Zooplankton diversity and their potential role in transmission of the bivalve parasite Marteilia pararefringens in Norwegian oyster polls

Marteilia pararefringens is a potentially lethal parasite of mussels Mytilus spp., listed by the World Organisation for Animal Health (WOAH) due to its impact on wild and cultivated populations. The life cycle of M. pararefringens is not known, hindering effective disease management. In Norway, M. pararefringens was first discovered in 2016 and has almost exclusively been detected in polls - small, semi-closed, warm estuarine pools where high parasite abundance is common. Polls are unique ecosystems, but remain relatively poorly studied, especially when it comes to zooplankton diversity. The presence of M. pararefringens in polls represent a unique opportunity to study both the parasite's life cycle in a natural system as well as provide comprehensive data on the diversity of the zooplankton communities. In this study, we used traditional stereomicroscopy together with high-throughput DNA metabarcoding to resolve zooplankton community structure. This combined methodology successfully overcame identification challenges associated with cryptic taxa and larval stages typical of estuarine environments, confirming polls unique species composition but low diversity. Following this, targeted Marteilia PCR on taxonomically sorted samples from 27 sites identified several candidate intermediate hosts of M. pararefringens. The copepods Paracartia grani and Acartia margalefi were the most frequently PCR-positive species for the parasite. Prevalence and abundance of P. grani was associated with presence of M. pararefringens across the sites. Parasite DNA was also detected in sorted samples of Oithona nana and planktonic larvae of benthic decapods (Athanas nitescens and Carcinus maenas) and Musculus subpictus. These findings provide the first evidence of candidate intermediate hosts of M. pararefringens in its northernmost range, and will act as a baseline for further studies on this parasite's elusive life cycle.