By Courtney Beneteau
You’ve probably heard of the zebra mussel, but have you ever heard of the purple wartyback, giant floater or round pigtoe? These are only a few of the interesting (and often fitting) names given to the 41 species of native freshwater mussels in Ontario, as opposed to the zebra mussel which is non-native.
Almost 70% of these freshwater mussels are showing signs of decline, and are, in fact, among the most endangered organisms in North America. This may be due in part to our ignorance of their very existence. If you’ve ever waded in a southwestern Ontario stream or river, chances are you’ve stepped on a freshwater mussel – and didn’t even know it!
What makes these organisms so fascinating is their unique life cycle. Adult mussels dig themselves into the streambed and take in and exhale water through siphons. This is how they filter their food out of the water, and how they reproduce. When male mussels are sexually mature they exhale their sperm into the water column to be taken in by downstream females to fertilize their eggs. Eventually, the fertilized eggs mature into glochidia while still contained in the mother’s gills.
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When conditions are favorable, the female mussel will release her glochidia into the water, where they attach themselves to the gills, fins and scales of nearby fish. This begins the parasitic phase of the mussel life cycle. The glochidia live on their “host” fish until they mature into microscopic juvenile mussels and drop off to start their sedentary life on the streambed.
Many species of mussel have evolved to grow on only one or two fish species. Therefore, if the host fish population declines, so will the dependent mussel species.
In order to lure fish and increase the chances that her glochidia will attach to the correct fish, the females of many mussel species have developed enlarged and specially-coloured mantles (this is the edge of the mussel where the two shells meet when closed). These specialized mantles can be bright orange, red or even resemble a fish or crawfish. The female mussel will display and systematically beat her mantle outside of the shell to attract fish. When a curious fish comes in to investigate, some mussels will actually clamp down on the snout of the fish to keep it close while releasing her glochidia.
These mussels are a very important part of aquatic ecosystems because they filter and clean the water and also provide food for fish, birds and mammals. Increasingly, aquatic habitat assessments are including not only fish and benthic invertebrates as indicators of stream health, but freshwater mussels as well.
Freshwater mussels are considered “fish” according to the Fisheries Act (Canada) and, as such, warrant the same protection. Like fish, increased suspended sediment in the water column resulting from in-water work or work around watercourses, can have detrimental impacts to mussels, especially because of their limited mobility. Projects that have the potential to impact mussel species-at-risk require a permit from the Department of Fisheries and Oceans (Canada) and the Ontario Ministry of Natural Resources and Forestry, depending on the status.
So next time you find yourself near a stream, river, lake or pond, think about the wealth of biodiversity camouflaged amidst the rocks.
Courtney Beneteau, M.Sc., is a fisheries biologist with Parsons. This article appeared in ES&E’s July/August 2015 issue.
I’m regularly wading through the Thames and Sydenham river and occasionally feel mussels mussels underfoot. I’d like to know if any volunteer groups are doing mussel counts? Butterfly and bird counts are very popular in our parks. For some, a mussel count could be just the thing for cooling off on a hot lazy summer afternoon. Safety first of course.
That’s a very interesting comment, not something I had ever thought of before. I’m are not aware of any volunteer groups doing mussel counts. There are a couple potential complications which come to mind… Not all mussels present in an area are visible from the surface – if the substrate allows, and every winter, they bury themselves completely. Protocol for mussel detection calls for excavation to a depth of 10 cm. Also, I wonder if a course in mussel handling would be required for anyone wanting to pull them from the streambed? I suggest this because they have a definite ‘upside’ or side that needs to be positioned upward so that they can open their siphons to feed, reproduce, etc., and a mussel has to expend quite a bit of energy to turn itself around.