Seawater adaptability, osmoregulatory function, and branchial chloride cells in the strain selected for high salinity tolerance of the guppy Poecilia reticulata

Seawater adaptability, osmoregulatory function, and branchial chloride cells were compared between the original strain (S3) and the selected strain for high salinity tolerance (S3-SR3) of the guppy. Survival time in 35 ppt seawater was significantly longer in the S3-SR3 strain than in the S3 strain, indicating the higher salinity tolerance of the S3-SR3 strain. When fish were acclimated to 15 ppt dilute seawater, LD sub(50)(ppt) of the S3 and S3-SR3 strains gradually and significantly increased with increase in the acclimation periods in parallel, and the S3-SR3 strain acquired seawater adaptability earlier than the S3 strain. Blood osmotic pressure after direct transfer to 15 ppt dilute seawater of the S3 strain significantly increased 1 day and 2 days after the transfer while that of the S3-SR3 strain did not significantly increase throughout the experiment. Branchial chloride cells of the S3-SR3 strain increased about 50% in number and about 15% in size in comparison with those of the S3 strain in fresh water. These results suggest that genetic increase in the number and size of branchial chloride cells in the S3-SR3 strain caused the improvements of salinity tolerance, seawater adaptability, and osmoregulatory function.