Morphometric variables and individual volume of Eurytemora americana and Acartia tonsa females (Copepoda, Calanoida) from the Bahía Blanca estuary, Argentina

Abstract

Eurytemora americana and Acartia tonsa are the two most important copepods from the Bahia Blanca estuary plankton. For this study, 30 females were selected from recently preserved samples. Prosome length (PL), width (PW) and height (PH), urosome length (UL) and width (UW) measured in each female and subsequently the following ratios PL/PW, PL/PH, PW/PH and PL/UL were determined. Individual volumes for E. americana and A. tonsa females were estimated following the morphometric method in order to obtain individual biomass values. Regression tests were conducted for both species between individual volume and prosome length or width as independent variables, fitting data to a power model. Regression lines were compared through covariance analysis and single pair comparisons in order to evaluate the temporal behavior of the relationships. PL/UL was the best ratio for separating one species from the other. Average individual volume and std. error estimated for E. americana and A. tonsa were 0.0749 (±0.0097) mm3 and 0.0399 (±0.0049) mm3, respectively. Strong differences between both copepods morphometric variable and rate values were observed. A size and volume decrease was detected associated with higher temperatures and less phytoplankton available for food. According to R2 values of regression lines prosome width and prosome length were good volume predictors on different dates for E. americana, however, prosome width was the best predictor on the majority of dates for A. tonsa. Differences in slopes and mean variances found between regression lines demonstrated seasonal variability. General regression equations between volume and PL or PW for each species could be derived.