Abstract
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, LD50 (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.
