THE DURATION OF COPEPOD LIFE STAGES ESTIMATED FROM STAGE-FREQUENCY DATA

Current methods to determine the stage duration of copepods from stage -frequency data are often based on inappropriate distribution function s of the stage development time; they also lack a clear probability mo del that describes the random noise in the data. This study aims to es timate the duration of copepod life stages, using an asymmetrical prob ability distribution function of development time. Data on stage frequ ency were collected from cultured cohorts of Acartia clausi. Temora lo ngicornis and Pseudocalanus elongatus. In addition, data from cultures of Calanoides carinatus and Calanus australis by Peterson and Paintin g (J. Plankton Res.. 12, 283-293, 1990) were used. Following Landry (L imnol. Oceanogr., 20. 854-858, 1975). for each stage the number of ani mals that did not yet pass that stage was plotted as a function of tim e. A gamma distribution function appeared to describe the expected pro portion of these animals versus time. The number of animals is assumed to follow a binomial distribution and the model parameters are estima ted by maximizing the likelihood. This method enables individuals with a retarded development, which are usually deleted from the data mater ial, to be dealt with. The median development time was estimated from the gamma distribution functions. from which the duration of the stage s was derived. Isochronal development was confirmed for most stages. H owever, the first feeding naupliar stage. as well as the last copepodi te stage (CV), took longer in most species, whereas the pre-feeding na upliar stages had a shorter duration, particularly in P. elongatus. Di fferences between sexes were not apparent. Contradicting results among cultures and with published results by other workers suggest that par t of the observed deviation from near-isochronal development may be du e to the culture conditions.