Got Mussels? Understanding Why Turbid Rivers Have More Bivalve Shellfish

The Eastern Pond Mussel (photo by the Smithsonian Environmental Research Centre, CC BY 2.0)

Does size matter? Aquatic ecologists want to know if the size of suspended particles can explain the mystery of why mussels, a type of bivalve shellfish, thrive in turbid rivers.

Unionid mussels are native freshwater mussels that are suspension (filter) feeders. They are sometimes called pearl mussels because the inside of their shell is pearl-like, and they were once popular with the button industry. This group of mussels used to be widespread in both lakes and rivers across North America but are now highly imperilled.

“Unionids are largely vanished from the Great Lakes, in part because of the zebra mussel invasion. Now they are mostly just found in rivers,” says Shaylah Tuttle-Raycraft, a mussel enthusiast and recent graduate student in the Department of Integrative Biology.

Theoretically, unionids should survive best in clear waterbodies. This is because in cloudy water, mussels presumably need to spend more time cleaning their gills, resulting in less time for feeding. But it turns out that this long-held theory may not measure up.

“In Ontario, our largest and most diverse population of unionid mussels is in a turbid river that has a high level of suspended solids,” explains Tuttle-Raycraft. This unexpected phenomenon has researchers like Tuttle-Raycraft and Prof. Joe Ackerman trying to explain why the unionids are thriving in this sub-optimal environment.

The most logical theory is that this phenomenon can be explained by the size of the particles in the turbid rivers – in other words, they are too small to affect the mussel’s rate of feeding. However, Tuttle-Raycraft and Ackerman have challenged the so-called “size theory” with their recent publication demonstrating that the quality of particles may be much more important.

The pair conducted laboratory feeding experiments with four unionid species collected from different rivers in Southern Ontario. Each species was exposed to four sediment types, ranging from clay (0-5 micrometers in diameter) to fine silt (5-38 micrometers) and coarse silt (38-63 micrometers), as well as a mixed sediment treatment with all sizes – each type was supplied at a concentration known to reduce mussel feeding. To ensure the unionids were exposed to as natural a feeding environment as possible, researchers used sediment obtained directly from the rivers where the unionids were collected. Green algae were added to each sediment fraction to ensure that the unionids had a high quality food source. The unionids in each treatment were then observed to see how particle size affected their overall rate of feeding.

Tuttle-Raycraft and Ackerman initially predicted that the smallest particles wouldn’t affect mussel feeding. To their surprise, however, it was the medium sized particles that had no negative impact on feeding.  This begged the question:  if particle size doesn’t affect mussel feeding, then what does?

The duo decided to dig deeper and examine the nutritional quality of the different sized particle fractions by measuring their organic, algal, protein and lipid content. This analysis revealed that the medium particles (those 5-38 micrometers in size) were in fact the most nutritious, containing the most algal particles and the highest levels of proteins and lipids.  The bottom line? Quality of the particle has a greater impact on unionid mussel feeding than the size of the particle.

The results of this study will help direct conservation efforts for unionid mussels, as well as future reintroduction and relocation studies.

“Globally, 70% of unionid mussels have some kind of conservation concern, which I think is a strong motivating factor to look at their habitat,” explains Tuttle-Raycraft.

The Ackerman lab continues to study the feeding ecology of unionid mussels in rivers to better understand their ecology and assist in their conservation.

“Somebody once told me that ‘healthy rivers have mussels’. If mussels are that important, then we need to understand what affects them – because whatever affects mussels is probably affecting other organisms, too,” says Tuttle-Raycraft.

This study was funded by the Ontario Ministry of Natural Resources and Forestry, Fisheries and Oceans Canada and the Natural Sciences and Engineering Research Council of Canada.

Read the full study in the journal Freshwater Biology.

Read about other CBS Research Highlights.