Seawater adaptability, osmoregulatory function, and branchial chloride cells were compared be tween 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, in dicating 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 sig nificantly increased 1 day and 2 days after the transfer while that of the S3-SR3 strain did not significant ly 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 func tion.
The present study examined a factor in seawater which stimulates salinity tolerance and the num ber and size of branchial chloride cells in the guppy. Salinity tolerance was measured as LD50 24 h after transfer to serial concentrations of salinity. LD50 significantly increased from 25.8 ppt to 43.3 ppt 5 days after exposure to 15 ppt dilute seawater. Thus, increases of salinity tolerance were examined on the basis of 5 days' exposure to various solutions which were contained within 15 ppt dilute seawater. LD50 significantly increased in 0.180 M NaCl and 0.023 M MgCl2 but not in 0.012 M Na2SO4, 0.004 M CaCl2, or 0.004 M KCl. During 5 days' exposure to 0.090 M MgCl2, 0,090 M Na2SO4, 0.090 M CaCl2, or 0.180 M KCl which contain the same Na+ or Cl concentrations as 0.180 M NaCl, all guppies died in 0.180 M KCl. Using mixtures of NaCl and KCl, it was revealed that more than 0.020 M K+ caused death of the guppies. LD50 did not change after exposure to 0.090 M Na2SO4 which contains the same Na+ concentra tion as 0.180 M NaCl but significantly increased after exposure to 0.090 M MgCl2 and 0.090 M CaCl2 which contain the same Cl concentration as 0.180 M NaCl. The number and size of chloride cells did not change in 0.090 M Na2SO4 but significantly increased in 0.180 M NaCl, 0.090 M MgCl2, and 0.090 M CaCl2. These results suggested that environmental Cl stimulates salinity tolerance and the number and size of branchial chloride cells in the guppy.
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