Aplacophora

Image: NOAA PMEL Vents Program

Proof that molluscs can worm with the best of them!

Speaking as a human being, I can't help but find molluscs an exceptionally strange group of animals. I don't just mean all those tentacles and eye stalks, I mean the strange fact that a smart-aleck octopus and a slovenly snail are both molluscs. They seem about as different as different can be.

Compare it to arthropods. Sure, all those insects and spiders and lobsters are different from each other, but I can at least imagine them all getting together for an extended-family dinner. Catering would be a nightmare, but at least they could have a grand, old time recounting their very different life experiences.

What could an octopus and a snail possibly talk about?

Image: Arctic Ocean Diversity

Clearly, this is all ignoring the fact that I myself am a chordate, not just a mammal or a vertebrate. I can extend some kind of imaginative hand of understanding to a differently-abled salamander or fish, but it's a lot more difficult to do so with a sea squirt or salp.

Aplacophorans then, are the octopus' tunicate. They're the molluscs that lie beyond a divide that the tentacle of friendship just can't cross. How can an octopus with all its intellect, eyesight and ebullience possibly understand something that found inspiration in the lifestyle of a worm?

Image: G. Rouse

About 320 Aplacophorans have been described so far, but there are probably many more hidden away on the sea floor. Most are only a few centimetres (an inch or two) long, while the smallest are just 1 mm. The biggest ones can reach a length of 30 cm (a foot). I can't be sure, but I have a suspicion that that's horrifying...

There is one thing Aplacophorans and most octopus have in common, and that's the lack of a shell. You might remember the ancient Chitons, who are molluscs clad in a shell made of eight, interlocking parts. They were in a group called polyplacophora, which means "bearer of many plates". Well, here's Aplacophora, "bearer of no plates".

Image: Barwick & Cadien

Aplacophorans aren't completely soft... but they are fuzzy. This peculiar, furry look is achieved by a covering of bristles called spicules, which are embedded in their skin.

Image: Barwick & Cadien
Spicules

These things are calcified, so they're pretty tough. I'm sure our octopus would have a hard time dealing with all this, but it only gets worse when it's revealed that Aplacophorans lack both eyes and tentacles. I don't even mean octopus tentacles, I mean the little ones snails have on either side of their mouth for feeling things.

"Have you taken leave of your senses?!" rebuked Mr. Octopus.
"Well... yes" replied the Aplacophoran.

There are two groups within Aplacophora: the worm-ones, and the not-quite-as-worm-ones

Image: Barwick & Cadien

Caudofoveata contains the worm-ones. They use their spicules to burrow through the mud until they poke their mouth up through the surface to feed on passing detritus.

Image: M Bright

Solenogasters are somewhat different. They don't burrow since they're carnivores who feed on corals and the like. They don't move much...

Image: NOAA Okeanos Explorer
Aplacophoran besmirches a Bubblegum Coral

So they tend to just hang out on their food. Or coil around it if they're all long and horrible. Along the length of their underside is a long groove which seems to be all that's left of the molluscan foot. It can produce mucus and Solenogasters can even move around on it, they just prefer not to because they're rubbish at moving.

Image: Barwick & Cadien
Radula

Many, though not all, Aplacophorans come armed with a radula. This is the tough, toothy tongue other molluscs use to feed. Some Aplacophorans don't bother with it. These body parts are dropping like flies! How terrifying/liberating!

Oh, and of course reproduction consists of dumping their sex cells into the sea. In some species the female keeps the eggs in a cavity in her body, in others they're just cast out and left to somehow not die, hopefully.

Image: Barwick & Cadien
The height of modernity

For a long time it was thought that Aplacophorans might be incredibly ancient molluscs who first arrived before molluscs got their shells, and have since clung to life by the skin of their spicules.

It seems, however, that they're actually quite recent. They evolved from something that had a shell much like a Chiton, and the two groups within Aplacophora may not even be particularly closely related.

Such is the Worm's Path. As you push forward into the illustrious heights of wormdom, those that look on from the outside mistake your innovation for reversion. And, in the end, through great sacrifice and with your own body parts strewn behind you, you finally enter the Hall of Worms. And as you sit with your worm-brothers and worm-sisters, those nay-sayers on the outside cannot even tell you apart.

For you are Worm.