Sex ratios, intersexuality and sex change in copepods

Female-biased adult sex ratios are common in copepod species, and have been observed both in wild populations and in cultures. Biased sex ratios are most commonly explained by sex- or stagespecific differences in longevity or mortality. However, neither differential longevity nor differential mortality fully explains skewed sex ratios in planktonic copepod populations. We propose that sex change is an important mechanism determining the adult sex ratio. Though sex change has been proposed for only a few copepod species, intersexuality is widespread. We review the occurrence and causation of intersexuality in planktonic copepods, which is a manifestation of late sex change during development. By way of example, we demonstrate that skewed sex ratios of Acrocalanus gracilis, a common tropical paracalanid copepod, could be explained by sex change alone. Our observations suggest that sex determination in copepods is under strong environmental control, and we argue that the food limitation is an important determinant of sex change.