Invertebrate herbivores influence seagrass wasting disease dynamics

Abstract

Although invertebrate herbivores commonly impact terrestrial plant diseasesby facilitating transmission of plant pathogens and increasing host susceptibil-ity to infection via wounding, less is known about the role of herbivores inmarine plant disease dynamics. Importantly, transmission via herbivores maynot be required in the ocean since saline ocean waters support pathogen sur-vival and transmission. Through laboratory experiments with eelgrass (Zosteramarina), we showed that isopods (Pentidotea wosnesenskii) and snails (Lacunaspp.) created grazing scars that increased disease severity and thus indirectlyfacilitated transmission of Labyrinthula zosterae (Lz), a protist that causesseagrass wasting disease. Experiments also quantified different feeding prefer-ences among herbivores: Amphipods (Ampithoe lacertosa) selectively con-sumed diseased eelgrass, while isopods and snails selectively grazedasymptomatic leaves, suggesting different herbivore taxa may have contrastingimpacts on disease dynamics. Our experiments show no sign that herbivoresdirectly vector Lz from diseased to asymptomatic eelgrass. However, we iso-lated live Lz from isopod, amphipod, and snail feces and detected Lz withquantitative polymerase chain reaction in amphipods and snails, suggestingthat herbivores eating diseased eelgrass could pass the live pathogen. Finally,field surveys demonstrated a close association between seagrass wasting dis-ease and invertebrate grazing scars; disease prevalence was 29 ± 4.7% (95% CI)higher on eelgrass leaves with herbivore scars. Collectively, these findingsshow that some herbivores can increase eelgrass disease risk by facilitating thespread of an important pathogen via wounding, but not via direct transmis-sion. Thus, herbivores may play different roles in plant disease dynamics interrestrial versus marine ecosystems depending on the pathogen’s ability tosurvive and transmit without a vector.