Hippichthys heptagonus Raw sequence reads. Hippichthys heptagonus

This study aimed to describe the adaptation to salinity changes in freshwater pipefish (Hippichthys heptagonus) embryos and juveniles using transcriptome data. Embryos exposed to 30 ppt salinity showed downregulated cell cycle-associated genes, which play an important role in embryonic development. In addition, retinol metabolism, neuroactive receptor interaction, and peroxisome proliferator-activated receptor signaling pathways were significantly upregulated in the embryos. Interestingly, there was no change in the expression of ion transport and energy metabolism genes. In contrast, juveniles exposed to 30 ppt salinity showed upregulated ion transport genes and immune-related signaling pathways, including lysosomes, phagosomes, autophagy, and mitophagy. Additionally, carbohydrate metabolism genes were upregulated, whereas oxidative phosphorylation genes were significantly downregulated. We believe that our study makes a significant contribution to the literature because it shows that brood pouch protection in the embryo phase and salinity adaptation in juveniles might be a strategic adaptation to freshwater adaptation in Hippichthys heptagonus.