Parasite infection shapes the pathobiome and behavior of marine zooplankton

When assessing the total impact of disease in a host it is important to consider not only the disease-carrying agent, but all symbionts, as they affect and are affected by the course of disease. This concept of a pathobiome is increasingly recognized in disease ecology, but is not well-investigated in natural systems. Copepods are key organisms in marine ecosystems and host a variety of symbionts, including bacteria and eukaryotic parasites. We investigated the impact of a taxonomically uncertain yellow-hyphal parasite (YHP) on its copepod host \textit{Calanus helgolandicus} with an incubation experiment, comparing survival, behavior and microbiomes of healthy and infected hosts. Infected host had a higher mortality, and altered behavior which can increase predation risk. The microbiomes differed between infected and uninfected hosts, and we identified several potential contributing taxa to the \textit{Calanus}-YHP pathobiome using model-based ordination. The pathobiome taxa found could grow opportunistically in the presence of infection, while some taxa associated with healthy copepods could protect against toxins, parasite infection and epibiotic growth on the copepod surface. We propose that infection can shape the animal pathobiome in natural systems through the interaction between host and parasite physiologies.