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 also 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 Calanus helgolandicus with an incubation experiment, comparing survival, behavior, and microbiomes of uninfected and infected hosts. Infected hosts suffered higher mortality, and altered behavior which can increase predation risk. The microbiomes differed between infected and uninfected hosts, likely caused by a combination of direct effects of infection, and environmental effects driven by parasite‐induced behavioral change. We identified several potential contributing taxa to the Calanus ‐YHP pathobiome using model‐based ordination.