While catching tadpoles in the pond across the street from me, I’d occasionally net what looked to be a small tan pebble. Closer examination revealed something different though. Fingernail Clams, also called Pill Clams or Pea Clams, are members of a family of small to tiny freshwater clams that are common, but rarely noticed.
Like other clams, Fingernail Clams have two mirror-image shells that are hinged together that are generally rounded to triangular in shape. Their shells are thin and fragile, often translucent, the outsides typically shiny, fairly smooth, and gray, tan, yellowish, or whitish. Fine growth lines are often visible, but they are not strongly raised, ridged, or rough.
Fingernail Clams are found in a variety of aquatic habitats, usually in places with mucky or sandy substrates, and usually at or just under the bottom substrate. Some species of fingernail clams are adapted to live in vernal pools — temporary ponds, usually in woodlands, that fill with water in Springtime, then dry up during Summer. As the pool dries, the Fingernail Clam burrows down several inches into the soil, where it is moist enough for it to survive until the next Spring.
Like other clams, scallops, mussels, oysters, and most other bivalve mollusks, they are filter feeders that remove suspended organic materials from water, such as algae, tiny planktonic animals, and plant and animal detritus.
They have tubelike siphons that draw in water and tiny, drifting food particles into their shell cavity. A pair of flaplike, mucous-covered gills collects the food and channels it into the animal’s mouth.
Like its relatives, the Fingernail Clam has a foot, which is used to dig down into the pond bottom where it can burrow. Fingernail Clams (like most other mollusks) are hermaphrodites: each individual can produce both eggs and sperm. Fingernail clams are able to fertilize their own eggs.
Apparently Fingernail Clams can move from one isolated vernal pool to another by clamping on to a finger or toe of a salamander, frog, or other animal. Since they can fertilize their own eggs, only one individual can start a population.