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 suggest 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.