The genetics of phenotypic plasticity in the adaptation of freshwater colonization in anadromous fish can provide important insights into the generating of teleosts diversity and invasion niche. Here, we conducted a comparative population genomic analysis of two lineages of tapertail anchovy (Coilia nasus), the anadromous and the lake-resident populations, which complete their lifecycle independently in freshwater lakes. We tracked C. nasus during their migratory period and compared genome-wide genotyping of the freshwater population with physiological traits that colonization contributes to osmotic adaptation. Through genome scans between anadromous and freshwater populations identified osmoregulation and immunoregulatory pathways evolving by strong natural selection, whereas association mapping revealed loci that contribute to freshwater adaptation. Our findings suggest that patterns of divergence in physiological capabilities are consistent with parallel adaptation to local niches. Notably, a lot of cortisol-regulated genes (e.g., ATP1α1) appeared to mutate and dynamics expression in different forms, which enriched in the ion channels, implying that the reproductive phenomenon of osmoregulation in anadromous is critical for the adaptive divergence of freshwater environments. Taken together, these results enhance our understanding of the genetic architecture variation of teleosts adaptation to novel environments, enabling cross osmotic boundaries, which leads to population divergence and freshwater colonization.

Ecological stoichiometry is a scientific discipline that studies the balance of energy and multiple chemical elements in biological systems, and it has been widely applied in fish research. However, there are few studies on fishes with different reproductive modes, such as Coilia, which experience habitat shifts during their life history and serve as important ecological links between marine and freshwater ecosystems. To address this gap, we compared the ecological chemical elements of four Coilia species (C. nasus, C. brachygnathus, C. mystus and C. grayii) to understand their organismal stoichiometry pattern in terms of changes in C, N, and P. The result shows, 1) The C content of Coilia had a wide range and was higher than the general fish, ranging from 40% -70%, while N% and P% were 4%-12% and 0.5%-3%, respectively. 2) Diadromous populations showed wider variations of elemental stoichiometry during migration than non-migration populations, with a higher coefficient of variation of elements and C:N:P than other Coilia species. In particular, the C content of diadromous Coilia nasus was the highest (>60% in the East China Sea) when the fish was ready for spawning migration, while N and P were opposites. Diadromous fish might have evolved a mechanism to store as much lipid as possible in their bodies to meet the energy demand during their long migration journey; 3) There were certain similarities between the biochemical composition and elemental changes. The trends of total fat and C content curves were consistent, with anadromous Coilia nasus storing a large amount of fat before migration to meet their energy needs during long-distance migration. The variation trends of crude protein and N content curve were similar to some degree, with crude protein and total fat having opposite trends, meaning that increased with a decrease in total fat, and vice versa