Morphological and biochemical differentiation in Antarctic krill

During the February 1981 cruise FIBEX MD-25 between 30–50°E and 61–64°S, hydrography showed the presence of two gyres, confirmed by the geostrophic circulation relative to 1000 m from Levitus climatology, at the borders of these gyres concentrations of highly morphologically differentiated krill were found. Gaussian component analysis of krill samples, pooled by sectors, showed three cohorts of Euphausia superba in the western sector and one in the eastern sector. Across the sampling area, Thysanoessa macrura and E. superba occurred at separate stations. Analysis of cohorts in T. macrura separated two size groups in both the western and the eastern sectors. The use of a Differentiation Index (D.I.) [Färber-Lorda, J., 1990. Somatic length relationships and ontogenetic morphometric differentiation of Euphausia superba and Thysanoessa macrura of the southwest Indian Ocean during summer (February 1981). Deep-Sea Res. 37, 1135–1143.], based on somatic lengths, allows studying certain morphological differences within the populations sampled. Morphologically different and bigger males II (D.I. from 2.8 to 3.5) were present only in the southern transect while smaller males I (D.I. from 3.5 to 5.0) were present over the entire area. Biochemical composition of both species showed significant differences among stations for protein, lipids, and carbohydrates. A significant difference in lipid content was found between males I, and males II. For T. macrura, percentage of lipid content in mature animals was much higher than that in E. superba. The D.I. size distribution showed that when populations of E. superba were highly differentiated (corresponding to mature animals) in morphology, lipid content was high, and they were located near a gyre. Differences in morphometry can influence distribution of the species, because different developing stages have different swimming capacities. It is shown that, together with hydrography and trophic conditions, lipid content and morphometry of krill populations, are different but complementary aspects that help to understand krill ecology and distribution.