Infectivity is influenced by parasite spore age and exposure to freezing: do shallow waters provide Daphnia a refuge from some parasites?

Parasites impact ecological and evolutionary dynamics of Daphnia populations, their interactions with other food web members, and even ecosystem-level processes. Thus, there is interest in understanding factors that influence the infectivity and yield of transmission stages (spores) of common Daphnia parasites. We tested the impact of spore age (from 1 day to 1 year) and storage temperature (-20 degrees C vs. 4 degrees C) on infectivity and spore yield using two common parasites: the bacterium Pasteuria ramosa and the fungus Metschnikowia bicuspidata. Pasteuria spores remained infective even after 1 year at both temperatures, though spore yields dropped over time. In contrast, Metschnikowia spores were killed within days at -20 degrees C, indicating that they are intolerant of freezing. Previous studies of lake populations of Daphnia have found that Metschnikowia prevalence can be high, whereas studies of ponds and rock pools have found low prevalence. The high sensitivity of Metschnikowia to freezing might help explain this pattern. Our study also found differences in spore survival at 4 degrees C: while Pasteuria spores remained highly infective even after a year, Metschnikowia spores were no longer infective after 4 months. This finding suggests that Pasteuria spores might remain infective for longer in sediment spore banks in stratified lakes.