Mysterious microsporidians: springtime outbreaks of disease in Daphnia communities in shallow pond ecosystems.

Parasites can play key roles in ecosystems, especially when they infect common hosts that play important ecological roles. Daphnia are critical grazers in many lentic freshwater ecosystems and typically reach peak densities in early spring. Daphnia have also become prominent model host organisms for the field of disease ecology, although most well-studied parasites infect them in summer or fall. Here, we report field patterns of virulent microsporidian parasites that consistently infect Daphnia in springtime, in a set of seven shallow ponds in Georgia, USA, sampled every 3-4 weeks for 18 months. We detected two distinct parasite taxa, closely matching sequences of Pseudoberwaldia daphniae and Conglomerata obtusa, both infecting all three resident species of Daphnia: D. ambigua, D. laevis, and D. parvula. To our knowledge, neither parasite has been previously reported in any of these host species or anywhere in North America. Infection prevalence peaked consistently in February-May, but the severity of these outbreaks differed substantially among ponds. Moreover, host species differed markedly in terms of their maximum infection prevalence (5% [D. parvula] to 72% [D. laevis]), mean reduction of fecundity when infected (70.6% [D. ambigua] to 99.8% [D. laevis]), mean spore yield (62,000 [D. parvula] to 377,000 [D. laevis] per host), and likelihood of being infected by each parasite. The timing and severity of the outbreaks suggests that these parasites could be impactful members of these shallow freshwater ecosystems, and that the strength of their effects is likely to hinge on the composition of ponds' zooplankton communities.