Microplastics and early life stages of freshwater mussels: What we know to date

To date, much of what we know about how microplastics interact with animals in lakes and streams rests with testing zooplankton and fish. Much less is known about how benthic organisms interact with microplastics. Benthic organisms are those that live at or on the bottom section of a waterbody, often burrowing in the sediment to find food or simply to position themselves right at the sediment-water interface. An important benthic organism that is under-represented in microplastic research to date is the freshwater mussel, specifically species found here in Southern Ontario that are part of the Unionidae family (unionids).

A petri dish of juvenile mussels

Unionids are important players in whichever waterbody they inhabit, since they filter overlying water and ensure that the sediment is re-worked so that other aquatic invertebrates can easily live in the sediment. They are also highly valued since they have proven to be very sensitive to a variety of anthropogenic contaminants. Unfortunately, because of this, they are one of the most imperiled groups of aquatic organisms in North America. Their unique life cycle, which is dependent on the maturation of a parasitic larva (glochidium) on the gill of a fish host, also reinforces their dependence on a fully functioning ecosystem. To understand the full impacts that microplastics may have on freshwater mussels, we looked at the effects that multiple polymer types and sizes may have on three distinct unionid life stages.

The first round of testing focused on the earliest life stage, the glochidium.

Glochidia exposed to various types of microplastics

By exposing these glochidia to different types of microplastics including polystyrene and polyethylene spheres and polyester fibers, we found that there was no significant effect on their survival following an acute (short-term) test. These effects were seen when using glochidia from both Lampsilis siliquoidea (one of the most far-reaching mussels in Canada), and Lampsilis fasciola (a mussel current listed as special concern under the Species at Risk Act).

Example of an experimental set-up where juvenile mussels were exposed to microplastics through their food

Next, we exposed juvenile L. siliquoidea mussels which were approximately 1-2 years old in sub-chronic (slightly longer term) tests. Here we found that microplastics did not affect mussel survival, growth or their ability to bury in the sand. What remains to be seen is at what rate juvenile mussels took up the microplastics that were in their food, and how quickly these same mussels are able to clear the plastics from their system. Preliminary data shows that they are very efficient at clearing (depurating) polystyrene spheres. Future work will focus on determining the depuration capability for other microplastics tested, to better characterize the risk that microplastics may pose to these early life stages.