Sex differences in habitat distribution of a planktonic copepod, Eudiaptomus gracilis

Abstract

The spatial distribution of adult males and females in a population of the copepod Eudiaptomus gracilis was studied throughout one year. The sexes were markedly segregated in spring and under ice in late winter. This skewed distribution was mainly due to a strong concentration of females in deep water in the lake centre. Males were more evenly distributed but dominated in relative terms strongly over females near the surface and the littoral zone. Among females, those carrying eggs were most concentrated in deep regions. Also during summer, females in the lake centre showed a marked abundance peak in deep water but the distribution of egg-carrying females did not differ from other females. During autumn and early winter, differences in the distribution of the sexes were minor. The distribution of the sexes is discussed in relation to predators, temperature, food, and mates. Female copepods are susceptible to fish predation when carrying eggs. Fish may contribute to the over-representation of females in deep regions by eating or scaring away this category of vulnerable prey from littoral areas and from the surface waters. The fish predation hypothesis does not accurately predict the seasonal occurrence of the skewed distribution, however. Feeding rate of fish is most likely low under ice. Reproductive advantages in the warmer deep water may have contributed to female choice of habitat in late winter, in spring and late winter proportionally fewer females near the surface and shore carried eggs, but they carried more spermatophores. This indicates that these females were more receptive to matings. Phytoplankton biomass was higher near the surface. Thus, males may have gained mating advantages and more food by avoiding deep water in the centre of the lake.