Small language joke there…
Cephalopods–octopi, squid, cuttlefish, nautiluses, and a few others–are fascinating. They are likely the smartest invertebrates on this planet, and are probably bright enough to give many vertebrates a run for their money. But they are also, at least compared to us and what we are familiar with, *profoundly* alien.
This was originally going to be a giant article about all the fascinating things about various cephalopods, but honestly, it’s far too huge of a subject, so I’m making it a series instead. This one is basically just an introduction.
Let’s start at the beginning. Literally. Though most modern cephalopods don’t have (external) shells, their ancestors definitely did. As far as we can tell from the fossil record, they started out as a mollusk that was probably something kind of like a snail, until they developed some tricks with their shell anatomy that let them fill them with air, and thus use them to control their buoyancy. Somewhere along the way, and it’s hard to tell exactly when because soft parts don’t usually fossilize well, they probably went from being more or less snail-shaped to having distinct tentacles.
For a large chunk of Earth’s early history, various species of cephalopods, particularly ammonites, were among the largest animals on the planet. They were also abundant and diverse enough that their remains are frequently used as index fossils–that is, fossils used to help date rock layers.
At some point in the process, they developed various tricks to keep their shells from being too awkward to maneuver. Some coiled them up. Others actually wrapped their bodies around their shells, and used enzymes to break off the tips.
This later strategy eventually lead to the coleoids, ancestors of most modern cephalopods. Instead of merely wrapping their mantle (basically the cephalopod equivalent of a torso) around the shell temporarily to break it off, they wrapped their mantles around their shells permanently. Their shell was eventually reduced to the cuttlebone of cuttlefish, the gladius or pen of most squid, and the cartilaginous or absent shell of octopi.
Major Clades
I figured this would be a good place to talk about the major divisions within the extant cephalopods. I won’t talk much about the extinct ones, except for what I already said about them, both because we don’t know as much and because there’s so much to say about the ones that are still alive.
As I implied earlier, the first major division is between the nautiloids and the coleoids. There is only one extant family of nautiloids, the Nautilidae, with only about 6 extant species (depending on how you define them, scientists argue a lot about where the borders of a species are). They are the only cephalopods that still have external shells, though the so-called “paper nautilus“, a type of octopus, has a pseudo-shell.
Among the coleoids, the next division is the octopodiformes (octopi and vampire squids) and decapodiformes (everything else). Most cephalopods that aren’t octopi are called squids, even the ones that are more closely related to one of the other taxa (like the vampire squids).
There is actually a lot of debate about how the various groups of decapodiformes should be divided, but there are basically 3 orders to which most of the extant decapodiformes belong–the two major orders of squid, Myopsida and Oegopsida, and the cuttlefish, or Sepiida (though this clade includes the so-called bobtail squid).
Science is Really Weird 