Counterproductive: coinfection of a water flea by a fungus and a microsporidium reduces the reproductive outputs of all parties.

Organisms are often coinfected by more than one parasite species. These coinfections can alter the fitness of both host and its coinfecting parasites. Changes in host fitness are usually estimated by comparing parasite virulence under coinfection to the virulence of each parasite under single infection. While many studies focus on host survival as proxies of virulence, virulence can be expressed via reduced host fecundity. Here, we studied the outcome of coinfection of water fleas (Daphnia magna) by two microparasitic species, the fungus Metschnikowia bicuspidata and the microsporidium Hamiltosporidium tvaerminnensis. We found that Metschnikowia expressed its virulence mainly via host mortality and Hamiltosporidium expressed it mainly via host reproduction. Despite their competitive interaction, indicated by their reduced spore production, both parasites were able to fully express their virulence during coinfection. Host survival was dominated by Metschnikowia while clutch-size was dominated by Hamiltosporidium. Furthermore, coinfected hosts experienced increased virulence that was manifested only by additional reduction in reproduction (less clutches were released). Our results demonstrate that focusing only on the survival component of host fitness can miss important outcomes of coinfections. Since reduced host fecundity is a common outcome of parasitism, the influence of coinfection on host reproduction deserves more attention.