Walk Softly – Freshwater Mussels

by Geoffrey Carpentier

I remember, as a kid, spending countless summer hours fishing and enjoying nature. One thing I recall encountering, with some frequency, were the dark black mussel shells which littered the lake and river bottom, once the living organism inside had died. They were simply a curiosity to me, and a hindrance at the time, as I often cut my foot on their sharp edges. Many years later, as I revisit some of my childhood memories, I now wonder how these animals live, why there are so many shells left behind and are they an important part of our ecosystems?

Freshwater clams, more correctly called mussels, inconspicuously spend most of their lives buried in the mud and sand, at the bottoms of rivers, streams and lakes. Yet, they are among the most important animals in freshwater ecosystems. Most people are familiar with mussels from the seashore, as they formed a part of our coastal adventures and were honoured in whimsical songs (i.e. in the unofficial Irish anthem Molly Malone, where she offers “Cockles and mussels, alive, alive, oh”).

However, freshwater clams belong to a different group which has adapted to a freshwater life. Their hard shells protect a soft body, equipped with a muscular "foot" which allows them to slowly burrow through the bottom sediments. Although they seem immobile, they can quickly move surprising distances, especially when searching for suitable habitat.

Filter feeding, a single adult clam can pump litres of water through its body every day. As water passes over its gills, the clam removes tiny particles, such as algae, bacteria, and organic matter, while taking in oxygen. In effect, a healthy population of clams acts as a natural water treatment system, by keeping waterways cleaner and clearer, while benefiting fish, aquatic plants and countless other organisms. They are also excellent indicators of environmental health. Since they are long-lived and remain in one place, they cannot easily escape pollution or habitat disturbance. If water quality declines, clams disappear, long before the effects become obvious. In many ways, they serve as living report cards for freshwater ecosystems.

Unlike marine mussels, many freshwater species depend on fish to complete their development. After fertilization, the female releases microscopic larvae, called glochidia, which cannot survive on their own but must attach themselves temporarily to the gills or fins of a suitable fish, causing no harm to the fish in the process. Eventually, they drop off onto the stream bed, where they begin life independently. Many female clams have developed elaborate ways of attracting the preferred fish host, with some displaying colourful fleshy extensions which closely resemble small minnows, worms, or aquatic insects. When a curious fish strikes at what appears to be an easy meal, the clam releases its larvae at exactly the right moment. It is one of nature's more ingenious examples of deception!

Unfortunately, freshwater clams are among the most endangered animals, due to habitat destruction, dams, pollution, invasive species and climate change. Excess nutrients from agriculture can reduce oxygen levels in the water, while sediment washed in from eroded land can smother young clams.

Conversely, in North America, the Zebra Mussel has become a particularly serious threat. Unlike native freshwater clams, zebra mussels attach themselves directly to the shells of living clams, often in overwhelming numbers. Burdened by these ‘hitchhikers’, native clams struggle to move, feed or reproduce, and many eventually die.

When the animals die, empty shells are evident, each one a record of an animal which likely lived for several decades. Some species can survive for 50 years or more!

Ontario has one of the richest freshwater mussel faunas in Canada, with 41 native species recorded in the province. Native freshwater mussels occur throughout Durham Region (e.g. Duffins Creek, Oshawa Creek and Wilmot Creek) and the City of Kawartha Lakes (e.g. Trent–Severn Waterway, the Scugog River, Pigeon Lake, Sturgeon Lake, Balsam Lake and Cameron Lake).

So, the next time you wander along the shores of one of our beautiful creeks, rivers, or lakes look down and you might see these busy little creatures cleaning the water right before your very eyes!

Geoff Carpentier is a published author, expedition guide and environmental consultant. Visit Geoff on-line at www.avocetnatureservices.com and on LinkedIn and Facebook.