PHYSIOLOGICAL PLASTICITY TO TEMPERATURE IN CALANUS-FINMARCHICUS - REALITY OR ARTIFACT

During 1989 ovigerous females of Calanus finmarchicus were sampled in fjord areas near Tromso, northern Norway, at seawater temperatures of 4-5-degrees-C. The offspring were cultivated at 1.5-2-degrees-C to nau pliar stage III-VI, and then separated into three different temperatur e regimes almost-equal-to 40 days after hatching. A significant differ ence in abundance and stage composition in the three different experim ental series evolved within 6 days after differentiation in temperatur es, and greater similarities were found for those series which experie nced a temperature increase. The stage composition of C. finmarchicus in the three different series was equal at the start of the experiment s with the dominance of nauplius stage IV (NIV). In one series, with a constant temperature increase of 0.2-degrees-C day-1, the populations gradually changed into copepodites and a small proportion appeared as copepodite stage IV (CIV) in the period 18-24 days after incubation. At a lower rate of temperature increase (i.e., 0.1-degrees-C day-1) CI V did not appear in the populations, and only three copepodite stage I I (CII) and one copepodite stage III (CIII) were found at the end of t he experiment. In a third series at a constant temperature of 2-degree s-C population abundance decreased constantly throughout the experimen tal period, and only a low proportion of the population appeared as co pepodite stage I (CI) at Day 18. A tendency for arrested development w as observed since CI was not present in the populations at Day 24. The present experiments demonstrate that C. finmarchicus is dependent on a certain temperature increase for successful stage development at low temperatures. However, the results differ from earlier experiments co nducted in 1980 on the same species from the same region, since the na upliar developmental rates at 2-degrees-C obtained in 1980 were defini tely higher than those obtained in 1989. The overwintering temperature , which female C. finmarchicus experienced, differed markedly between 1980 and 1989. This indicates that different overwintering temperature s experienced by adults could shift the low temperature tolerance of t he offspring. The formulated hypothesis for ontogenetic differences in the physiological response to temperature in C. finmarchicus (Tande, 1988) must also be considered in relation to the possibility that the maternal acclimation to the overwintering temperature could modify the rate-temperature response in the offspring later in the spring. This means that caution should be applied when adopting the commonly used e mpirical temperature functions for estimating stage durations of copep ods in high-latitude environments until the possible effect of acclima tion has been ruled out in quantitative terms.