Oil sac volume, gonad size and moulting patterns were investigated in the copepod Calanus euxinus inhabiting deep and shallow zones of the Black Sea and penetrating into the Marmara Sea. In summer the C. euxinus population in deep layers of the Black Sea was dominated by pre-diapause and diapausing postmoult copepodite stage V (CV) with small sexually undifferentiated gonads and mean lipid content of 14.1 +/- 6.0% of body volume. The lipid content of deep-living females was 7.2 +/- 4.2% of body volume. At the same time, intermoult and premoult CV with enlarged gonads and low lipid content (7.7 +/- 5.1% of body volume) and females with oil sac volume of 1.4 +/- 1.0% were found at shallow stations. Premoult CV with oil volume of 0.6 0.8% and mature females with little visual evidence of substantial lipid storage dominated in the Marmara Sea. The differences in moulting patterns and oil sac volumes of C. euxinus from deep zones and shallow regions suggest that vertical migrations to the oxygen minimum zone (OMZ) are necessary for formation of large lipid reserves providing high reproductive potential of this species. On the basis of an energy balance model it was shown that under low phytoplankton concentration of about 30 mu g C 1(-1) preadults and adults migrating to the OMZ could accumulate lipids (up to 5% of body energy content daily), in contrast to copepods constrained to shallow oxic water columns of the Black Sea and from the Marmara Sea. (c) 2005 Elsevier B.V. All rights reserved.