Nucleic acid indices of egg production in the tropical copepod Acartia sinjiensis

Two experiments were conducted to evaluate the effects of food and temperature on the nucleic acid content and egg production (and their relationship) of the tropical copepod Acartia sinjiensis. Experiment 1 evaluated the effect of food quality (as different algae species) on the relationship between nucleic acid content and egg production. In Experiment 2, the main and interaction effects of food type, food concentration and temperature on the total, Carbon and Nitrogen specific egg production and nucleic acid indices were evaluated in a factorial experimental design. Food quality, concentration and temperature significantly affected the nucleic acid content, egg production rate and the nucleic acid–egg production relationship of A. sinjiensis. RNA indices were correlated with egg production in females fed Pavlova salina, Tetraselmis chuii and Chaetoceros muelleri, but not in females fed Isochrysis aff. galbana. The slopes of the linear regressions of RNA indices as predictors of egg production were similar in females fed different algae species, suggesting that the slope of these relationships might be independent of food quality. The DNA content of females was significantly affected by food and temperature, suggesting that it is not a good index of cell number in this species. Nevertheless, the RNA:DNA ratio was as good a predictor of egg production as total RNA content. Egg production showed a weakly positive correlation with temperature. On the other hand, total, C- and N-specific nucleic acid indices had a strong negative correlation with temperature. In addition, temperature had a non linear effect on the slopes of the regression lines of RNA content and RNA:DNA ratio as predictors of egg production—slopes were similar at 25 °C and 30 °C, but significantly lower at 20 °C. Furthermore, the predictability of egg production was improved when the interaction term of nucleic acid indices with temperature was used instead of the nucleic acid indices alone in linear regression models. Our results suggest food quality has a limited influence on the nucleic acid–egg production relationship, and that temperature should be accounted for in models using nucleic acid indices as predictors of egg production in A. sinjiensis.