The personality of Japanese flounder (Paralichthys olivaceus) and gene expression related with osmoregulatory capacity in the gills

Personality is defined as consistency in an individual's behavioral responses towards a given situation over time. Previous studies reported the presence of bold personality (BP) and shy personality (SP) in the olive flounder (Paralichthys olivaceus). Links between personality and the stress response have been studied in teleost fish; however, the response of SP and BP flounder to salinity stress is unknown. In this study, salinity tolerance of larvae at 25 days after hatch (DAH) and juvenile (50 DAH) flounders was investigated at salinities of 3, 5, 25, 45 and 50 parts per thousand (ppt). On 25 DAH, at salinities of 5 and 25 ppt, both SP and BP flounder keep 80% survival rate, while BP flounder larvae had higher survival rates at salinity of 45 ppt when compared with SP flounder. On 50 DAH, salinity tolerance of SP and BP flounder greatly improved, with 70% survival at salinitie of 3, 5 and 25 ppt but low survival rates (35%) at 50 ppt. At 45 ppt, BP exhibited slightly higher survival rate (80%) than SP flounder (60%) after a 96 h challenge. In addition, juvenile BP flounder (8 months) had lower plasma cortisol level, were more motivated to feed and showed more active responses during net confinement. Furthermore, Na+/K+/ATPase α1 (NKAα1) mRNA expression showed no significant difference between SP and BP flounder in SW, but was up-regulated after transfer to FW in SP and BP flounder. BP flounder expressed higher levels of gill Na+/K+/2Cl− cotransporter (NKCC1) mRNA than SP flounder in seawater (SW), while there were no differences between groups after transfer to freshwater (FW). Taken together, BP flounder had a low plasma cortisol level and an elevated appetite, as well as hyper-osmoregulatory ability. Selection for BP flounder in aquaculture is potentially important to improve animal welfare and increase production efficiency.