ORIGINAL PAPER K.N. Kosobokova The reproductive cycle and life history of the Arctic copepod Calanus glacialis in the White Sea Accepted: 27 April 1999 Abstract Seasonal variations in the gonad development and sex ratio of copepodite stage V (CV) and adults were examined from February to November in order to un- derstand the reproductive cycle and the life history of Calanus glacialis in the White Sea. Gonad maturation, sexual dierentiation and moulting to adults take place during the 2nd year of development. Energy accumula- tion takes place in the spring and summer of the 2nd year. The following autumn/winter is the major period of CV maturation, which occurs independent of food supply. Maturation of males precedes that of females by 2± 3 months. The maximum proportions of CV and adult males are found in the population in October and No- vember. The onset of female maturation is observed in February and March, ca. 2 months prior to the spring phytoplankton bloom. Reproduction takes place be- tween April and June. Its termination in the second half of June coincides with the warming of the surface water layer where egg laying takes place. Variations in the gonad morphology throughout the year suggest long life spans and iteroparity of females of C. glacialis in the White Sea. Many of them survive for several months after reproduction and are able to overwinter again. Therefore, females with dierent life histories co-occur in the population in winter: ``young'' females recently moulted from the overwintering CVs, and ``old'' females which have spawned at least once in their life, after which they return to overwintering conditions. In contrast, males have shorter life spans of 3±4 months resulting in a sex ratio skewed toward females at all seasons. Introduction The Arctic calanoid copepod Calanus glacialis Jaschnov is one of the dominant species in the plankton communities of the Arctic Basin, its peripheral seas, and the adjacent areas of the Atlantic and Paci®c Oceans. The distribution of C. glacialis is related to the distri- bution of the Arctic water masses in the northern hemisphere (Jaschnov 1970; Conover 1988). In the Atlantic Ocean this species is found as far south as Nova Scotia (Runge et al. 1985), and in the Paci®c in the northern part of the Sea of Okhotsk (Jaschnov 1955). In the White Sea C. glacialis is one of the major components of zooplankton, accounting for 30±45% of its total biomass (Pertzova 1971). This Arctic sea was formed during the last interglacial period and had sub- sequently lost its direct connection with the Arctic re- gion as a result of the warming of the climate and the penetration of the warm North Cape Current into the southern part of the Barents Sea. Thus, the White Sea population of C. glacialis is a relict of the glacial period, isolated at present from populations inhabiting the largest part of its distribution area. The closely related boreal species C. ®nmarchicus seldom occurs in the White Sea. It does not reproduce there and has never been found there in numbers (Jaschnov 1966; Kosobokova and Pertzova 1990). The estimated duration of the life cycle of C. glacialis in the dierent parts of its area is still under dispute (Tande 1989; Smith and Schnack-Schiel 1990). The life cycle is generally thought to be biennial north of the polar front and annual over the rest of the area (Conover 1988). In the northern Barents Sea and the Arctic Ocean generation time is apparently 2 years, with CIV being the main overwintering stage (Pavshtiks 1977; Kosobokova 1981, 1986; Tande et al. 1985). A 1-year cycle has been suggested for the Greenland Sea (Smith 1990), Davis Strait (Huntley et al. 1983) and the Sea of Okhotsk (Safronov 1984). In some areas C. glacialis demonstrates equivocal characteristics suggesting a combination of an annual and biennial cycle (Grainger 1963; Maclellan 1967; Melle and Skjoldal 1998), which probably represents survival of early and late cohorts (Hirche 1989). In the White Sea the life cycle of C. glacialis is considered to be biennial, with spawning in Polar Biol (1999) 22: 254±263 Ó Springer-Verlag 1999 K.N. Kosobokova P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, 36 Nakhimov Ave., 117218 Moscow, Russia