Viral Community Profiling of <scp>RNA</scp> Viruses in Lesion Tissues From <scp> <i>Hyriopsis cumingii</i> </scp> With Epidemic Disease via Metatranscriptomics and <scp>VirID</scp> ‐Based <scp>RdRP</scp> Mining

To identify enriched pathogens and characterise the viral community associated with epidemic disease outbreaks in the freshwater mussel Hyriopsis cumingii, we performed metatranscriptomic sequencing combined with VirID-driven RNA-dependent RNA polymerase (RdRP) mining and phylogenetic analysis using hepatopancreas and intestinal samples from six severely infected individuals. Clinical observations were consistent with hallmark features of epidemic outbreaks. The sequencing yielded 86.2 Gb of raw data, of which 97.1% passed quality control, resulting in 77.7 Gb of high-quality clean data. Taxonomic annotation identified 182 viral species, predominantly unclassified viruses (45% Transcripts Per Million, TPM), followed by members of the phyla Lenarviricota (28%) and Uroviricota (17%). Phylogenetic analysis of RdRP sequences revealed 13 viral supergroups, with the Picorna-Calici supergroup showing the highest abundance (26.2% of annotated viruses) and reaching a prevalence of 39.3% in sample HcAV3. Notably, 89.6% of the identified viral RdRPs exhibited less than 70% amino acid identity to known viral sequences, highlighting the presence of extensive "viral dark matter" in this host species. This study establishes the first viral profile associated with epidemic disease in H. cumingii, providing a baseline for further etiological research on this high-mortality aquaculture disease.