Transcription
Mollusks are usually the simplest of creatures, often not having blood vessels and sometimes even lacking a proper brain. But one group of animals challenges this: squid and their other cephalopod cousins, cuttlefish and octopus.
All of these animals are bewitchingly alien with their multiple limbs, hearts, and brains, but are far from simple creatures. Their family tree branched away from ours over half a billion years ago, before trees had even evolved, and yet developed eyes independently from our own. And unlike their modest cousins, are capable of some of the same intellectual feats as mammals, but developed all of these features on their own and in their own way.
With three brains and blue blood, cephalopods contain octopus, squid, and cuttlefish, but also their very distantly related cousin, the nautilus. The nautilus is a living fossil, with fossils of very similar looking creatures found in rocks dating to more than 400 million years ago in the Ordovician period. It is a survivor of a very different type of cephalopod that was more common back then. It has very primitive eyes that lack a lens and are just an exposed hole, but more importantly, it had a shell, as the modern and more common form of shell-less cephalopod that we are used to today, like squid and octopus, all descended from shelled ancestors not too different to the nautilus.
The name cephalopod means "head feet," pointing out the way that this group of creatures have limbs surrounding their mouths coming from their head. And although it may not look like it, these tentacles are actually a highly modified version of the molluscum foot that is found on the bottom of most mollusks. As many scientists think that squid and all cephalopods are closely related to a group of mollusks not too different in appearance to limpets called monoplastophorans, specifically a monoplastophoran that lived in what is modern-day Antarctica over 500 million years ago back in the Cambrian period called *Nitoconus* that had an extremely exaggerated conical shell, previously only known from fossils.
The monoplastophorans were thought to have gone extinct hundreds of millions of years ago, but were discovered to have survived until the present day, living at the bottom of the ocean. They are thought to be the ancestors of squid because these shelled mollusks share many features with the earliest confirmed cephalopod known, the tiny *Plectronociris* that lived 480 million years ago in the early Ordovician period. And if *Nitoconus* was the ancestor of the cephalopods, it would mean that the cephalopods are basically just a highly evolved limpet-like animal.
The problem with this theory is that there is another older, stranger animal known as *Nectocaris* that might actually be the earliest cephalopod known. And along with its two tentacles, it had no shell, meaning if this animal was a cephalopod, then cephalopods may not have inherited their hard coverings from a monoplasta foreign ancestor, and these shells were a later innovation all on their own. However, there are parts of this puzzle that don't quite fit, and many scientists doubt that *Nectocaris* was a cephalopod, and more fossils will be needed to know for sure.
*Plectronocirus* was tiny and would have been difficult to spot on its ancient sea floor habitat, but hidden inside its shell was one of the most useful pieces of equipment that early cephalopods had at their disposal, known as a sci-funcle. A scifuncle is a little tube that pierces through all the chambers found inside a cephalopod's shell, that the creature is able to flood with salty blood, which dehydrates the chambers, leaving them filled with gas, so the protective shell can be used as a flotation device and can be finely tuned to accurately control buoyancy in the water. So a sci-funcle functions in a similar way to how a fish uses its swim bladder.
These early shelled cephalopod were known as nautaloids, and the two species of nautilus alive today are a lineage of one of these ancient creatures that survived into the present day, and this can be seen as the nautilus still uses its shell as a flotation device to this day. In the Ordovician period, 440 to 480 million years ago, their populations exploded, becoming some of the most common animals around at that time. Their shells would have given them protection from predators, and their ability to float and move in all three dimensions would have allowed them to seek out food more easily.
But also, they were pioneers in very advanced features not seen before, like developing one of the world's first circulatory systems. Unlike snails, for example, when the blood can flow freely throughout their body. Unique among all mollusks, cephalopods have a series of blood vessels and capillaries that transport the blood from their gills around their body, similar to vertebrates, but with a twist.
Squid and other cephalopods don't have the same blood as humans or any vertebrates, for that matter, as it is bright blue. This is because while most vertebrates rely on the iron-rich hemoglobin to transport oxygen throughout their body, cephalopods rely on a substance known as hemocyanin that is copper based, and so when it is oxidized, it goes blue rather than red. So despite this incredibly different starting point that squid were left with, unique among any of their blue-blooded relatives like horseshoe crabs, cephalopods landed on the same method as vertebrates to increase efficiency in their respiration. So it is like a mirror of our own circulatory system that has evolved to do basically the same thing, but within a very alien body.
These features would have given them an edge over their mollusc cousins, contributing to their success, often being found at the top of the food chain, sometimes getting incredibly big, like *Cameroceras*, that may have grown to the same length of a great white shark and was one of the first large animals to inhabit the planet. Among all of these new coned nautiloids, some of them evolved to have coiled shells. There are multiple advantages to having a coiled shell over a conical shaped one, which is why so many cephalopods trended to coiling up their shell, and this trend isn't even isolated among cephalopods because other mollusks have made this adaptation as well, like snails. Limpets are in fact primitive snails that have little cone-shaped shells. The later, more advanced descendants like regular snails eventually adapted into a coil.
Coiled shells require fewer resources to form because as they overlap, the chambers inside the shell share the same walls. Plus, the more circular shape is better at withstanding higher pressures because the pressure is more evenly distributed among the shell with no weak spots. But the primary reason why so many cephalopods started to coil their shell is probably because they were more stable in the water. As they use their shell as a flotation device, it makes more sense having it nearer their center of gravity. There are many creatures living around this time that looked like they were halfway between having their shell coiled up, and if these creatures were changing their shells to be more stable in the sea, just a slightly curved shell would have advantages over a straight one.
400 million years ago, one of these coiled cephalopod groups would evolve into the next stage of cephalopod evolution: the coleoids, that is the group that contains most cephalopods alive today, like cuttlefish, squid, and octopus. Unlike their ancestors, these creatures started to trend towards losing their shells. Cuttlefish have a chambered shell with a sci-funcle like older cephalopods, only it is hidden under a layer of skin. Squid have a small hard part inside their body that is a leftover from when they once had a shell like a cuttlefish, but it is now vestigial, and they do not use it to control their buoyancy like older cephalopods did. And octopus have taken this a step further, as they have lost their shells entirely.
Gone are the days of bobbing up and down at the surface of the ocean, and instead, squid dart around the ocean a lot more like fish, using a large fin to push themselves, or occasionally jet propelling themselves to safety when pursued by predators. The earliest colioids known were called the *Melanocetus* that dated back to the Devonian period, about 400 million years ago, and looked like squid, although they were most closely related to the cuttlefish and had a similar internal shell with chambers that is what usually fossilizes.
But the sudden appearance of this new range of cephalopods that were less reliant on their shells was thought to be spurred on by another new arrival on the Devonian scene: the first jawed fish. They would have competed with cephalopods for the same resources, forcing them to change into becoming faster and more agile. Plus, some Devonian cephalopods would have been forced deeper underwater where their fish competitors could not as easily follow. The higher pressures of the deep would mean that their shells were at risk of cracking and breaking, and so they may have had to lose them to colonize these new deeper habitats.
And the process of losing their shell served them well because the K-T extinction that saw the extinction of the dinosaurs was the final nail in the coffin for the shelled cephalopods, and the nautilus survives to this day as a reminder of these more ancient shelled creatures, but never again would they dominate. Whereas to this day, colioid cephalopods, including squid, are some of the most successful creatures around, using their advanced respiratory systems and superior intelligence to give them an edge in their marine environments.
So, surviving multiple mass extinctions, bouncing through the turmoil of the planet's changing environments, the cephalopods were shaped and walked from an often simple group of creatures into very advanced forms. Some incredibly alien to us, like their multiple brains and limbs, while others were convergent evolution, like their closed circulatory systems and camera-style eyes, but always with a twist, reminding us how distantly related these creatures are from us.
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