Abstract Two different Calanus finmarchicus (Gunnerus)cohorts originating from 60 N (Bergen) and 69 N(Tromsø) were investigated in equal environmentalconditions to study their different physiological responsesto the same environment. A two-plus-two-bagmesocosm study was carried out between March andJuly 1998, in Ha?køybotn, Tromsø, in order to determinedevelopment and mortality rates of the two parallelcohorts of C. finmarchicus. For practical reasons, thecohort from Bergen was incubated 10 days earlier thanthe Tromsø cohort. Consequently, they were exposed toelevated food conditions as compared to the Tromsøcohort. A high initial mortality among the Bergen cohortcould clearly be ascribed, by genetic discrimination,to ‘‘contamination’’ with C. helgolandicus. After thisinitial mass mortality, the mortality was constantly 0.03–0.04 day)1. In cohorts starting from naupliar stage I,there was no significant difference in development orgrowth, the median development time (NI–CIV) differingby only 7 days ( 6%). The difference in developmenttime can be explained to a large extent ( 4 days)by temperature differences. This is substantiated withmodel simulations using a physiological model developedfor C. finmarchicus. There was a time lag in incubationbetween the two cohorts, resulting in elevatedtemperature during incubation of the Tromsø cohort. Afraction of both cohorts differentiated sexually atstage CV, with males differentiating before females.Females from both cohorts produced eggs, but specificegg production differed significantly (P>0.001, t-test).This was supported by elevated RNA:DNA ratio in femalesfrom the Bergen cohort. Both cohorts demonstratedquite similar development and physiologicalgrowth rates and, consequently, are considered to belongto the same genetic population inhabiting theNorwegian Shelf. The study demonstrates that C. finmarchicusis capable of adaptation to changes in environmentand, thereby, demonstrates a significantphysiological plasticity.
