Abstract
European sea bass (Dicentrarchus labrax), a major aquaculture species, is distributed along the coasts of the North-Eastern Atlantic Ocean, Mediterranean and Black Sea. D. labrax enter lagoons and estuaries where salinity fluctuates and sometimes reaches levels over 60%o, notably in Mediterranean lagoons. Keeping in mind that European sea bass are genetically subdivided in an Atlantic and a Mediterranean lineage, we compared fish from Atlantic (A) and West Mediterranean (M) populations regarding their capacity to tolerate hypersalinity with a focus on the kidney, a key organ involved in water reabsorption at high salinity. Fish were analyzed following a two-week transfer from seawater (SW, 36%o) to either seawater (SW, 36%o) or hypersaline water (HW, 55%o). Plasma osmolality was significantly increased in the MHW group compared to the other groups. Plasma sodium levels were significantly increased in hypersaline water compared to seawater in both lineages whereas plasma chloride levels showed an opposite trend. In order to estimate water filtration at the kidney level, the size of renal glomeruli was investigated and showed a decreased glomerulus perimeter and area in hypersaline water compared to seawater. NKA was highly expressed in all kidney tubules notably collecting tubules and ducts. There was an effect of salinity on renal nka alpha 1a mRNA expression with slightly lower transcript levels at 55%o compared to 36%o. Relative protein amounts and activity of NKA however were significantly higher in fish exposed to hypersalinity regardless of their origin. AQP1a immunolabeling differed between proximal tubules subtypes and only faint AQP1a was detected in subapical parts of cells lining collecting ducts. The transcript levels of renal aqp 1a were lower in the HW group than the SW group whereas the expression of other aqp paralogs (aqp 1b, aqp 8b) did not change according to the analyzed conditions. This study showed an efficient acclimation of sea bass to high salinity by increasing active ion transport at the kidney and by decreasing the size of filtering glomeruli to minimize water loss through urine. Despite Mediterranean D. labrax are supposed to more often encounter high salinities in their habitat, their high blood osmolality in hypersaline water indicates that their overall response to hypersalinity seems not improved compared to the Atlantic lineage. However, at the kidney level, the traits analyzed differ slightly between genetic lineages, potentially as a response to high blood osmolalities in MHW.