PHENOTYPIC VARIATION IN METAMORPHOSIS IN 4 SPECIES OF FRESH-WATER COPEPODS

1. Physiological metamorphosis accompanied by an ecological habitat sh ift is a widespread life-history phenomenon, and both age and size at metamorphosis are highly variable in many organisms. In this study, ag e and size at metamorphosis (defined as the transition from the last n aupliar to the first copepodite stage) were quantified for four specie s of freshwater copepods to determine the scale on which these two tra its vary, if age and size at metamorphosis are equally variable, and i f variation at metamorphosis is related to variation in newborn size. 2. Measurements of laboratory-reared and held-caught individuals show that age and size at metamorphosis, together with newborn size, vary a mong siblings, between families within a population, between populatio ns of one species and between closely related species. 3. In all popul ations, age at metamorphosis was the most variable trait, a result obs erved in many other organisms. Most of the variation in age at metamor phosis could be explained by differences between families within a pop ulation, while differences among siblings from the same clutch account ed for most of the variation in size at metamorphosis. 4. Although new born size was variable, differences in this trait could not fully acco unt for variation observed at metamorphosis. Newborn size differed amo ng populations, but most interpopulational differences disappeared by the time metamorphosis was reached. In particular, size at metamorphos is appears to be tightly constrained in freshwater copepods. 5. Age an d size at metamorphosis were not equally variable among species, eithe r. Species-specific metamorphic envelopes (joint distributions of age and size at metamorphosis) result from differences in trait means, var iances and covariances, and suggest very different larval growth traje ctories among three of the species examined.