Growth and development of Calanus helgolandicus reared in the laboratory

Two Calanus helgolandicus cohorts were reared under laboratory conditions (150 l tanks) at 15°C with the dinoflagellate Prorocentrum micans as food, at high (278 μg C l^-1, Cohort H) and low concentrations (77.5 μg C l^-1, Cohort L), respectively. The study focused on the copepodite stages and for each stage, development time, total and structural growth rates, ingestion rate and gross growth efficiency were estimated. The growth rate of each stage was estimated using 2 methods, the classical one (Method I) and a new method (Method II), which considers the initial weight of the 2 successive stages rather than their mean weights. The initial weight corresponds to that estimated at the day (t_ini) when 50% of the individuals in the cohort entered the stage. Using the new method, the increase in lipid deposition through copepodite stages (increase of the C:N ratio from 4.3 up to 4.7 and 6.9 in Cohorts L and H, respectively) and the effect of food concentration on the growth rate from early CI to CV are clearly shown in the values of the growth rates (the CV growth rate in terms of carbon was 0.063 d^-1 in Cohort L and 0.115 d^-1 in Cohort H). Both of these features are not apparent for the key stage CV when using the classical method (0.041 d^-1 in Cohort L and 0.035 d^-1 in Cohort H). Female weight increase after moulting, by up to -60% in Cohort L and -40% in Cohort H, is also demonstrated by the new method. Using Method II on field data is however limited to some specific cases in which the cohort development is synchronous; this might be particularly the case for some large copepods. Food supply influences the development time of stages CIV and CV, and differences are also found in the development time of each stage indicating a non-isochronal development. Food concentration also influences the gross growth efficiency (GGE) of all copepodite stages, with significantly higher GGE at the low food concentration. GGE varies with stage, mostly at low food concentration (for GGE in terms of carbon: 0.135 to 0.502 in Cohort L and 0.147 to 0.308 in Cohort H). Finally, we propose an index that can be used to indicate the position of an individual within its stage.