The survival and structural changes in gill of juvenile Eleutheronema tetradactylum under different salinities

【Objective】In order to explore the characteristics of the survival and gill histological structure of Eleutheronema tetradactylum juveniles cultured at different salinities, to provide effective technical support and theoretical reference for the production of E. tetradactylum.【Method】Five salinity groups of 0, 5‰, 10‰(control), 15‰ and 20‰were set up in the experiment. The juvenile E. tetradactylum temporarily raised in 10‰ salinity water was directly put into other salinity water bodies. The living gill tissue was sampled on the 15 thday of the experiment. The histological structure of gill of E. tetradactylum juveniles under different salinities were observed by using the histological methods.【Result】Under 0 salinity condition, E. tetradactylum juveniles all died within 12 h, at 5‰ salinity condition, the mortality rate was 6.6%, no death in the control group, the mortality rates of 15‰ and 20‰ salinity were 33.3% and 60.0% respectively.Compared with the high salt environment, E. tetradactylum juveniles was more adaptable to low-salt environment and could not survive in freshwater environment. The microstructure observation results showed that, at 10‰ salinity condition, the gill lamella of E. tetradactylum juveniles closely arranged closely, mitochondria-rich cells were distributed at the base of the gill patch. With the increase of salinity, the length and spacing of gill lamellae decreased, while the width of gill lamellae increased. At 5‰ salinity condition, the length, width and the intervals of gill lamella were 45.19±3.18 μm, 5.32±1.02 μm and 17.65±1.43 μm respectively. At 20‰ salinity condition, the length, width and the intervals of gill lamella were 36.67±3.81 μm, 8.78±2.22 μm and 11.56±3.81 μm respectively, the differences were all significant(P<0.05).At 20‰ salinity condition, mitochondrial rich cells could be seen at the distal end of the gill patch and the pavement epithelial cells appeared to be detached or separated. The surface scanning of the gills showed the mitochondrial rich cells could be divided into two subtypes:subtype Ⅰ(embolic apical crypts) and subtype Ⅱ(shallow apical crypts). Under 5‰salinity condition, there were two types of mitochondria-rich cells. Most of which were type Ⅱ, the apical membrane had a large opening and there were a large number of microvilli on the surface. The surface of pavement epithelial cells was uplifted by the microridge, forming a fingerprint-like circuit. Under 10‰ salinity condition, mitochondria-rich cells showed type Ⅰ structure, the microridge on the surface of pavement epithelial cells was irregular and the intercellular boundary distribution was not obvious. Under 15‰ and 20‰ salinity conditions, mitochondrial rich cells were dominated by type Ⅰ, with the change of salinity, the apical crypts gradually deepened, and the number of mitochondria-rich cells increased. The surface of the pavement epithelial cells had small ridges and the fingerprint-like circuit was not obvious.【Conclusion】When the water osmotic pressure changes, the survival rate and the number distribution and morphology of the cells on the gills of juvenile E. tetradactylum, the histological structure of gill are affected by environment salinity.