Physiological responses and adaptive strategies to acute low-salinity environmental stress of the euryhaline marine fish black seabream (Acanthopagrus schlegelii)

Abstract

The physiological metabolism of marine animals farmed in coastal areas is influenced greatly by extreme weather and frequent aqueous layer exchange, with short or long periods of heavy rain reducing seawater salinity. Black seabream (Acanthopagrus schlegelii) is a euryhaline fish, that are farmed in China under both normal/ambient salinity in seawater cages and low salinity conditions in pond culture. To explore the effects of an acute change in salinity on physiological metabolism, juvenile black seabream (weight 19.80 ± 0.37 g) was subjected to low salinity (4–5 psu) over five time points including 1 h (H1), 6 h (H6), 12 h (H12), 24 h (H24) and one week (W), with 0 h (initial, natural salinity of 22 psu) as control group. Results indicated that acute exposure to low-salinity resulted in various responses related to oxidative stress (OS) and endoplasmic reticulum stress (ERS). The key markers of OS including reactive oxygen species and malonaldehyde were significantly increased with acute salinity stress up to 6 h, then decreased to return to normal levels. A similar pattern was recorded for total antioxidant capacity, which showed significantly higher activity in the H6 group. All parameters related to ERS increased with duration of the low-salinity stress, which stimulated expression of key genes including nuclear factor kappa B (nf-κb) and c-Jun N-terminal kinase (jnk), triggering inflammation and apoptosis. However, total apoptotic cell rate decreased markedly as the duration of salinity stress increased up to one week. Gill expression of genes of long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis was up-regulated by low salinity stress with levels of docosahexaenoic (DHA), eicosapentaenoic (EPA) and arachidonic acids (ARA) being highest in the H12 group, which positively contributed to enhanced ability of osmoregulation. In conclusion, acute low-salinity stress stimulated various stress responses but, as the duration of salinity stress increased, the fish showed signs of physiological adaptation to the environmental change. During the adaptive process to maintain hemolymph osmotic pressure regulation, LC-PUFA were rapidly enriched in gill, which might be a crucial strategy allowing A. schlegelii to adapt to low-salinity stress. These findings provided further insight and understanding of the physiological responses and underlying adaptive strategy of euryhaline fish to salinity stress, and confirmed time-dependent effects.