The histological and molecular response of acute hypoxia and reoxygenation in rainbow trout and steelhead.

Hypoxia is a critical environmental stressor affecting fish survival and physiological function. Landlocked rainbow trout (Oncorhynchus mykiss) and anadromous steelhead (Oncorhynchus mykiss), two ecotypes of the same species, exhibit distinct physiological adaptations to fluctuating oxygen levels. To investigate their responses to acute hypoxia in these fish, brain histology and transcriptome analyses were conducted before stress, after hypoxia (12 and 24 h), and after reoxygenation (24 h). The results showed that the number of vacuolated nerve cells in the brain increased with the duration of hypoxia and decreased after 24 h of reoxygenation in both fish. RNA-Seq analysis identified 8409 differentially expressed genes (3952 in rainbow trout and 4457 in steelhead), one hypoxia-related module (MEred), and two significantly expressed gene clusters (cluster 3 in rainbow trout and cluster 8 in steelhead) through differential expression analysis, weighted gene co-expression network analysis (WGCNA), and Mfuzz clustering. KEGG pathway enrichment analysis revealed that signal transduction, immune response, angiogenesis, and apoptosis were significantly influenced by hypoxia in both fish. Distinct hypoxic responses were observed: in rainbow trout, ferroptosis, necroptosis, phosphatidylinositol signaling, calcium, and apelin signaling pathways were enriched, whereas in steelhead, the enriched pathways included platelet activation, Rap1, PI3K-Akt, Hippo, relaxin, and oxytocin signaling. Steelhead display marginally greater hypoxia tolerance than rainbow trout, although the difference is minimal, likely owing to their classification as the same species. These findings provide insight into the physiological and molecular mechanisms underlying hypoxia and reoxygenation stress in salmonids, contributing to a broader understanding of oxygen homeostasis in fish.