Molting and mortality depend on age and stage in naupliar Calanus pacificus: implication for development time of field cohorts

Molting success and survivorship increased during naupliar development in the marine calanoid copepod Calanus pacificus studied in the laboratory. However, both rates decreased with age within each instar. Simulations of the development of different naupliar stage III (NIII) age-groups suggest that mortality can shorten estimated cohort development times below the minimum possible in individual nauplii. The magnitude of this reduction depends strongly on differences in initial mortality rate among instars. The suggestion that copepod generation times in the field are independent of food availability can be reconciled with the well-established relationship between stage durations and food concentration if a distinction is made between individual and cohort development rates. Individual development rates decrease at low food levels, but with the death of a sufficiently large fraction of slowdeveloping individuals, cohort development rates may remain maximal.