November, 1999 Body size affects survival probabilities, reproductive output and individual fitness in many organisms. In freshwater zooplankton, traits ranging from demographic rates to community composition depend on body size, and predation is most often identified as the selection pressure determining body size. We examined the extent to which stage-specific growth trajectories and body sizes of copepod crustaceans are constrained, independent of selection. We used exuviae shed at each molt to quantify the relationship between size at molting and growth during the subsequent instar for two common, herbivorous calanoid copepods, Boeckella triarticulata and Diaptomus leptopus. Individuals of both species were raised under diets of different food quality or quantity, and at different temperatures. Size at molting varied little among individuals of both species, as a consequence of a persistent negative relationship between size at molting and subsequent (absolute) growth increment. Individuals that were small when they molted grew more during the subsequent instar than individuals that were large. This relationship was statistically significant for nearly all instars of both species raised in different food or temperature conditions, and not affected by food quality, food quantity or temperature. Our results indicate that body size is constrained or regulated over much of the copepod life cycle, independent of the effects of environmental conditions (food, temperature, predation).Keywords: body size, size regulation, copepod crustaceans, growth and development, molt increment.To be published in Oecologia. organisms. In freshwater zooplankton, traits ranging from demographic rates to community composition depend on body size, and predation is most often identified as the selection pressure determining body size. We examined the extent to which stage-specific growth trajectories and body sizes of copepod crustaceans are constrained, independent of selection. We used exuviae shed at each molt to quantify the relationship between size at molting and growth during the subsequent instar for two common, herbivorous calanoid copepods, Boeckella triarticulata and Diaptomus leptopus. Individuals of both species were raised under diets of different food quality or quantity, and at different temperatures. Size at molting varied little among individuals of both species, as a consequence of a persistent negative relationship between size at molting and subsequent (absolute) growth increment. Individuals that were small when they molted grew more during the subsequent instar than individuals that were large. This relationship was statistically significant for nearly all instars of both species raised in different food or temperature conditions, and not affected by food quality, food quantity or temperature. Our results indicate that body size is constrained or regulated over much of the copepod life cycle, independent of the effects of environmental conditions (food, temperature, predation).
Metamorphosis is a common life-cycle transition in organisms as diverse as amphibians, insects, fishes and crustaceans, and the timing of this transition often affects an individual's fitness. Here, we manipulated the timing of the metamorphosis in the freshwater copepod, Diaptomus leptopus, and then followed individuals over their entire life cycle to assess the fitness consequences of variation in age and size at metamorphosis. In 3 separate experiments, individuals were raised in different food conditions: low food (0.2 J..Lg C/ml) switched to high food (0. 7J..Lg C/ml), or high food switched to low food, at several different larval and juvenile stages. Control individuals were reared on high or low food concentrations over their entire life cycles. For each individual, we measured age and size at metamorphosis and age and size at maturity; for females we also measured total lifetime egg production and calculated a composite fitness measure, A.. Statistical analyses showed few effects of treatment on age or size at metamorphosis; of these two traits, only age at metamorphosis correlated significantly with age at maturation, suggesting fitness effects. Changes in food conditions during juvenile stages also had no effect on size at maturity, but caused significant differences in age at maturity. Only age at maturity and egg production covaried significantly with A.. Because egg production was significantly correlated to age at maturity, the fitness differences we observed may be primarily due to variation in developmental rates and age at maturity. These results suggest that natural selection should favor rapid development in field populations, and that larvae should metamorphose (and perhaps mature) at minimum possible sizes. They also suggest that body size plays a different life-history role in these organisms than is recognized in most poikilotherms... AbstractMetamorphosis is a common life-cycle transition in organisms as diverse as amphibians, insects, fishes and crustaceans, and the timing of this transition often affects an individual's fitness. Here, we manipulated the timing of metamorphosis in the freshwater copepod, Diaptomus /eptopus, and then followed individuals over their entire life cycle to assess the fitness consequences of variation in age and size at metamorphosis. In 3 separate experiments, individuals were raised in different food conditions: low food (0.2 ).lg C/ml) switched to high food (0. 7 ).lg C/ml), or high food switched to low food, at several different larval and juvenile stages. Control individuals were reared on high or low food concentrations over their entire life cycles. For each individual, we measured age and size at metamorphosis and age and size at maturity; for females, we also measured total lifetime egg production and calculated a composite fitness measure, A.. Statistical analyses showed few effects of treatment on age or size at metamorphosis; of these two traits, only age at metamorphosis correlated significantly with age at maturation, suggesting fitnes...
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