An environmental factor stimulating salinity tolerance and branchial chloride cells in a euryhaline teleost Poecilia reticulata

The present study examined a factor in seawater which stimulates salinity tolerance and the number 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 Naf or CI-concentrations as 0.180 M NaCl, all guppies died in 0.180 M KCl. Using mixtures of NaCl and KCI, 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+ concentration as 0.180 M NaCl but significantly increased after exposure to 0.090 M MgCl2 and 0.090 M CaCl2 which contain the same CI-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.