Marine herbivores facilitate transmission of a seagrass pathogen

Herbivores commonly vector terrestrial plant pathogens, though their role in transmitting marine plant pathogens remains unknown. Here we show that amphipods ( Ampithoe lactertosa ) consumed more eelgrass ( Zostera marina ) infected with the protist Labyrinthula zosterae (Lz), the causative agent of seagrass wasting disease, then asymptomatic control leaves. Isopods ( Pentidoea wosnesenskii ) and snails ( Lacuna spp.) grazed more asymptomatic leaves. We isolated Lz from herbivore feces, indicating that herbivores that eat diseased eelgrass can pass the live pathogen; qPCR detected Lz in herbivores. Experiments demonstrated isopods and snails indirectly facilitated pathogen transmission by creating grazing scars that increased disease, but did not directly transmit Lz from diseased to asymptomatic eelgrass. Field surveys demonstrated a close association between disease and herbivore grazing scars. Disease risk increased by 29% on grazed leaves. This is the first report of herbivores facilitating the spread—and magnifying the impacts—of a marine plant pathogen.