Like corals, tridacnids can bleach for a number of reasons, losing their complement of energy-producing zooxanthellae, and are susceptible to attacks by numerous bacteria and other organisms. Of all the things that can trouble them, however, I think the two problems that most commonly plague hobbyists' clams are infestations by parasitic snails and pinched mantle disease. I will provide in this article some information about both problems, and discuss the best ways to treat the conditions. Fortunately, both conditions can be prevented and treated effectively if you can properly identify the signs or the organisms that cause them.

Pyramidellid Snails:

A few types of generalist predatory snails will attack and eat the flesh of tridacnids, but these are very uncommon in aquariums as far as I am aware. For example, some snails of the genera Chicoreus and Cymatium are known tridacnid predators (Govan 1995), but I don't recall ever seeing one of these snails for sale at a shop (they're predatory, after all), and have never seen one in an aquarium even after being in the hobby/business for almost 15 years. In fact, with all the years I've been photographing marine invertebrates, I don't even have a photo of one (but an Internet search for them using the "images" feature at will work if you'd like to see what they look like).

To the contrary, the parasitic snails that attack tridacnids, however, are actually quite common and are, thus far, more troublesome for hobbyists (and clams) than the above mentioned generalist predators. These parasites are called the pyramidellid snails, better known as pyrams to hobbyists, and include members of the genera Pyrgiscus, Turbonilla and Tathrella (Cumming 1998). These parasitic varieties aren't just common, they also are very small, difficult to see and can reproduce prolifically.

Pyramidellid snails are actually much smaller than the generalist predatory species, and rarely reach a maximum size of around 7mm in length (Cumming 1988, Boglio & Lucas 1997). They're also lightly colored, and usually hide under a clam's shell in the petal-like scutes present on some species' shells, or in the sand/gravel substrate during the day, and thus are very hard to see. Finding them is easy enough if the tridacnid specimen isn't attached to the substrate, but far more difficult when a clam is firmly affixed to a piece of rock that prevents its removal. If a clam is attached, the best method to find them is to watch for the snails at night using a flashlight.

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Pyram snails hiding on the underside of a small Tridacna derasa. The right photo shows a close-up of the snails.

Rather than munching on a tridacnid's tissues, these parasites use a trunk-like snout called a proboscis to puncture the soft, extendable mantle near the shell's upper edge, and then feed on the victim's bodily fluids (Cumming 1988). So that's where you need to look for them if a clam can't be removed. It's also interesting to note that by some means the snails can prey on a clam without eliciting a defensive response from the clam. As such, any effects of their activities may not be detected until a clam is severely weakened and starts to gape or bleach. Gaping means its shell is open wider than normal and its mantle tissue is not extended, or is lying flaccid within the shell, while bleaching can be seen as a loss of color due to a loss of zooxanthellae.

Pyram snails can also move from one clam to another if they choose and, as mentioned, they can reproduce rapidly if left alone. On tridacnids' shells they can produce numerous small, gelatinous, egg masses, which are transparent and, again, difficult to spot (Cumming 1988). Each egg mass can produce up to a couple hundred baby snails within just a few days and they are direct developers on their host (Cumming 1988). As if that's not bad enough, the rate at which they can produce these egg masses will make you cringe, as a mature snail can produce one of these egg masses every few days (Cumming 1988)! The newly-hatched offspring can then feed on a clam and start to lay their own egg masses in as little as a few weeks (Knop 1996).

In the wild, many of these snails die and/or get eaten by other reef animals before reaching maturity. In an aquarium that lacks numerous natural predators, however, many, many more will survive and live to bear their own offspring. Such unchecked reproduction can lead to an absolute explosion of these snails in a tank and any and all clams present can be killed within a matter of weeks even if only one pair of snails is inadvertently introduced and begin mating. Larger clams can deal with these little parasites far better than small clams can, but as you can imagine, if there are hundreds of hungry snails in a tank, even a big clam will eventually succumb to them.

An easy and effective way to get rid of pyrams is to break out an old toothbrush and start scrubbing.

So, what can you do about these snails? The first thing to mention is quarantine. Keeping clams under close watch in a quarantine tank allows you to be sure that any snails and/or egg masses have been eliminated, rather than adding a new clam to your tank and worrying about the possibility of two undetected snails going in with it. But, I know a lot of hobbyists, and I know good and well that many still won't quarantine clams anyway…

Regardless of whether or not you quarantine a specimen, the most effective thing you can do to eliminate snails and egg masses is to vigorously scrub a tridacnid's shell with a toothbrush or some other brush with firm bristles with care to ensure that you don't scrub any of the soft tissue that's exposed on the underside of most tridacnids. Injuries to this tissue can lead to an infection that can kill a clam faster than the snails.

The Sixline wrasse is a natural predator of pyram snails, which can be added to a reef aquarium in order to help get rid of the parasites.

If you scrub a clam but somehow miss some snails or eggs and they do get into your tank, you can also try adding a Tail-spot wrasse (Halichoeres melanurus), a Green wrasse (H. chloropterus), a Sixline wrasse (Pseudocheilinus hexataenia), a Fourline wrasse (P. tetrataenia) and/or a Red Coris wrasse (Coris gaimard) (Knop 1996, Neigut 2005, personal observation), all of which are the snails' natural predators. Still, this may not be entirely effective since these fishes tend to feed during the day and may not reach all the snails in a tank if they are too well hidden and protected. Thus, it's also a very bad idea to impatiently skip the scrubbing and quarantine protocols, thinking that you'll just add a new fish if problems arise later. There's also the question of incompatibilities with other invertebrates you might have as these fishes eat more than pyram snails. Size is an issue, too. Some of these fishes may be acceptable in size when they're young, but it may not be too wise to have an adult 40cm wrasse in your tank as the fish matures. Thus, you'll need to do a little homework on a species that you might be thinking about adding before actually doing so (researching them at Reefkeeping Magazine, and/or is a good place to start).

Another effective predator of pyramidellids is the crab, Thalamita sima (Cumming 1988), if you can find one. These grow to only about 2.5cm in size, and they have been known to eat small tridacnids along with the snails, but generally only prey on small specimens that are much smaller than what are typically available in the trade; so, no worries there. Still, I've never tried one of these crabs as a snail predator, and don't know of anyone else trying one in a reef aquarium, either. Thus, there may be some unknown incompatibilities with other invertebrates, as well.

Regardless of what biological controls you try, you still should carefully watch any new tridacnid(s) for a period of several weeks just to be sure that no snails are present. If possible, it's a good idea to lift a clam up and look at its underside too, because the snails also like to hide there more than anywhere else. Start scrubbing if you find anything, but avoid the soft tissues!

Pinched Mantle Disease:

    An example of pinched mantle.

Unfortunately, at this time no conclusive evidence for the cause of pinched mantle disease has been found. However, tridacnids can be plagued by a number of microorganisms, and the pinched mantle disease may be the result of an attack by an unidentified protozoan, according to Barry Neigut, owner of Clams Direct. Hopefully, a definitive answer will be found in the future.

Protozoans are a mixed bunch of single-celled organisms, and as is the case with bacteria, lots of these organisms occur in marine environments and can live in/on tridacnids. It seems that, in some cases, these protozoans that live with tridacnids are parasites that don't cause any serious trouble when present in low numbers, but when pinched mantle disease strikes, the end result is almost always death unless it is treated in a timely manner.

This condition causes the mantle's smooth, curving edges to be pinched and contorted. A specimen may look like it is doing its best to stretch out its thin mantle tissue, but the margins just won't extend fully the way they should. According to Neigut, it affects T. crocea most often, with the other species being more resistant, but not immune. And, it can spread to other clams at times and is nearly 100% fatal, usually within a week or two of the first signs. Very bad, indeed! Fortunately, an easy treatment actually works well: an affected clam is given a freshwater dip for approximately 30 minutes in freshwater of the same temperature and pH as the tank from which it came. The only other thing to do is to thoroughly shake the clam for a few minutes while it is submerged in the freshwater to make sure that the water makes its way into all areas of the clam's body and into the spaces between the mantle tissue and the shell.

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More examples of pinched mantle disease.

If done correctly, it may take a clam a day or two to show any signs of recovery and re-extend its mantle, and maybe a couple more days to regain a completely normal appearance. This treatment is highly effective, and while it may be stressful, it shouldn't kill any clams that aren't already seriously compromised. It might sound hard to believe that a tridacnid can survive such a long period in freshwater, but you should note that in their natural habitat many live in very shallow water and often become exposed to air at low tide and can end up sitting in the rain for up to several hours before the tide comes back in. Dick Perrin, owner of Tropicorium, also told me a story about putting a few clams in a bucket of freshwater and actually forgetting about them for several hours, but they all survived, too. The pinched mantle condition is consistently fatal unless treated, so even if a freshwater dip is stressful to a clam, you've got nothing to lose by trying.

The freshwater dip works, but Neigut also experimented with the medication metronidazole, a drug usually employed to kill protozoan and bacterial infections in fishes, but apparently it can also treat clams. However, he did find that the clams temporarily lost their bright colors for some unknown reason. The colors reportedly came back eventually, but for a period of several days the clams looked very dull (but not bleached). Very strange, but thankfully it's only temporary.

In any case, metronidazole is readily available to hobbyists, and each product has its own usage instructions. I don't add any sort of drug directly to a reef tank unless there's no other option, whatsoever. However, its use in a quarantine tank, on the other hand, should pose little problem (other than the possible loss of color), and I should also mention that any drug should be inactivated by adding some bleach to the treatment tank after the treatment is finished, before the water is discarded. Again, pinched mantle is almost always fatal, so action of some sort must be taken if you expect a clam to survive, regardless of possible side effects.

For more information about tridacnids and their care, including a whole chapter about tridacnid troubles, watch for my new book, "Giant Clams in the Sea and the Aquarium," due in October of 2006.

If you have any questions about this article, please visit my author forum on Reef Central.

References and Sources for More Information:

Boglio, E. and J.S. Lucas. 1997. Impacts of ectoparasitic gastropods on growth, survival, and physiology of juvenile giant clams (Tridacna gigas), including a simulation model of mortality and reduced growth rate. Aquaculture 150: 25-43.

Cumming, R.L. 1988. Pyramidellid parasites in giant clam mariculture systems. In: Copeland, J.W. and J.S. Lucas (eds.) Giant Clams in Asia and the Pacific. ACIAR Monograph Number 9, Canberra. 274pp.

Fatherree, J.W. 2006. Giants Clams in the Sea and the Aquarium. Liquid Medium, Tampa. 227pp.

Govan, H. 1995. Cymatium muricinum and other ranellid gastropods: major predators of cultured tridacnid clams. ICLARM Technical Report 49, ICLARM, Manilla. 136pp.

Knop, D. 1996. Giant Clams: A Comprehensive Guide to the Identification and Care of Tridacnid Clams. Dahne Verlag, Ettlingen, Germany. 255pp.

Neigut, B. 2005. Personal communication.

Perrin, D. 2005. Personal communication.

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Troubles with Tridacnids: a Look at Two Common Problems by James W. Fatherree, M.Sc. -