Muscle fiber plasticity, stress physiology, and muscle transcriptome determine the inter-individual difference of swimming performance in the large yellow croaker (Larimichthys crocea)

Swimming performance is extremely important to critical activities of fishes, such as foraging, anti-predator, and migration. As one of primary stock enhancement fish species in China, it is imperative to study on swimming performance of the large yellow croaker (Larimichthys crocea). In the present study, the swimming performance of juvenile large yellow croaker was investigated, including the behavior, muscle morphology, stress physiology, and genetic mechanisms underlying the individual difference. Juveniles of the croaker were individually sub-jected to the critical swimming speed (Ucrit) test, and thereby divided into fast (36.47 cm/s) and slow (18.65 cm/ s) groups. The juveniles in the fast group had significantly higher tail beat frequency, smaller muscle fiber diameter and more muscle fiber number, as well as lower plasma cortisol and corticotropin releasing hormone (CRH) levels than those in the slow group. Meanwhile, expression levels of hsp70 and pcna in the brain were significantly higher in the fast group than in the slow group, however no significant difference was observed in syt1a expression. Furthermore, the key genes and pathways involved in the swimming performance were assessed by mapping the white skeletal muscle transcriptome. We found that a total of 157 transcripts was differentially expressed between the fast and slow groups, enriched in oxidative phosphorylation, glycosami-noglycan biosynthesis, nicotinate and nicotinamide metabolism, cellular senescence, and cardiac muscle contraction pathways. A multitude of mitochondrial NADH dehydrogenase subunits and solute carrier family genes were especially related. In conclusion, we for the first time found that fish swimming performance directly links to muscle fiber plasticity, stress physiology, and molecular base divergence in the large yellow croaker. The present results lay a foundation for ethological-character-based selective breeding for the large yellow croaker in the regards of swimming performance.