Variable development and excystment of freshwater pearl mussel (Margaritifera margaritifera L.) at constant temperature

The highly endangered freshwater pearl mussel (Margaritifera margaritifera L.) has strongly declined throughout Europe and is a priority species in aquatic conservation. The complex life cycle of M. margaritifera includes an obligate development phase of glochidia larvae on a suitable host fish. Knowledge on the progression of the parasitic phase and on the factors governing excystment of juvenile mussels are particularly crucial for artificial breeding and conservation measures. The core objective of this study was to study excystment of M. margaritifera after maintaining the infested hosts under constant water temperatures between 11 and 12 °C and to determine the sum of daily water temperatures (day degrees) required by M. margaritifera for completion of metamorphosis. In a standardized laboratory experiment, excystment of juvenile mussels from brown trout (Salmo trutta) was found between 1700 and 3400 day degrees post infestation, indicating highly variable development times of individual glochidia and the absence of a previously postulated threshold temperature of ≥15 °C for successful excystment of living juveniles. Consequently, the parasitic phase does not seem to limit the current distribution range and reintroduction of the species into cool headwater areas, as well as the culturing under constant water temperature conditions in typical salmonid fish hatchery setups. The concept of day degrees of development may also be useful to test the ecological implications of observed genetic differences among different populations.