Effects of reduced salinity on survival, growth, reproductive success, and energetics of the euryhaline polychaete Capitella sp I

Physiological adjustment to water of reduced salinity requires energy expenditure. In this study we sought to determine the fitness costs associated with such adjustment in the euryhaline polychaete Capitella sp. I, and the extent to which such costs could be explained by increased rates of energy expenditure. In a series of experiments conducted at 20 degreesC, salinity was reduced from 30 parts per thousand to either 25, 20, 15, 12, or 10 parts per thousand within 72 h after the larvae had been induced to metamorphose. Juveniles were reared on line, organic-rich sediment. Over the next 15-30 days, we determined survival, growth, fecundity, and rates of respiration and feeding (via fecal pellet production). Larval salinity tolerance was also determined. Juvenile survival at salinities as low as 12-15 parts per thousand was comparable to that at 30 parts per thousand. The lower limit of salinity tolerance was 10-12 parts per thousand at 20 degreesC for both larvae and juveniles. Juveniles grew significantly more slowly at 12-15 parts per thousand in six of the seven experiments. Fecundity, however, was generally highest at intermediate salinities of 20-25 parts per thousand, and comparable at 30 and 15 parts per thousand. No individuals released embryos at 12 parts per thousand over the approximately 30-day observation periods in any of the three experiments in which the worms were reared at this low salinity. Reduced growth rates were not explained by differences in rates of respiration at different salinities: at reduced salinity, respiration rates were either statistically equivalent to (P > 0.10) or significantly below (P < 0.05) those recorded for animals maintained at 30. Lower growth rates at lower salinities were best explained by reduced feeding rates. Further studies are required tc, determine whether digestive efficiency, growth hormone concentrations, or reproductive hormone concentrations are also altered by low salinity in this species. (C) 2000 Elsevier science B.V. All rights reserved.