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 naupliar stage III-VI, and then separated into three different temperature regimes almost-equal-to 40 days after hatching. A significant difference in abundance and stage composition in the three different experimental series evolved within 6 days after differentiation in temperatures, and greater similarities were found for those series which experienced a temperature increase. The stage composition of C. finmarchicus in the three different series was equal at the start of the experiments 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) CIV did not appear in the populations, and only three copepodite stage II (CII) and one copepodite stage III (CIII) were found at the end of the experiment. In a third series at a constant temperature of 2-degrees-C population abundance decreased constantly throughout the experimental period, and only a low proportion of the population appeared as copepodite stage I (CI) at Day 18. A tendency for arrested development was 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 conducted in 1980 on the same species from the same region, since the naupliar developmental rates at 2-degrees-C obtained in 1980 were definitely 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 temperatures experienced by adults could shift the low temperature tolerance of the 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 empirical temperature functions for estimating stage durations of copepods in high-latitude environments until the possible effect of acclimation has been ruled out in quantitative terms.