When ponds are fertile, mussels aren’t

Posted on December 16, 2014

Freshwater mussels help to maintain water quality by filtering suspended particles and reducing concentrations of heavy metals, pesticides and other pollutants. Unfortunately, however, freshwater mussels are experiencing dramatic declines around the world. It’s been suggested that heightened levels of suspended solids, associated with agriculture and urban development, are responsible for the decline, but the mechanism has remained unclear. To see how mussels functioned at different turbidities, researchers added different amounts of NPK fertilizer to a series of 0.1 hectare ponds to create environments with a range of suspended solid concentrations of up to 50 mg/l. Experimental chambers containing pondmussels (Ligumia subrostrata) were placed in the ponds to measure filtration rates over a seven-month period. Mussel reproduction relies on male animals releasing sperm into the water, and on females capturing sperm on their gills in the process of filter feeding. Fertilisation then occurs within the female. Therefore, the researchers used a syringe to extract fluid from the gills of female mussels on a regular basis to check for the presence of eggs and larvae. The results of the experiment showed that rates of mussel growth and fecundity were unrelated to levels of suspended solids, probably because the food supply wasn’t limited in any of the ponds. In contrast, suspended solids had a strong negative impact on reproduction: at levels of 10 mg/l, almost all females contained fertilized eggs, but there was complete reproductive failure above 20 mg/l. And above 8-15 mg/l, the filtration rate of mussels fell sharply. So it appears that, when the concentration of suspended solids (and therefore the energy intake) is high, mussels respond by lowering their pumping rate, which reduces the chance that females collect and fertilise suspended sperm. These findings show that mussel reproduction can be compromised even with relatively modest increases in suspended solids caused by human impacts.

Reference: Gascho Landis, A.M. et al. 2013. High suspended solids as a factor in reproductive failure of a freshwater mussel.
Freshwater Science 32(1), 70–81. http://www.bioone.org/doi/pdf/10.1899/12-093.1