Before studying an organism, you have to know a bit about where it sits in the phylogenetic tree (that is, how it relates to other animals in the grand scheme of evolution.) Phylogeny is determined by studying morphology, physiology, and (more recently) genetics of organisms. As new organisms are discovered and known ones are studied, biologists fit the into phylogenetic categories that show approximately how they are related to each other. The Tree of Life website (http://tolweb.org/) attempts to construct a complete, interactive phylogenetic tree – it’s pretty fun, and I’ll be using their diagrams throughout this post.
Let’s get started, shall we?
I’ll start by saying that cephalopods are molluscs. They have, like all molluscs, a bilaterian body plan (that is, their bodies are basically laterally symmetrical,) a chitinous shell (although this has been internalized in most cephalopods,) a mantle cavity, and two pairs of main nerve cords.
Cephalopods are differentiated from other molluscs in that they all have a funnel derived from the molluscan neck region, a ring of arms derived from the molluscan foot, chitinous beaks, a shell, and image-forming eyes (although I seem to remember reading about an octopod that only has cup eyes – that is, eyes with no lens, which cannot form an image.) As a heuristic, you can think about cephalopods as including squids, octopuses, or nautiluses.
Within Cephalopoda, there are 5 subdivisions; of those, Nautiloidea and Coleodia still have living representatives, while endoceratoidea, ammonoidea and actinoceratoidea are all extinct nautilus-like groups.
Nautiloidea contains the extant and extinct nautiluses, animals with a chambered spiral shell and a funnel that is not fused but made of two flaps (you’ll see what I mean later – squids and octopods have funnels that are smooth tubes.) The nautilus is often spoken of as the most primitive or most ancient of the cephalopods, due to its resemblance to extinct cephalopods that appeared early in the fossil record.
Notice the funnel, the many arms, and the eye – importantly, it has no lens. It forms images the way a pinhole camera does; thus it is called a pinhole eye.
Now we get into the good stuff. The living Coleoidea are divided into decapodiformes (or decapods) and octopodiformes (or octopods). Decapods have 8 arms and 2 tentacles (for a total of 10 appendages) while octopods have 8 arms and no tentacles (with the exception of the vampire squid, which has 2 modified tentacles). Decapods include squids and cuttlefishes, while octopods include all octopuses.
Let’s start with the decapod family tree.
Decapods are divided up into 6 groups:
Bathyteuthoidea are a group of small squids that live mostly in the open ocean.
Idiosepiidae are a small group of cuttlefish (only 8 species) that live on west pacific coastlines. Their distinguishing feature is an organ on their dorsal side that they use to anchor themselves to seaweed.
Myopsida contains two subgroups – Australiteuthidae and Loliginidae. Australiteuthidae are a type of miniature squid found off of the coast of Australia (if you couldn’t tell from the name.) Loliginidae contains a variety of genera, and these are generally what you think of when you think of squid. Of particular note, Loligo vu
lgaris is in this group. This species is widely exploited for food (historically, for its ink,) and has been widely studied by marine biologists and neuroscientists. It was this guy that the squid giant axon was isolated from (I’ll write a separate post about that.)
Oegopsida contains a large variety of open-ocean squids – notably, a lot of really cool deep-ocean squids like the Glass Squid and the Giant Squid. I won’t get further into Oegopsid systematics now, as this is already a long post.
Sepioidea contains the cuttlefishes, most of which have internalized shells called “cuttlebones.” These guys hold the title of the cutest cephalopods, at least in my book. If you don’t believe me, check out the Striped Pyjama Squid. Even the name is cute!
Lastly, spirula contains a single species of squid that has a unique internal coiled shell.
Now, moving onto the octopods. These are my personal favorite. I think they are the smartest (or at least the most behaviorally adaptable) cephalopods.
The living octopods are all divided into two groups: octopoda and vampyroteuthidae. Vampyroteuthidae contains a single species, the Vampire Squid. Although this guy is undoubtedly cool (as seen in this clip from Planet Earth), I find octopoda to be more interesting, if only because they have been studied much, much more.
Octopoda is further subdivided into cirrata (which are a small, poorly studied group of deep-water octopods) and incirrata (which are “conventional” octopuses.) There are a great variety of species in the group incirrata, but the most important one to note (in terms of neurobehavioral research) is Octopus, particularly Octopus vulgaris. A great deal of research has been done on this guy, including a comprehensive anatomical study of the central nervous system by J. Z. Young (one of the greats of neuroscience – I’ll have to write a separate post about him, too.)
There you have it. This was a great review for me, and it should set the stage for any other discussion of cephalopod behavior or physiology. It’s immensely important in biology and neuroscience to think about the organisms you study in terms of their evolutionary history, and phylogeny guides us through that history. Hopefully this was informative for you!
Also, let me know if I’ve made any glaring mistakes, please!