Vermetid Snails in the Aquarium

As a keeper of marine aquariums for many years, one of the more interesting categories of animals I’ve encountered are Vermetid snails. They are not your typical snails, as perhaps can be seen from the picture below. In breaking down my aquariums for a move recently, I discovered some nice examples of shells from a larger species and thus felt it worth sharing my experiences with them.

Vermetid snail shells sitting in a blue plastic lid that is approximately 2.5in. in diameter. The white spirals are Spirorbid worms (another filter feeding animal) that settled on the Vermetids.
Vermetid snail shells sitting in a blue plastic lid that is approximately 2.5in. in diameter. The white spirals are Spirorbid worms (another filter feeding animal) that settled on the Vermetids.

Adult Vermetids are filter feeders. The shells are anchored in the rockwork and the snail secretes a slime net to capture particulates in the water. Often the exist in an aquarium totally unnoticed, although large populations and adults of large species may cast visible slime nets.

There are several common species of Vermetids in the aquarium trade, ranging in size from a few millimeters as adults to nearly 2 inches. Shell shapes range from an extended spiral as in the photo above to a more messy, spaghetti-like appearance.

Life Cycle

Vermetids start out as a regular free-roaming tiny snail. In fact, the are often confused for other snails like Cerithoids when they are in this phase. Although I had these snails in my aquarium for years, it took me a very long time to realize that the tiny conical snails that would periodically roam the glass were actually juveniles of the large Vermetid species I saw poking out of various rocks. Juveniles for smaller species can be nearly invisible due to their minuscule size. The free-roaming phase is actually quite easy to see in large specimens though, such as the ones in the photos here. It is the part of the shell that is “normal” before curling off in a crazy way. Once the snails find a spot in the rock to tuck in, they begin their lives as a stationary filter feeder.

Free-roaming stage of shell highlighted in red.

The Good

Vermetids are often a sign of a tank with a healthy ecosystem, as is the case for many filter feeders. Partciularly in smaller aquaria, obtaining high enough water quality without it being “too clean” (devoid of plankton and other edible particulates) is a challenge. Healthy populations of Vermetids often accomany populations of other filter feeders, such as small clams and mussels.

Vermetids are typically harmless, although some exceptions involving corals and equipment are noted below. They are preyed upon by many crabs and some fish. Since they are often readily able to reproduce in stable reef aquariums, sustained populations represent natural food source for other reef animals such as hermit crabs and wrasses.

Many people have these snails in their tanks and never know it. There are other animals that secrete slime into the water for various reasons, so it can be difficult to tell whether a trail of slime blowing in the water is a Vermetid feeding or something else. For the most part, Vermetids are just cool little animals that help filter your water and make the biology of the tank more diverse.

Of course, as with many marine organisms, they can have a slight dark side too…

The Bad

If your tank has “dirty” water with too much suspended waste, it is possible to end up with an over-abundance of Vermetids. This has happened to me on more than a couple of occasions in the multitude of aquariums I’ve kept. Although this doesn’t directly harm anything, there are two main problems associated with these little snails:

  1. Visible slime nets waving about in the water. This is an eyesore to some people, but more importantly it’s an indicator that you have way too much particulate food floating around in the water.
  2. Clogging filters, power heads, and other water circulation tubes and pumps. These animals LOVE to grow inside canister filters and pumps when space allows for it. The smaller species in particular can get into water circulation systems and clog up the tubes or even cause impellers to get stuck.

Vermetids aren’t the only culprits for the second issue. Many other filter feeding animals that produce calcareous shells cause the exact same thing, and typically when I crack open a piece of equipment that has been Vermetid-bombed, it is also full of these other organisms. Sometimes the populations have been so dense that the equipment had a continuous inner coating of shells that had to be chipped away to return it to its normal functionality.

The Ugly

While I’ve had no really terrible problems with the smaller species of Vermetids, the larger ones have caused me some grief with something very particular: polyp bailout on stony corals. Large-polyped corals that secrete a skeleton will sometimes evacuate their skeletons in the event of sever stress or irritation, and the condition is typically fatal to the polyp (in the wild, some corals will send out free-roaming polyps to relocate and start a new colony, but drifting polyps almost never settle successfully in captivity).

One year I had a series of inexplicable incidents of polyp bailout on several large-polyped stony corals. In each case, the skeleton contained a collection of tiny snail shells in the cavity where the polyps would normally be rooted. All were very much dead though upon discovery in the coral skeletons, which was a puzzle indeed. I initially thought this was some sort of parasitic snail that had been trying to feed on the coral tissue, but I had no luck with identifications and eventually gave up. Much later, after finally finding a complete shell of the large Vermetids pictured here, I discovered that the “parasites” that savaged my corals perfectly matched the free-roaming stage evidenced in the adults’ shells. Culprit identified!

My best hypothesis for these repeated polyp bailout events with snails at the bottom has been that the free-roaming snails burrowed between the polyp tissue and the skeleton looking for a good place to live. Either they had been smothered by slime from the coral or the coral had secreted something else that killed them. Probably the irritation from this activity (and perhaps from the resulting decay) was too much and the polyps bailed as a last attempt at survival. It is worth noting that there were very heavy filter feeder populations in this tank, so the Vermetid populations were likely well above what would be found in the average reef tank.

The Conclusion

Should you find these unique snails in your tank, please take a moment to appreciate the beautifully weird convolutions of their shells and, if you can, leave them alone to live out their weird little existences. Dislodging them will almost certainly kill them – if not from breaking the shell, then from the fact that they will no longer be in a desirable feeding position when moved. In most aquariums they are totally harmless and interesting animals and problems generally only arise when you let them get a bit out of control due to too many particulates in the water.

If you want to read more about Vermetids, Ron Shimek has a lovely article on them: Snails that Worm Their Way into Tanks.

Comments 2

  • My tank is over a year old. The vermitid snails came on the back of Turbo/Astrea Snails ordered online. Was told on a forum not to worry about them if not planning to get corals. Fish only for now, but in future want to start to get some corals. The mucous nets coat everything. Cant even see the animals. See their nets, but where is the animal? Must be micro. As soon as cleaning ensues, nets in the 1000s. Told my wife I’m planning to remove rock, snails, and sand to kill these vermin. Hope not to offend. Will lose Coraline, snails, and bacteria. My Margarita Snails have spawned babies that have gone from micro to 1/4 inch.

    The slime coats filter socks, sump, even durning water changes can see it running down glass. How does it go thru the filter sock is a mystery. It catches fish waste and collects everywhere.

    Purchased 6 line wrasse that constantly eats something from the rock. Have 1 bumble bee snail. Cut food to my starving pets. Vermetid snails still here. Am i missing another control animal? This hobby is too rewarding to quit. Moved to the city so cant garden, this replaced. Anything you can offer, besides starting over with more dry rock? Thanks

    • Normally I also advocate leaving Vermetids alone, but when in small numbers where they’re just a curiosity. You are unfortunately seeing their nasty side, which does require intervention of some type or you will be slimed into a mess. The filter sock phenomenon isn’t entirely weird; the slime isn’t really solid so it can pass through a lot of media when there is a current forcing it through.

      I would advise against rock replacement except as a last resort, since starting over with rock won’t necessarily clear them if you’re replacing it within the same tank rather than a full breakdown – Vermetids have a motile stage that typically lives in the sand before finding a place in the rock to settle down. If you’ve got a significant sand bed, their babies will be in it. Your new rock also won’t necessarily be free of them unless you get the man-made stuff. The fact that your snails spawned and successfully grew indicates that you have a very healthy collection of microscopic water column fauna – and that’s what the Vermetids will be feeding on too. So, another thing to be aware that if you take out the Vermetids with will likely put an end to any snail breeding and also remove a food source from any other filter feeders you have (tube worms, etc.). Ideally it would be best to identify where all the nutrients are coming from that is supporting the population and control it. The root of the problem is that somewhere there is too much of something being “fed” to the tank, whether light promoting phytoplankton or actual physical food being added. If the wrasse is finding enough to eat just from the rock, it seems unlikely to me that your other animals would be starving unless you have some very specific/finicky feeders in your stock. Most marine fish in the hobby are omnivores who actually prefer to eat rock critters when available and you may have plenty of that there. Very sparsely stocked tanks may only need to be fed once a week or even less. On the flip side, it’s also possible the tank is overstocked and therefore being over-fed even when fed minimally for the fish themselves, in which case the “food” that needs cutting to the system is really fish waste (the only solution for which is really less fish per water volume). Not knowing exactly what you’re stocked with and how big the system is, I can’t really guess one way or the other. Point is, it’s best to try to address the root issue of too much “food” before trying other things. For the following other suggestions, I’m basically assuming you just got unlucky and have a relatively sparsely stocked tank that is just a plankton paradise.

      Vermetids live in the rock – often in places where you’ll never see them unless you break down the tank and start taking the rocks out, and then only the large species are easily findable. Small to medium hermit crabs with claws that have a cup-and-pin or cup-to-cup type shape for at least one claw, usually the smaller claw, love to eat encrusting animals like Vermetids since they essentially have a tool designed for popping them out of the rock. If well fed and supplied with sufficient clean spare shells they will also leave other invertebrates alone (with a possible exception of Nerites for some species). Calcinus hermit species in my experience are very good rock-pickers with C. seurati (zebra hermit) being one of the best at clearing encrusting animals since it is very active, although it also one of the more aggressive species. Mythrax crabs will also eat them to some degree (and have a cup-to-cup claw shape) but have a bit more trouble getting into crevices. However, they’re less likely to start a crab war than C. seurati if you’ve got an existing clean-up crew well-stocked with hermits and if the tank isn’t very big.

      Another thing to consider, although potentially rather more drastic than crabs, would be a UV-sterilizer on one of your lines to/from the sump. UV sterilizers will nuke your microfauna and kill larval forms of good and bad alike – but that may be the lesser of two evils compared to having everything get smothered in slime regularly. So, they can be used to basically de-plankton systems to a degree, although it typically also needs to be done while controlling the amount of other light the system receives. With a UV sterilizer you’ll also see a waste spike as all that stuff dies if it’s actually having an impact, so ammonia/nitrite should be carefully monitored. UV is also most effective on smaller systems. With a nano they can be very effective (hence why they are drastic) but on larger tanks that will drop off. I would only ever recommend using something like this temporarily; they should not be a part of healthy systems.

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