Last summer (2024) I had the privilege and pleasure of attending a 6 week class at Bamfield Marine Sciences Center (BMSC) and I learned about many awesome marine organisms. So I'm thinking I want to tell people about them, because they're not big or flashy or anything like that, but they are still cool! Little uncharasmatic beasties living their best lives. So I'm going to start a small series and teach you all about the critters I met.
I want to start with worm snails (Vermetidae). Not snail worms, that something else, but worm snails: little snails that resemble tube worms but are clearly molluscs. Also, I think they're really cute. Like, look at that face:
Look at it! It's just a little guy!
This is very long, here's a readmore so in don't 'color of the sky' your dash with this:
Now unfortunately, I don't have a size scale for any of the images I took, it wasn't necessary for the project we were doing, but as a reference it lives happily on the back of one of these snails, Red Turban Snails (Pomaulax gibberosus), a marine snail found
With about 11 of its closest friends, 700 tube worms, patches of bryozoans and a variety of other critters. They are not large animals (or at least this species specifically is not large, other species can get a bit bigger, I'm not immediately seeing any size descriptions about it right now.)
So how do we know that this thing is a Mollusc (like a snail) and not an Annelid (like a worm)? Let me introduce you to HAM:
HAM, the Hypothetical Ancestral Mollusc is an approximation of the OG Mollusc. All Snails, clams, squid and octopus descend from this guy. (He kind of resembles a abalone tbh, or a limpet). This is a Very Average Mollusc.
And this is your average Annelid:
Image source
So we can see that these are two different body plans even at a glance right? But how do we know the snail worm is a Mollusc and not an Annelid. Because like, this is an Annelid too:
And it's also a little guy that lives in a tube. Tube worms! Fantastic.
So how do we know one is a snail worm and one is a tube worm?
Well, there's a few things we can look at. So on the tube worms tube, the tube is composed of 2 layers and it's dull inside, whereas the snail worm is shiny inside and it's composed of 3 layers.
The second thing is the thing that drew me to this little dude in the first place: the behavior.
Now, both of these animals stay in their little tubes their whole lives, so they have that similar behavior, but check this out (and I apologize for the potato quality of the video, I was using my phone focused through a dissection scope and the creature I was videoing was on the back of a living snail that was moving around is dish, making focusing quite difficult.)
So what are you looking at here. The most obvious feature is the thing coming out of its mouth, kind of rhythmically right? Well, that's is radula! If you look at the Mollusc vs Annelid images above, you see the Mollusc has a radula in its mouth and the Annelid does not. (A radula is like a tongue with teeth on it).
A tube worm, as we can see in the images above, doesn't have a radula, it has structures called radioles at the top of its head that capture food particles and also act as gills for breathing, and they just passively sick out in the water. When the worm is disturbed it'll quickly retract into the tube to hide until the danger has passed:
The worm snail, instead of retracting all the way deep into the tube, has a little door, called an operculum, that it will close to protect itself, you can see it at the lower left of the snail gif. This feature can also be seen on things you will more readily recognize as a snail too:
A mystery snail (common in fresh water home aquariums) with visible operculum. Image source.
Now, operculum are not an exclusive feature of snails, snails just happen to have them nearly across the board as they are useful little doors that protect them when they close up shop to hide. Tube worms even sometimes use operculum! But it's not a standard feature in tube worms like it is with snails and their kin.
What does a snail worm eat with that weird little radula, seemingly upsidedown, just kind of licking the open water? Well, it looks like this is an area that requires further study. With a lot of these small ocean organisms, even reasonably readily accessible ones like these, the answer is 🤷♀️. It probably either secretes a mucous net, captures food (tiny plankton) from the water with that and slurps it up, or draws water in to its mantle cavity (inside its shell) and filters the plankton out with its gills, because as of this writing those are the two methods of feeding other species in this family use.
Vermetidae as a whole are under studied animals, even though they are reef building organisms in their own right and ecosystem engineers, they're often overlooked and the literature around them outside of ones that live in the Mediterranean Sea, is slim because they're just not that interesting to the casual observer, a curse of the uncharasmatic to go ignored and understudied. For all we know this species only lives on the back of these turban snails in this one place in British Columbia and this might be the only writing on them ever. I hope I did them justice, I think there really neat.
Snails with worm-like shells, usually found growing cemented onto a hard surface, or cemented together in colonies. The inner surface is smooth, unlike the tubes of the better-known Tube Worms. Since the shape of the tubes is highly variable, Vermetid taxonomy is a nightmare, even with the living species.
According to one source I've looked up, there are 135 recognized living species of Vermetidae and 16 genera. The Paleobiology Database recognizes 11 fossil genera and 114 fossil species of Vermetidae. They've been around since the Triassic (250-201Mya) - the fossil up the top is from the Late Oligocene, in the Waitakian Stage (25.2-21.7Mya).
Since Vermetids can't move from where they grow attached to the seafloor, they feed by capturing plankton - either filtered from the water passing over the gills, or by spinning a mucus net in which plankton is trapped.
Unusually for a sessile animal, vermetids are of separate sexes, with sperm packets captured in the feeding web of females and eventually fertilizing her eggs. Fertilized eggs are brooded in her mantle cavity, and released as crawling or free-swimming larvae.
University of Otago Geology Museum, Dunedin, Aotearoa New Zealand
