MEPS 470:15-29 (2012)  -  DOI: https://doi.org/10.3354/meps09907

ABSTRACT: Mortality, respiration rate, and body mass density of the estuarine copepods Calanipeda aquaedulcis and Arctodiaptomus salinus and their eggs were studied over a wide range of salinities. Empirical hydrodynamic modeling was applied to calculate body mass density using the sinking speed of the copepods. According to the index of median lethal salinity (LS₅₀), the salinity tolerance ranges of C. aquaedulcis and A. salinus reared at 18 psu were 0.1 to 50 and 0.1 to 35 psu, respectively. No significant effect of salinity on the specific oxygen consumption rate was found in the range from 0.1 to 40 psu in both species, while mean body mass density increased iso-osmotically with water salinity from (mean ± SD) 1.053 ± 0.007 to 1.077 ± 0.005 g cm⁻³ in C. aquaedulcis and from 1.039 ± 0.004 to 1.062 ± 0.007 g cm⁻³ in A. salinus. However, body density contrast and sinking speed were stable in both species, allowing them to avoid buoyancy problems with changing salinity. The mean mass density of resting eggs in A. salinus gradually increased iso-osmotically following gradual changes in salinity. In contrast to resting eggs, the mass density of subitaneous eggs was not altered by changes in the surrounding salinity, suggesting a hysteresis response in these eggs.

KEY WORDS: Calanipeda aquaedulcis · Arctodiaptomus salinus · Salinity tolerance · Respiration rate · Body mass density