like creatures into swimming animals — first with long, straight, conical shells, and later, with those shells taking on all kinds of wild shapes. Ammonites were among the most successful of these early cephs; we know of more than 10,000 different species that lived for hundreds of million years, all across the globe.
Chambered nautilus. Image from Monterey Bay Aquarium
Today, the only cephs with visible shells are the nautiluses. They putter around backwards and occasionally bump into things, shell first. The shell itself is made of a series of chambers, and a tube running through all the chambers helps balance levels of gas so the nautilus can control its buoyancy. Squids and cuttlefish still have shells too, though they’re completely internal. You might have come across cuttlebone in a pet store — that’s a cuttlefish’s shell, and it’s packed with calcium and perfect for pet birds chew on. Octopuses are the only cephs that don’t have a shell at all anymore; their only hard body part is their beak.
Squids have eight arms and two tentacles. Image from the Smithsonian Institute.
The most obvious feature of any ceph is the collection of limbs. Octopuses famously have eight, squids and cuttlefish have ten, and nautiluses can have fifty or more. You might have heard “arms” and “tentacles” used interchangeably, but there is a difference: arms have suckers along their entire length, while tentacles only have them at the end (or not at all, in the case of the nautilus). Some cephs also have hooks or tendrils on their limbs, like the colossal squid or the mysterious vampire squid, or large stretches of tissue in between limbs, like the female blanket octopus.
We know this blanket octopus is a female because the males are less than an inch long. This species has such extreme sexual dimorphism that scientists initially thought males and females were different species! Footage by Joseph Elayani.
A cuttlefish’s eye. The wavy pupil shape allows them to see forward and backwards at the same time. Photo by Sergio Hanquet.
Cephs have some of the best eyes of all invertebrates — even the deep dwellers who live in total darkness. Most cephs can move the lenses of their eyes in and out to adjust their focus, kind of like how you can zoom and focus a camera lens. We vertebrates do that by changing the size of our pupils. The exception is, once again, the nautilus, with less complex eyes that work more like a pinhole camera.
Footage by Roger T. Hanlon, from a TED talk by David Gallo.
It may not surprise you at this point that nautiluses are also the outliers when it comes to changing color. Most other cephs have little pigment-filled pockets throughout their skin, which they can spread out or suck in to change color. Octopuses and cuttlefish are especially good at changing their appearance. Some species can even scrunch up their skin to appear a different texture. It can be almost impossible to spot a fully camouflaged octopus, so major kudos to the guy who spotted that one octopus in the Galápagos and pointed it out to me. Their color-changing abilities are all the more impressive when you realize that cephs don’t even see in color! Scientists are still trying to work that one out.
People who keep octopuses need to provide them with lots of entertainment and enrichment. Gif from Futurism (original video unavailable)
Color-changing and camouflage is essential for survival when you’re essentially a squishy bag of protein — and so is intelligence. Octopuses are particularly known for their observational learning, tool use, and puzzle-solving abilities, although cuttlefish can do all of those things too. Not only do they have big ol’ brains in their heads, but there are also brain cells throughout the entire body. All the limbs and even all the suckers have their own “mini-brain” controlling them. A lot of intelligent animals are (a) long lived, and (b) social — think apes, whales, elephants, and crows. But octopuses are neither, which makes them even more fascinating (more on that in my animal intelligence post).
Firefly squid. Image from ANA.
Some squid species, on the other hand, are very social, living and traveling in large groups. Coordinating a big group requires communication, and cephs might do this with glowing spots, like the firefly squid, or by flashing different color patterns at certain times. I have fond memories of swimming through a school of foot-long, torpedo-shaped squid while on a snorkeling trip when I was a teenager. Each squid had dark bars undulating across its pale surface, and with hundreds of them all together, the effect was mesmerizing. Some squids, like the enormous humboldt, are known to hunt in packs of thousands, and divers know to get out of the way when the swarm of “red devils” head their way.
Painting by Martin G. Roper, via the Smithsonian Institute.
Those big groups usually stick to the sunlight zone. Down in the deep is where you find the true giants — the colossal squid among them, the most massive invertebrate in the world (that we know of). Bus-sized creatures, with rotating hooks on their arms, basketball-sized eyes, and glowing spots. Animals that regularly do battle with another amazing, intelligent giant of the ocean: sperm whales.
From the massive squids of the deep sea to the tiny octopuses that make homes out of clamshells, from the clumsy nautilus to the graceful cuttlefish, these mollusks really do it all. Aren’t we lucky, to live in a world with something as wonderful as cephalopods?
A dumbo octopus swims along. Gif from apolonisaphrodisia.tumblr.com, via tenor