Mollusks: Mussels, Snails, Clams, and Co

Today, we are talking about one of my favorite groups of animals: mollusks. Yep, the group that contains snails and all that slimy stuff, but also octopuses and other cool stuff. Plus, snails are cool if you get past their sliminess. And I’m a marine ecologist, so the slimy bit doesn’t really concern me, as the snails I encounter are wet anyway.

I’ve made it through two of the three weeks of full-time lectures and am out of spoons, probably in spoon debt by now, so if I sound a little exhausted, it’s because I am beyond exhausted. And there are so many exciting things to tell you that all have to wait until I get my brain back. Damn it. Right, mollusks, let get into that.

Mollusks make up the largest marine phylum, with about 23% of the organisms. They also are the second-largest animal phylum, with over 100,000 species. And there’s a lot of diversity here, from mussels to octopuses. They live almost everywhere you find water, and some are even terrestrial.

First, let’s get that spelling issue out of the way: Mollusca, the taxon name, is spelled with a -c. There’s no debate about that. When it comes to the common name, mollusks, it’s not that clear-cut: The US seems to prefer the spelling with a -k, but the rest of the world spells it with a -c. I so want to spell it with a c, but as I have chosen American English a long time ago, I have to live with mollusks with a -k, I guess.

But what even are mollusks? Well, from dinner plates all around the world, we know a lot of the members of this taxon, as people like to eat them. Mussels, clams, scallops, oysters, squids, and octopuses, and some others, all make it into human consumption. Then there’s the fact that we’ve learned a lot of what we know about mollusks from shell collections. I’m sure you’ve collected seashells at the beach at one point or another, too. This means that a lot of people know quite a bit about this taxon—once dead.

In general, mollusks are invertebrates with soft bodies. Some of them (like the mussels, clams, etc.) have shells, other members don’t.

Unfortunately, there’s no single distinguishing feature that all mollusks have. Many of the apomorphies of the Mollusca have undergone a reversal in some of the Mollusca. Most mollusks have a primitive head, a foot, a body called a mantle, and a raspy tongue called a radula.

Anyway, there are a few features that are common enough to talk about them anyway:

1. Mollusks have a specialized foot that’s used for movement or digging and grabbing things. This foot, however, looks very different in different groups. I mean, in a snail, finding the gliding foot is easy, but considering the arms of an octopus as a single specialized foot or finding them in a clam might not be as easy. In some bivalves, the foot looks more like a paddle for digging through sediment than an actual foot.

2. Mollusks have a mantle. That’s what you call their body. It’s the soft stuff that encloses the essential bits, like organs. Some species produce hard shells from their mantles, others don’t. In German, this mantle is called Eingeweidesack, so organ sac. I definitely think mantle sounds nicer.

3. Many mollusks have a radula, a kind of raspy tongue used as a feeding tool. They scrape the radula along surfaces in characteristic patterns. The major group of the bivalves doesn’t have a radula though, so don’t go searching for one in your mussels or clams. In those, the radula is completely reversed. And in some species, like the predatory cone snails, they have changed into a kind of stinger for toxin injection.

Actual Soft Blobs

There are a few groups in the Mollusca that are actually just soft blobs, so let’s get those out of the way with as few sentences as possible.

There’s three groups with similar names: First, there’s the aplacophora, small shell-less deep-water worm blobs. They can live inside the sediment or on it. Their feet have undergone partial reversal to the point where the foot is hard to find. They just look like worm blobs.

Then, there’s the monoplacophora, another deep-sea group, this time of blobs with a plate-like shell. They look a lot like snails, but they aren’t gastropods. Their definition is pretty vague and slippery, so let’s not even try. Non-gastropod snail things is fine with me. The only cool thing I could find about these is that they are considered living fossils, because they were known in fossil records for a long time before they found the first extant (living) species in the Pacific in 1952—at a depth of more then 3,500 meters, so pretty deep-sea. No wonder we didn’t find them earlier.

There’s a third group with -placo- in the name, but we’ll get to the chitons, the polyplacophora in a moment. First, let’s get the Scaphopoda out of the way: the Scaphopoda, the tusk shells with cool names like Sword Razor, are pretty much exactly what they sound like: soft blobs in a tusk-like shell. They look like someone modeled a snail house from an elephant tusk and stuffed a soft blob inside. Exciting, I know. Let’s move on to less boring things.

Polyplacophora: Underwater Hedgehog Knights

I actually remember when I first encountered chitons in real life. Chitons, so the Polyplacophora, the many-shelled blobs in the mollusk world, have shells comprised of eight plates (also called valves). They overlap like the plates on a knight’s armor. The extra cool thing is that they can actually roll into a ball, kinda like a hedgehog, for extra protection. Anyway, I first saw these off the coast of New Zealand at an island called Goat’s Island. There was a sign at the parking lot outlining what lived in the bay. One species was nicknamed the sea cockroach, and while I think they are a lot more like hedgehog knights than roaches, I see where the name comes from. When we went diving there, we saw a lot of these things on rocks everywhere. We were diving only a few meters deep at high tide, and the chitons enjoyed their favorite habitat: shallow ocean and intertidal zones. They do exist all over the world, though, from very cold to tropical. And while they are exclusively marine, there’s a lot of different species of them. And their colors vary widely from very boring grays and browns to pretty patterns.

They are primarily herbivorous, so plant-eaters, which they scrape off surfaces with their radulae. While their radulae are very well-developed, their nervous system is pretty simple.

Now that we’ve got the boring stuff out of the way—okay, okay, chitons are kinda cool—let’s talk about the remaining taxa of Mollusca: bivalves, the gastropods, and my absolute favorite: the cephalopods.

Gastropods: Snails, Slugs, and other Squishy Things

The gastropods are the most diverse group of the mollusks—and also the only one that has some representatives on land.

Their name essentially means stomach-footed because their digestive tract sits on top of their gliding foot.

Most gastropods secrete calcium carbonate shells, but there are definitely those without. The gastropods most people are most familiar with are probably the terrestrial snails and slugs that use their radulae to pierce plant parts.

But, I promise that there are way cooler gastropods than pond or garden snails—though I have to admit there are a lot of really cool ones there once you start looking.

The Nudibranchia alone make the gastropods worth looking into. They don’t have shells, but they do display a very stunning array of colors, shapes, and patterns, and fun names like Leaf Sheep and Pikachu. More on that in a bit.

There are a lot of ways to classify the gastropods. One way to look at things is by how they breathe, as there are the front-gilled (prosobranchia), back-gilled (opisthobranchia), and lunged snails (pulmonata).

Gastropods undergo a process known as torsion, so twisting, where their mantle rotates 180 degrees to one side. This moves their anus somewhere roughly above their head. Interesting choice, but okay. That’s also where some chemical and mechanical receptors end up, so the siphon can now filter water into the mantle cavity. Some extra senses for the little gastropod.

And, while you might be thinking that twisting and snails fit well together, keep in mind that this torsion is completely independent of the snail shell coiling. In the opisthobranchia, this torsion of the mantle is undone to varying degrees.

Oh, and like there are right-handed and left-handed people, there’s snails that have their shell open to the left or right, though right is way more common. And in the shelled snails, you’ll sometimes find something called an operculum, so a kind of trapdoor to seal off the shell.

Independent of shell or no shell, there’s a lot of pattern and color variation among the gastropods. Remember that Pikachu nudibranch, I mentioned. It shows off how poisonous it is with bright yellow colors. The Spanish Dancer is bright red, the leef sheep green and blue. I think I’ve seen just about any color while diving off California.

Gastropods are pretty weird when it comes to their sex stuff. There’s some that are hermaphrodite, so have both male and female parts, some with separate sexes. Most gastropods lay some kind of eggs, but there are species that give live births to little snail babies. They are very fittingly called Viviparus. There’s even a Viviparus viviparus in Central Europe.

Why is that fitting? Well, the English word for a species where the embryo develops inside the body of the parent is viviparous. The opposite of viviparity is oviparity, so egg-laying.

Can we return to nudibranchs now? They definitely are my favorite gastropods. We’ve already established that they look pretty cool with their colors and shapes and patterns, but they are also really cool when it comes to their behavior.

First, most nudibranch species are benthic, so ground-near inhabitants. Nonetheless, there are some nudibranch species that can swim and spend their time in the upper parts of the water column near or at the surface.

In either case, they are pretty impressive predators. Many nudibranchs like to munch on cnidarians. If you remember our excursion into the cnidarians, you’ll remember that they have characteristic cnidocytes, so stinging cells. When the nudibranchs eat those, they leave them intact and embed them into their own bodies for defense.

When we get to the environmental impact of humans on the different species we discussed, we’ll definitely talk more about the different kinds of gastropods. As you can imagine, humans have quite the impact on both the species we eat and the species we don’t eat… We’re nothing but efficient at species fuckups.

As I’m taking a class on snails this summer, I’ll probably have a lot more to say about them then. And there will definitely be more about nudibranchs in the future because I love those colorful pretty things and their diversity. But for now, enough about snails. Let’s move on.

Bivalvia: Mollusks with Doors to Shut

Describing the bivalvia as mollusks with doors to shut might be a little unfair to those of the gastropods that have the trapdoor-like operculum to shut their shells, but in the case of the bivalvia, I’m actually talking about them living inside two shell halves that they can open and close like a door on a hinge.

The Bivalvia include clams, oysters, mussels, scallops, and the like. And, I assume you know this, but as many humans only ever interact with the seashells they find on the beach: what you find there is usually half of the shell. Bivalvia have two valves, so two doors that close around them. The actual hinge line is a combination of a ligament, like in your own joints, and shell protrusions much like teeth, that can interlock. The interlocking mechanism keeps them from sliding left and right, while the ligaments keep them together. If you look inside an empty shell, you’ll probably find thin scar-like lines where the adductor muscle used to attach. Those are used to close the shell, so the default stage is open. That’s also probably why you have to discard the open mussels of your bunch. When they die, the muscles lose their strength and the bivalves open.

Bivalves exist all over the world in marine and limnic (that’s lakes and such) environments. We’ll get a lot more into a few of these, when we talk about the environmental impact of fun little bivalves like the Zebra mussel.

As far as I could find, all bivalves are sessile, so they live attached to something. Because of this low-movement lifestyle, they don’t really need that complex of a nervous system. No brains, just a simple network of nerves in so-called ganglia.

They also are really not that great at sensory stuff. They have chemoreceptors and mechanoreceptors, sometimes attached to short tentacles, sometimes lining the fringe of the mantle.

They can breathe through their gills (if they have them), but also through capillaries along their mantle cavity. So, essentially, they can breathe with their skin. Cool.

Thanks to their sealable doors, they do pretty well in intertidal zones and tide pools. They just seal off what they hold inside their shells and use that until they can open up again. Though, impressively, some can open their shells slightly and actually use air.

Another thing connected to the fact that mussels like to sit around is their byssus threads. Not all Bivalves have these, but it’s those sticky furry threads they use to attach to the substrate. You can’t eat them, but they are dead useful for sticky materials. And, if you wash off the sticky stuff, you can apparently make clothes from them. People will make clothes out of everything, won’t they? Byssus is pretty flexible and pretty sticky, so scientists really want to copy those properties.

Also, not present in all bivalves is the siphon. It’s essentially like a trunk they can stick out of their shell to feed.

Damn it, I didn’t want to let this one get this long again. I guess we are moving the octopuses to their own episode. I can’t just mention giant squids that reach a total length of up to 22 meters, and my beloved octopuses in the last sentences of a very long episode. I can’t do that to those of you who stopped listening already. You’d be missing the coolest things. No, can’t do that.

So, we’ll talk about cephalopods next time. And don’t worry, I haven’t forgotten about the insects and crustaceans, either. We’ll get there afterward.

And when we’re done with all of this—we are getting closer and closer if I stop splitting things into new episodes, that is—we’ll talk about how humans fucked things up for all of these taxa. Can’t wait!

Sources:

I study Marine Ecology at the University of Hamburg, so a lot of this knowledge comes from hours of research and sitting through lecture after lecture.

Going through the lecture slides from school is a process that involves a shit-ton of fact-checking, as a lot of what we learn is pretty outdated. So, all semester, I google things to death, read papers and essays, ask a million questions, and discuss things with friends and classmates.

Where the source isn’t our lecture slides or unidentifiable sources from hours of late-night knowledge hunts, I have linked them in the text.

Kate Hildenbrand

Kate Hildenbrand

Kate Hildenbrand is the writer behind the essays here, author of fiction novels, the creator of the Kate Hildenbrand podcast, and a student of marine ecology. At least, that's her on the surface.
Germany