Fingernail Clam – Species Profile

The Fingernail Clam (aka Pea Clam, Nut Clam, Pill Clam) is a generic name for many species of small freshwater bivalves of the family Sphaeriidae. In the US, there are 38 species of these clams. All are very small in size, when compared to other types of freshwater clams, and unlike those other species, they do not have a parasitic stage in their life cycle. This makes them a fish-friendly option for hobby aquarists.

Care Level: Intermediate to Advanced
Preferred Water: pH 7.0 to pH 8.0, Hard Water
Ideal Temperature: Temperate to Subtropical
Maximum Size : approx. 0.5″ (1.27 cm)
Average Lifespan: 1 to 2 years


Fingernail Clams can be found throughout the continental United States, with 20 of the 38 known species being found in the southeastern states. They prefer slow moving or still bodies of water. Because they bury themselves in the sediment, they prefer sandy, silty, or muddy substrate.

It is believed that freshwater clams first evolved in the Mississippi drainage basin, since this is where the greatest diversity can be found. However, freshwater clams of many species can be found all over the world. It is thought they have spread so far and wide because juvenile clams will attach themselves to plants. The plants are then carried away by birds and other animals, which then deposit them in new waterways. Some species of Fingernail Clams can even survive being swallowed by ducks! The birds then carry the little guys in their stomach to the next pond. When the ducks… do what ducks do, the clams are released into their new habitat.

For clams living in temperate zones with harsh winters, they can spend the off-season buried deep within the substrate. They dig as far down as 16 times their own body length to escape the cold. This behavior also protects them in other regions during the dry season, when their habitats may nearly completely dry.


These small clams are exclusively filter feeders, which means they obtain all of their nutrition by pulling in water through one of their siphons, filtering the organic matter, and releasing the “cleaned” water through a second siphon. This second siphon is also where they excrete their bodily waste.


Fingernail Clams are hermaphrodites, meaning they have both male and female sexual organs. They can also self-fertilize. Fertilized eggs will develop inside the body of the mother clam, and will be born looking like miniature copies of the adults. Each batch of young can range in size from 2 to 20 individuals. The range in batch size comes from the way young clams compete for space within their mother/father’s shell. The oldest and largest young will out compete their siblings, filling up the space.

In the Aquarium

Clams are a somewhat risky venture in the aquarium hobby. They can be easy to keep, if things go well. When things go bad, they can go very bad.

For starters, clams are filter feeds, which means they need a special diet of suspended organic particles in the water. Saltwater hobbyists who keep corals are familiar with the dietary demands of filter feeders. It won’t be enough simply to sprinkle some extra Tetra flakes into the water. Many use tubes and pipettes to blow specialized foods into the siphons of larger clam species.

If you’re a hobbyist who strives for a pristine aquarium environment, clams may not be the best choice. Clams are best kept in aquariums that aren’t “too clean.” Hobbyists who keep heavily planted tanks will have an easier time feeding their clams. Those that use the Walstad Method should have an even easier time. The more organic bits you have suspended in the water column, the happier your clams will be. They also prefer deep substrates, where they can securely bury themselves.

“So what’s that danger you were alluding to earlier?” Good question. The real problem with keeping clams in an aquarium is when they pass away. Either from lack of food or old age, a dead clam can be a disaster for your tank. The problem comes from the preference of clams to live buried in the substrate. If a large clam were to die while buried, it may be quite difficult to locate and remove. A rotting animal in your tank can trigger an ammonia spike, which comes with a whole host of other problems.

While the minute size of Fingernail Clams may reduce the severity of this problem, you are not completely out of danger. If conditions are good, you could end up with a large population of these clams in your substrate. In the wild, Fingernail Clams can number as many as 10,000 individuals in one square meter of sediment. If something were to cause some or all of them to die at once, it would be almost impossible to clear them out of your substrate without tearing the whole thing apart.

A solution that many people use with larger species is to place them within a small, porous container of sand or other fine sediment. Then that container can be buried in your typical aquarium gravel. This keeps the clam in one place, and makes it easier for you to feed and care for it. It also makes it much easier to locate and remove the clam, if it were to pass away.

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Let me know what you think about this species profile.

Featured image credit: Wisconsin Department of Natural Resources. Source: creative commons

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