Length-weight relationships and coefficient of condition of Euphausia superba and Thysanoessa macrura (Crustacea: Euphausiacea) in southwest Indian Ocean during summer

Abstract

Samples of antarctic euphausiids, Euphausia superba and Thysanoessa macrura were obtained during the MD 25 FIBEX expedition of the R. V. “Marion Dufresne”, in February 1981, to the southwest Indian Ocean. Individual and mean coefficients of condition (Km) were calculated from variations in weight as a function of total length and of carapace length. In E. superba, no significant differences were found as a function of sex or development stage in either total length and weight or carapace length and weight. In T. macrura, a significant allometric difference emerged in the wet weight of juveniles, males and females as a function of total length, and in wet weight as a function of carapace length between adults and subadults. In both E. superba and T. macrura, wet weight as a function of total length differed significantly among stations, and also between species, with a greater weight increase for T. macrura in summer. The coefficients of condition calculated for all wet weight-total length relationships showed that the morphologically different Group II males were heavier than Group I males and mature females. Km in T. macrura was higher for females than for juveniles or males, indicating a greater weight gain by the females. These differences probably reflect real differences in physiology and may affect the distribution of these two species.