A comparative study of body proportions in Euphausiids. Relationships with the physical environment

Dominant euphausiid species inhabiting the Tropical Mexican Pacific, the California Current, and the Antarctic regions were studied to understand differences in their development patterns. Body proportions were analyzed in each species using four measurements and the Differentiation Index (DI, after Färber-Lorda, 1990). The genus Euphausia showed a smaller DI in warmer waters. Differences in regression slopes between sexes within each species showed distinct morphological patterns across genera. The exponent b of the asymptotic regression between Abdominal Length (AL) and Carapace Length (CL) (AL = aCL^b) by sex was higher for males in all Euphausia species and lower in Nyctiphanes simplex (an egg-carrying species) and the genus Thysanoessa. Differences in the mean Differentiation Index (ΔDI) between males and females were calculated for all species in each area. Mean seawater density in the upper 400 m layer, also calculated for each sampled area, showed a similar trend to ΔDI, while mean temperature showed the opposite trend. The Antarctic Euphausia superba showed the highest ΔDI. A trend towards larger Abdominal Length and Eye Diameter, but smaller Carapace Length and lower lipid levels, was found in a group of mature males (Males II) of E. superba. Higher lipid content likely contributes to buoyancy, allowing distinct morphological differentiation patterns within the genus Thysanoessa; Nyctiphanes simplex, a sac-spawning species, also exhibits a different growth pattern. Physical environmental conditions and lipid content are assumed to regulate the morphological differences between sexes in these species.