Effects of water temperature on the larval parasitic stage of the thick-shelled river mussel (Unio crassus)

The complex life-cycle of freshwater mussels from the superfamily Unionoidea includes an obligate parasitic phase on a suitable host fish, which makes freshwater mussels particularly susceptible to disruption by environmental stressors. In particular, temperature stress due to rising ambient temperatures resulting from heated effluents, damming of streams and deforestation of banks or climate change may have detrimental effects on threatened mussel species. In this study, the effect of four different temperature regimes (12°C, 17°C, 20°C and 23°C) on the development and metamorphosis success of Unio crassus encysted on Phoxinus phoxinus was investigated. The highest metamorphosis success as well as the lowest host mortality were found at a temperature of 17°C. Duration of development was inversely related to the temperature at which the hosts were maintained. Excystment of living juvenile mussels was found between 265 and 357 degree-days for 23°C and between 433 and 632 degree-days for 12°C. In conclusion, water temperature during the parasitic phase of U. crassus plays an important role in development and metamorphosis and should be considered in the conservation of the species through catchment management and in the artificial propagation of this species as well as in other endangered mussel species.