Photo of three fingernail clams, Sphaerium corneum, on a finger.

Update on Fingernail Clams, Sphaerium corneum

Photo: Closeup of three Fingernail Clams, Sphaerium corneum, on a finger.

Okay, I thought I could get away with a short blog (see my previous blog:, but right off the bat its shortness and lack of natural history led to a question from Tim Gray, a fellow fish keeper and friend. He asked, “Do all mussels produce Glochidia?” Now this might sound like Greek (that’s because it actually is), but it’s a very good question. I’ll explain.

First, let’s discuss glochidia. An online dictionary ( provided a convenient definition, “a parasitic larva of certain freshwater mussels that attaches itself to the fins or gills of fish by hooks or suckers.” So, you can see why Tim might have asked if fingernail clams have glochidia. In a closed recirculating system (for a description of our filtration system see:, such as ours where we don’t used UV sterilization or ozone, glochidia could be a nightmare. Many mussels have glochidia, but fortunately fingernail clams don’t.

How do fingernail clams reproduce? I went to the United States Geological Survey (USGS) website ( and looked up Sphaerium corneum, our fingernail clam, and found this concise description of their reproduction, “Eggs incubated in a brood-sac in the parent; embryos develop and are released as miniature adults.  Sexually mature adults can carry 1–20 embryos, occasionally more.” Kind of neat and they don’t produce glochidia and their larvae aren’t parasitic.

For more natural history of the fingernail clam I’ll let you go the USGS site. But, I do want to cover one concern I have which was caused by this statement USGS made, “S. corneum hosts such digenean species as Crepidostomum transmarinum, Bunodera lucipercae, and Phyllodistomum simile in North America.” This made me do some more research.

It turns out that all three are parasitic worms requiring two alternating hosts, a mollusk (snail, clam, oyster, etc.) and a vertebrate (you, me, or fish). Wikipedia summarizes them well, “Digenea (Gr. Dis – double, Genos – race) is a subclass within the Platyhelminthes consisting of parasitic flatworms with a syncytial tegument and, usually, two suckers, one ventral and one oral. Adults are particularly common in the digestive tract, but occur throughout the organ systems of all classes of vertebrates.”

Could our fingernail clams be infected with any of these three parasites? USGS says about them, “These species have been recorded from the Ottawa River, which flows into the St. Lawrence River in Canada (Mackie 1976, Mackie 2000).” We’re pretty far from Canada, so it’s not likely our clams are infected. But, to be certain, we feed praziquantel, a wormer, to our fish every quarter year to prevent any kind of worm infestations. Our greenhouses aren’t impermeable to things such as frogs, snakes, turtles, and the such. Water snakes of the genus Nerodia have colonized our greenhouses and could possibly bring in parasites. (I really should blog about the snakes living in our greenhouses and probably will soon.) Bullfrogs and tree frogs also find ways in. So, we treat prophylactically with the wormer to insure our fishes’ health.

By the way, we buy praziquantel in kilogram lots. A kilogram lasts about four to six feedings, maybe a year to year and a half. The foods we feed our fish are oily so we simply place a batch of food in a gallon jar, add praziquantel, cap the jar, and shake until all the food is coated. Since I imagine praziquantel doesn’t taste very good (actually, I know it doesn’t), I starve the fish for two days before feeding. When fed after starvation, the fish go into a feeding frenzy (it helps that we crowd our fish and they always feed frenetically) and gobble down the coated food. Interestingly, enough the Tubifex worms (for a blog about them, see in our system survive, I assume since the fish eat the wormer and metabolize it and the Tubifex don’t get any of it.

Also, by the way, snails are mollusks and carry these worms too. And, lots of fish you buy are wild caught or pond raised and are likely exposed to mollusks and parasitic worms. You can buy wormers for your fish and it’s probably a good idea to occasionally use one. If your fish aren’t fat and healthy, you might want to consider the possibility of worms.

So, Tim’s question was pertinent and shows knowledge of fish parasites. Way to go Tim, make me work harder!

Good fishkeeping!