From expression profiling to functional validation: evidence hierarchy and biomarker readiness of circular RNAs in aquatic animals

Circular RNAs (circRNAs) are covalently closed non-coding RNA molecules that are mainly generated from precursor mRNAs through back-splicing. Owing to the lack of free 5' and 3' ends, circRNAs exhibit high stability and pronounced spatiotemporal expression specificity. In aquatic animals, recent advances in high-throughput sequencing and bioinformatic analysis have greatly expanded the catalog of circRNAs and uncovered their regulatory roles in growth, gonadal development, tissue regeneration, immune defense, and environmental stress responses. This review provides a systematic overview of circRNA biogenesis, representative molecular functions, identification strategies, and bioinformatic resources in aquatic animals, with particular emphasis on circRNA-mediated regulation during bacterial and viral infection, antiparasitic defense, trained immunity, and responses to physicochemical stressors. We further summarize the current evidence supporting the potential of circRNAs as candidate biomarkers for economically important traits, infectious disease monitoring, and ecotoxicological assessment. Importantly, we highlight the major bottlenecks that currently limit progress in this field, including incomplete genome annotation, algorithm-dependent identification bias, and the lack of robust in vivo functional validation. Overall, aquatic animals provide valuable non-model systems for expanding our understanding of circRNA biology, although many proposed regulatory mechanisms and translational applications remain to be further validated.