Mature size determination in copepods the adaptive significance of mature size in copepods: output or efficiency selection?

Abstract

Species of small crustaceans show changes in the size of mature females in successive seasonal generations. The proximal factors influencing seasonal variations in mature size may involve environmental conditions such as temperature and food conditions and biological factors such as predation. But the ultimate factors are not clearly understood at present, without the hypothesis that the mature size changes for escaping from visual predation. In the present study, the ultimate factors determining the variability in mature sizes of females in five copepod species were examined using previously published laboratory growth studies. The present study focuses on bioenergetics with the ultimate factor, apart from the predation hypothesis. The bioenergetical analysis of the published data demonstrates at least two strategies among these copepods with respect to the factor that ultimately determines the mature size of females under given conditions of temperature and food supply. One strategy is associated with an optimum mature size for maximizing total net production, i.e., output-selection, and the other is associated with an optimum size for maximizing net production efficiency, which is the ratio of net production to maintenance energy, i.e., efficiency-selection. In addition to the two strategies, one copepod species can change from one strategy to the other as a function of ambient temperatures or food conditions. Usually, the optimum body size giving the maximum efficiency is smaller than the one giving the maximum output, and body mass gets firstly the maximum efficiency and then the maximum output in the course of individual growth process. Therefore, the selected strategy depends on the timing to cease growing. Maximization of output or efficiency may be important as one of the ultimate factors that determine the mature size of females in some copepod species according to seasonally changing environments.