Freshwater Clams Filterfeeders That Cleanse the Waterways and Provide Food for Native Fish and Mammals

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Freshwater clams are filter-feeding mollusks that as part of the natural biologic process clean the water, consuming the free-floating phytoplankton that exists in the waterways. In this way, they are purifying the water and keeping it clean.

By keeping the water clearer, they allow sunlight to penetrate down deep and this in turn allow for aquatic plants to take hold and grow. River-bottom plants help to stabilize the river bottom and oxygenate and purify the water too. The clams themselves in their great numbers form a community called a ‘bed’ that actually help grip the river bottom secure and provide stability to the river-bottom, averting water erosion.

While not as palatable to eat for humans as the marine clam, they are edible. Mostly, animals such as fish, turtles and wild mammals use them as a major food source. Native American Indians and First Nations used freshwater clams for food, tools, and decorative ornamentation.

Fresh water clams are quite hardy but they can hardly withstand the onslaught of pollution, silting and in recent years they have become threatened by the invasive species Zebra Mussel.

Invasive Specie Zebra Mussels

In the Great Lakes region and tributaries, Zebra Mussels have arrived and they are in direct competition with the native clams for the same resources. The more prolific Zebra Mussels cause the death of native fresh water clams by over-competing with them for available phytoplankton and by their insidious attaching mechanisms. Also in the favor of Zebra Mussels is the lack of natural predators to keep their numbers in balance.

In another study by the Great Lakes Science Center, it seems to suggest that Zebra Mussels aren’t the threat to native clams that other sources have cited, claiming that in one study only about 1% of the 7000 native clams harvested for the study showed any measure of Zebra Mussels colonization.

Zebra Mussel over-population is insidious at any rate. Their population blooms and they glue themselves to any underwater surface available, often clinging to the very shells of native clams and drowning them. Their numbers allegedly causes the local species to not be able to open their bi-valve to feed, circulate water and thus to breathe.

If you have ever peered into a lake infested with Zebra Mussels you will notice two things right away. First, is that the water is very clear. Zebra Mussels will strip the phytoplankton from the water quite effectively and the lack of natural predators allows their numbers to quickly blossom out of control. Second, a Zebra Mussel infested body of water will have the shells of both the live and dead littered everywhere, attached in prolific numbers to any structure, plant or manmade object. What is not disputed is that Zebra Mussels attach themselves to water intake pipes and reduce the volume needed for municipal and agricultural use, causing the need for increased maintenance. The creation of super-slippery surface coatings on intake valves may make it difficult or impossible for Zebra Mussels to attach, thus potentially solving at least this problem.

Some years ago I walked along the beaches near Muskegon, Michigan, along the shores of Lake Superior and was taken at first by the cleanliness of the sand and clarity of the water. Upon closer evaluation, I noticed thousands upon thousands of black and white shells and shards litter along the waterline, many inches deep in places. I was told that native fresh water clams in the region were rare and had been so for some years in recent times. People blame this on the invasive Zebra Mussel.

Freshwater clams have lived for over 300 million years, but in recent times their existence is threatened like never before. Habitat loss due to logging and other industry on the rivers, water pollution from agricultural run-off and of course, Zebra Mussel invasions are among the main culprits. Overharvesting by man in the early part of this century contributed to this decline too.

Clam Shell Buttons

In the early 1900s there were over 200 button factories harvesting freshwater clams for the creation of buttons for clothiers. The Great Lakes Science Center in Ann Arbor, Michigan states that “Shell (the commodity) was so valuable that harvesters were known to use cannons to blow competitors' boats out of the water." The competition was that intense, the rewards that grand that it led to some 60,000 tons of freshwater clams being harvested annually.

Freshwater clams and mussels can have very long life spans. Some types can live for decades, other are thought to live for at least 100 years or more. For them, they saw a period before we came along and threatened their very existence.

Some species of freshwater clams such as the Rainbow clam in order to disperse their progeny have evolved a most peculiar method; they attract a fish to their open bi-valve with a lure resembling a worm, insect or other cetacean. Upon clamping down on head of the inquisitive fish, the female clam ejects her eggs (called “glochidia”) some of which cling to the host fishes’ gills to drop-off later and burrow into the mud to begin growing.

The overall health of the water is absolutely required for everything to work just right. Agricultural pollution, damning of the rivers, silt build-up from logging and runoff can totally disrupt this and other natural processes. Some clam and mussel beds still have thousands of live but aged specimens. They are not reproducing and going on for decades. Something is definitely wrong.

The ‘population-zero’ mussel beds are just one major catastrophe away from extinction in the local bed. One oil tanker spill, one municipal waste run-off incident, etc., the entire bed will be lost. If you find a clam or mussel bed with both adult and young specimens, the health of the bed is acceptable. They are reproducing. This is a good sign. Enjoy and protect this increasingly rare natural resource.


carol roach
Posted on Feb 5, 2010
Alma Galvez
Posted on Feb 3, 2010