When this summer’s drought left the riverbed dry, it afforded the opportunity to look for freshwater mussels. Members of the Phylum Mollusca, mussels are related to snails, slugs, clams and oysters, and even octopuses. Mussels are sometimes called living filters. They play an important role in aquatic ecosystems by cleaning the water. They also provide food for assorted fish and wildlife such as raccoons.
There are 41 native species of mussels in Ontario. Of these, 28 species are in decline or threatened with extinction. Mussels are among the most endangered organisms in North America, threatened by many human activities from pollution to habitat destruction.
Perhaps the most severe threat to the native mussel population has been the introduction of the zebra mussel, an invasive species. Zebra mussels attach themselves to the shells of native mussels by the hundreds or even thousands, causing them to die from lack of oxygen or food. Native mussels have been nearly eliminated from much of the Great Lakes system and St. Lawrence river, as well as watersheds where zebra mussels have been introduced.
Freshwater mussels are the largest and longest-living freshwater invertebrates in North America. Their life spans can reach many decades. They occupy a wide variety of habitats, from small streams to lakes, but have their greatest diversity in large rivers, which supply a constant supply of oxygen and food and a variety of habitat types.
Mussels spend their lives buried in the substrate of their aquatic home. They feed by drawing water in through a siphon and passing it across gills to filter out small particles of algae and bacteria. The reproductive cycle of freshwater mussels is amazingly complex.
During spawning, males release sperm into the water and females living downstream take in the sperm through their siphons. Eggs are fertilized in a specialized portion of the female’s gills called marsupia. Embryos remain in the gills until they have reached a larval stage called glochidium.
When conditions are right, depending on temperature, photoperiod and time of year, the female mussel releases her glochidia into the water where they must quickly attach themselves to the gills or fins of an appropriate fish host. The glochidia then become encysted in the tissues of the host fish and get nourishment from its body fluids for a time ranging from a week to over 6 months.
They transform into juvenile mussels during this parasitic phase. Once metamorphosis is complete, the juvenile ruptures the cyst and falls to the river bottom, where it burrows into the mud and remains for the next few years. Most mussel species have only a few specific host species and the chances of a glochidiium surviving are low. Mussels produce millions of glochidia to improve the odds of some reaching adulthood.
I found evidence of 3 species in our local riverbed. Some of the Eastern Floater shells were quite large and sturdy, while the single Eastern Lampmussel I found was small and more fragile, a real beauty with striking green rays.
Mussels are an example of the astounding lives lived by so many creatures to which most of us are oblivious. Perhaps if Canadians had a greater awareness of the wonderful richness and diversity of life that surrounds us, and how little we know and understand it, they might be less apathetic regarding cuts to scientific research and the protection of waterways.