The role of density in sex determination in Daphnia magna (Crustacea, Cladocera)

SUMMARY. 1. Cladocerans are generally believed to exhibit environmental sex determination; however, the evidence for this assertion is equivocal. Particularly, no studies have been performed which isolate potential effects of the environment on the sex of offspring (and hence sex ratio) from effects on the induction of sex (with the necessary appearance of males). We report the findings of a study of Daphnia magna Straus which is able to make this distinction.
2. We examined the sexes of broods produced by asexually reproducing females collected from high and low densities of D. magna in naturally occurring populations. We looked for an effect of density, a factor frequently implicated in male induction, on offspring sex.
3. We could find no effect of density on brood sex. Our results may indicate a true absence of interaction or may arise from our sampling procedure obscuring an interaction. More detailed field observations are suggested as a means of further examining the role of density in sex determination in Cladocera.