Diversity of Ascetosporeans Associated With Feather Hydroids in New Caledonia.

Ascetosporea is a group of parasitic protists within Endomyxa that primarily infect marine invertebrates and can cause serious damage to fishery resources such as oysters and prawns. Although approximately 50 species have been formally described, environmental DNA (eDNA) studies suggest that their diversity remains largely underestimated, with many lineages yet to be discovered. Their ecological roles and host ranges are also poorly understood, partly because no complete life cycle has been elucidated, likely due to the difficulty of maintaining live cells in laboratory conditions. In this study, we investigated feather hydroids as potential hosts of ascetosporeans in New Caledonia, where this group has not been previously examined. Using metabarcoding with ascetosporean-specific primers, we detected 46 operational taxonomic units (OTUs) from three hydroid individuals. Of these, 31 OTUs were affiliated with environmental clades ENDO-3a and ENDO-2, whereas two and four OTUs were assigned to Paradinida and Haplosporida, respectively. Notably, seven OTUs were consistently detected across all samples, suggesting that they may have been broadly associated with the feather hydroids in this environment. These findings indicate that ascetosporean diversity and host associations in marine ecosystems are substantially underestimated.