Distressed Clam? Take a Good, Hard Look

Pyramidellidea snails, commonly called “pyramid, pyram or rice snails” are one of the most common pests/parasites that keepers of Tridacnid clams will come across. There are over 8000 species of pyramidellid snails throughout the oceans, and as a group, these ectoparasites have been reported to feed on bivalves, gastropods, polychaetes, echinoderms and polyplacophorans (Robertson & Orr, 1961).

Pyramidellids are primarily nocturnal and tend to stick close to their host. They feed at night using their proboscis to pierce the host’s flesh and feed on the host’s body fluids. According to Robertson & Mau-Lastovicka (1979), some species can, and will, feed on unnatural hosts so there is the possibility of pyrams found on snails moving over to bivalves and visa versa. In small numbers, pyram snails can be tolerated by the host and seem to do little damage. Pyrams, however, multiply very quickly. Their egg masses can contain hundreds of eggs. These eggs hatch after about 15 days, become sexually mature at about 45 days, and have a life cycle of about 120 days. So, two pyrams today can become hundreds in a month and thousands in a few months. In larger populations these parasites will quickly weaken the host, leading to its death if left unchecked. Before placing any new clams or snails into your aquarium, inspect their shells well, looking all around the shell for these 1/8” snails and also look for clear jelly-like egg masses. If you find either, thoroughly clean the shell with a soft brush (as described below). Ideally, it’s best to place any new additions into a quarantine tank – whether they have pyrams or not. If pyrams are introduced into your display tank, it can be a long and tedious ordeal to completely eradicate them.

So how do you know that the snails you have are pyrams and not harmless rissoid? If you have a good magnifying glass or microscope, you can look at the orientation of the apical shell whorl. In pyrams, it’s perpendicular or turned 90 degrees, whereas rissoids' whorls are all the same (Shimek 2006.) Or you could say “those little white snails are getting a little too friendly with my clam, I bet they’re pyrams! Better get rid of them.”

What if you already have them in your aquarium? All is not lost. Take a deep breath, roll up your sleeves and gather some supplies.

The above photo is your pyram eradication kit. As you can see it contains a soft toothbrush for brushing around the shell's edge and the clam's byssal opening to avoid damaging any flesh. Use a stiffer brush for scrubbing the middle of the shell where you won’t be in danger of contacting the flesh, and a few pointy objects for getting into nooks, crannies and crevices where the brushes can’t reach. I like kabob skewers for this purpose because they are disposable. I’ve used nice sharp kitchen utensils before, but they end up getting me into the dog house with my spouse. It's also adviseable to find a well-lit area where you don’t mind splashing a little tank water or flicking snails in it.

Okay, so now you’ve chosen your weapons, what about your plan of attack? Remember that pyrams are nocturnal so we are going to use this to our advantage. About an hour or two after all lights are out in the aquarium the pyrams will start to come out to feed on their host. Fill up two containers that are large enough to hold the clam with aquarium water. Gently grab the clam and turn it so that the mantle is horizontal and slowly lift it out of the water. You never want to remove a clam from the water – especially a large clam – with the mantle pointing up (vertical) because clams can not support their mantles while out of the water. Bring the clam over to your work area, and give it a good once over to see how many pyrams you are dealing with.

Now grab the soft toothbrush, and with the clam’s mantle horizontal and facing slightly down, start brushing all along the shell's edge. Brush away from the clam so that the snails do not land back on the shell or go into the mantle cavity. Make sure you get into every nook and also brush areas that don’t appear to have pyrams because the egg sacks and newly hatched juveniles are nearly impossible to see. Use the pointy object of your choice to get clean out any areas the brush can’t. Continue all around the shell's edge to the byssus and up the hinge ligament. Then brush the middle of the shell with the toothbrush or the stiffer brush. When you are convinced you removed them all, turn the clam over and repeat on the other half of the shell. When you are finished, place the clam into one of the two containers of tank water you filled earlier and quickly brush the shell again to remove any eggs or snails you loosened earlier. Then place the clam into the second container.

Before you place the clam back into the display, if the clam was on the sand, scoop out the substrate from where the clam was to remove any pyrams that might have been hiding there. When replacing the clam back into the aquarium, lower it into the water slowly with the mantle still horizontal until fully under water, then turn the clam so the mantle is facing up and gently rock the clam back and forth to release any air that might be trapped under the mantle cavity (burp the clam). Another trick you can use is to fill a small Tupperware™ container with sand and place that in the aquarium and then place the clam in the container.

Why go through all of this? Because no matter how hard you tried, you missed a few snails and or eggs (trust me). So, for the next four to six weeks you will need to repeat this cleaning routine two to three times a week, discarding the substrate as well. Even if you don’t see any snails, do the whole routine. After six weeks you can slow down to once a week for another six weeks. And then from there periodically inspect the clam. I know, it sounds like a lot of work. All the more reason to set up a quarantine tank; everything you add to your tank may harbor unwanted pests.

Possible Predators

There are a few fish like the Six-line wrasse (Pseudochellinus hexataenia), the Four-line wrasse (P. tetrataenia) and the Yellow Coris wrasse (Halichoeres chrysus) that may eat pyrams, but at their best, these fish will only control the population, not eradicate them. Remember, pyrams are nocturnal (active at night) and wrasses are diurnal (active during the day). So you may find a fish that eats some, but if you’re depending on one of these fish to eradicate them, in my opinion, it’s not going to happen.

With any luck, you'll rid your tank of these pests forever.