Fish welfare: MS-222 sedation in juvenile Xenocypris davidi transport, dose screening, and exploratory gill transcriptomic profiling

Transport stress remains a significant challenge in aquaculture. To optimize sedation protocols, we evaluated MS-222 (tricaine methanesulfonate) for juvenile Xenocypris davidi by combining behavioral dose screening (32–50 mg L⁻¹) with gill transcriptomics and histology analysis. At 10 °C, behavioral screening identified 34 mg L⁻¹ as the minimum dose that induced stage A2 (deep sedation) without prolonging recovery. Following an 8-h simulated transport at this concentration, comparative gill transcriptomics revealed 1,220 differentially expressed genes between MS-222-sedated and unsedated control fish. The MS-222 group exhibited enriched glutathione metabolism and a concurrent reduction in transcripts associated with inflammatory signaling. Additionally, DNA repair and cell cycle pathways were positively enriched, whereas cell death, ion transport, and tight junction modules showed bidirectional modulation, suggesting complex tissue remodeling. Qualitatively, the MS-222 group showed a more regular lamellar arrangement. In conclusion, these behavioral and transcriptomic findings suggest that 34 mg L⁻¹ may serve as a potential sedation protocol for X. davidi transport at 10 °C. Furthermore, the molecular findings identify candidate gill pathways associated with MS-222 sedation, generating novel hypotheses for future unstressed-baseline studies.