Nature Nugget: This Bivalve Thrives in Seasonal Woodland Pools

Fingernail clams thrive in vernal pools.

Exploring vernal pools

Ever notice a pool of water in the woods that disappears by summer? These seasonal puddles, called vernal pools, form in small depressions (generally less than half an acre) following rain and snow melt, and evaporate by summer. There are thousands of vernal pools in Vermont, and they support all sorts of wildlife. Populations of fingernail clams, fairy shrimp, snails, water fleas, and copepods are typical. Frogs and salamanders use these seasonal wetlands as breeding grounds in early spring, laying their eggs there because there are no fish to eat them.

You’ll find little or no vegetation in vernal pools, but the surrounding forest is part of what makes these wetlands such important habitat. The tree canopy usually reaches over the pool (because it is so small). This keeps the water shaded and cool, preventing evaporation during mating season. The forest also provides habitat for the amphibians, which spend most of adulthood within 500 feet of the pools.

Meet the fingernail clam

What has two shells, one foot, and lives in the forest? You guessed it: the fingernail clam. This tiny clam, named because it is the size of a child’s fingernail, also indicates vernal pool sites.

While clams aren’t usually associated with the woods, this freshwater bivalve is related to the clams, oysters, mussels, and scallops that you’d find in the ocean. They’re all bivalves, or aquatic creatures whose bodies are protected by two shells hinged together. Like other bivalves, the fingernail clam has a foot, or a retractable muscle that helps it to move around or burrow. 

This vernal pool has 3 characteristic pool species - fingernail clam, wood frog tadpole and caddis fly larva.

In this photo are three species characteristic of vernal pools: the fingernail clam, wood frog tadpole, and caddisfly larva. Can you spot them all?

How do fingernail clams survive Vermont winters?

To survive both winter and the seasonal drying that happens in summer, fingernail clams will burrow up to eight inches into the muck—more than 16 times their body length. There, they become inactive until autumn rains refill the pools. Then they survive on algae and leaf litter. 

Fingernail clams can be found in other temporary wetlands, but the bedrock that forms the pools must be rich in calcium so they can build their shells. Additionally, the water cannot be too acidic, or their shells will dissolve. 

[Related: learn how fingernail clams end up in these woodland sites.]