Seagrass Wasting Disease and the Causative Agent, Labyrinthula zosterae: Detection, Quantification, and Potential Mitigation

Abstract

Eelgrass (Zostera marina) is an ecologically important seagrass providing numerous ecosystem services to coastal habitats globally. Z. marina is currently experiencing universal decline due to multiple factors, including degrading environmental conditions and disease. One disease commonly found in Z. marina beds is seagrass wasting disease (SWD) caused by the opportunistic pathogen Labyrinthula zosterae. SWD has previously caused mass die-offs of seagrass beds. Seagrass beds are well-surveyed in the Chesapeake Bay but SWD presence and prevalence remains unknown in the region. In this present study, seagrass beds were sampled in the Chesapeake Bay for SWD presence, lesion severity, and infection intensity. SWD and L. zosterae were present at four sites with one site having significantly higher disease severity and infection intensity. An undertested disease mitigation strategy is the use of farmed and naturally occurring oysters in wild populations, near eelgrass beds. The Pacific oyster, Crassostrea gigas, has been previously found to reduce and transmit SWD in Z. marina during laboratory trials. However, the density threshold of C. gigas needed to significantly reduce SWD is still unknown. Previous lab experiments also used higher doses of L. zosterae compared to recently determined environmentally relevant levels. Thus, understanding the impact of oyster densities on SWD at previously tested pathogen concentrations and more environmentally realistic concentrations is essential for a better understanding of the dynamics between disease, host, and a nonhost receptor. A mesocosm experiment was conducted with Z. marina co-cultured with two densities of oysters and exposed to two concentrations of pathogen, measuring their effect on blade growth, lesion severity, and infection intensity. High oyster density impeded growth while Z. marina growth benefited from low oyster densities. High pathogen concentrations increased SWD severity and L. zosterae infection intensity. High oyster densities significantly reduced the likelihood of L. zosterae infection presence. Results from this experiment confirmed that bivalves can reduce SWD under environmentally realistic pathogen concentrations and paves the way for integrated disease management. Further field testing is required to validate in-situ oyster filtration as a viable method for SWD mitigation.