Spawning in a threatened freshwater mussel shifts to earlier dates as a result of increasing summer mortality.

Freshwater mussels are among the most imperilled groups of animals on the globe, however, the drivers of mussel declines are still poorly understood. Here we show that in a seasonal environment, Unio crassus females can initiate spawning from late winter to late summer, implying a very high phenotypic plasticity of their spawning date. However, they shift their reproductive effort to earlier dates and make greater investments in early broods in order to adapt to high spring temperatures and higher levels of summer mortality. Thus, the reproductive investment during early spring may credit energy to be invested later in self-maintenance, in order to alleviate the impact of mortality. As a result, mussel reproduction is being progressively compressed into earlier and shorter periods. Such constraints must reduce reproductive output and contribute to the decline of this already endangered group. The seasonal pattern of reproductive effort can always serve as an excellent indicator of the conservation status of a given population.