Tiny Clams That Thrive in Dry Ponds

by Shan Leung and James Darling

The Herrington’s fingernail clam (Sphaerium occidentale) is one of the smallest bivalves in Canada. At only 7 mm long, it is smaller than—you guessed it—a fingernail!

It is also the only bivalve in Canada that lives exclusively in vernal pools: small, temporary bodies of water that form in the spring and dry out by mid- or late summer. While they last, vernal pools harbour a number of amphibious and dry-resistant species, including tadpoles, salamanders, insect larvae, fairy shrimp and water fleas.

This pool forms every spring and disappears every fall.

This pool forms every spring and disappears every fall. It is located on the museum’s property in Gatineau, Quebec, in the sector of Aylmer. Image: André Martel © Canadian Museum of Nature

This year, as summer students, we are putting on our rubber boots and bug jackets to collect these tiny clams. We’re working at the museum’s research and collections facility, on the 76 hectare property. Joining us are Noel Alfonso, an ichthyologist and head of the Environmental Monitoring Program at the Canadian Museum of Nature, and André Martel, a leading Canadian malacologist.

By sectioning the vernal pools into quadrats from which to sample, the team will estimate population density on the property. Preliminary results suggest there may be as many as 200 of these tiny animals per square metre of pond!

Noel Alfonso (left), Shan Leung (middle), and James Darling (right) take a look at their collection.

Noel Alfonso (left), Shan Leung (middle), and James Darling (right) take a look at their collection. Image: André Martel © Canadian Museum of Nature

Living in vernal pools presents significant challenges for the Herrington’s fingernail clam.

First, it must survive when its habitat dries out each year. To this end, the Herrington’s fingernail clam is the most amphibious bivalve in Canada, spending most of the year locked in dry land (although the mud in which it buries itself may remain damp for the rest of the summer).

Second of all, new generations of the clam must somehow disperse if it is to colonize new pools. How the clam does this is something of a mystery. Here’s a possible explanation: it takes to the skies.

The Herrington’s fingernail clam may not have wings but its tiny larvae are able to hitch a ride on passing Wood Ducks, other waterfowl, or even large flying insects to new pools where they can colonize. Once there, a single larva can mature and, once an adult, reproduce clonally (a process called parthenogenesis) to colonize the entire pool.

Herrington's fingernail clam (Sphaerium occidentale) at five different ages. Individuals can live up to a few years, overwintering several times during this period.

Herrington’s fingernail clam (Sphaerium occidentale) at five different ages. Individuals can live up to a few years, overwintering several times during this period. Image: André Martel © Canadian Museum of Nature

Despite the challenges presented by life in vernal pools, such a lifestyle also presents one momentous advantage. Temporary bodies of water in temperate areas do not sustain fish populations, which are the major predators of these bivalve molluscs. By spending their entire lives in vernal pools, fingernail clams have found a special kind of sanctuary, safe from their erstwhile predators.

Our team is looking forward to uncovering more about these tiny clams over the summer!

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