Factors Affecting Fertilization Success in Freshwater Mussels,
an article summary by Daniel L. Graf, BIOL 490 Fall 2014

Mosley et al. (2014) examined the effects of male density and proximity to females, as well as flow velocity, on the fertilization success of freshwater mussels. Freshwater mussels (Bivalvia: Unionidae) are perhaps best known for their imperiled conservation status. These mollusks also have a complex life cycle involving “spermcasting” by males and fertilization within the mantle cavities of the females. The authors were motivated by a concern for Allee effects on mussel populations. That is, that low densities in shrinking populations could lead to reduced fertilization efficiency and faster rates of decline. Mosley et al. (2014) used Lampsilis straminea in an artificial river simulating the mussels’ natural habitat. Females were placed 1, 10, and 25 m downstream of males, and experimental treatments varied the presence/absence of flow, male density, and the presence/absence of upstream females (i.e., 25 m only). All experiments ran for at least three months during the breeding period of L. straminea, and two-factor ANOVA was used to analyze the data. Neither presence of flow nor fertilization distance had significant effects on the proportion of females that had eggs fertilized, the proportion of eggs that were fertilized, or individual fecundity. However, both male density and the presence of females upstream (intercepting sperm) generally impacted fertilization success, although the magnitude and direction of effects varied. More research is needed over a broader range of these independent variables to determine the critical densities at which Allee effects might impact freshwater mussel populations.

Mosley, T.L., W.R. Haag & J.A. Stoeckel. 2014. Egg fertilisation in a freshwater mussel: effects of distance, flow and male density. Freshwater Biology 59: 2137-2149.