Density-dependent sex ratio in Echinomermella matsi (Nematoda), a parasite of the sea urchin Strongylocentrotus droebachiensis

SUMMARYWe investigated the adult sex ratio in 70 infrapopulations of the nematode Echinomermella matsi, a parasite of the green sea urchin Strongylocentrotus droebachiensis. The adult sex ratio was skewed towards female dominance at low adult intensity and towards male dominance at high adult intensity. We hypothesize that this is due to differences between the sexes in development and mortality rates, or that female recruitment is density dependent. A model with differences between the sexes in developmental and mortality rates may develop the observed sex ratios if the female developmental and mortality rates are several times that of the males. A large difference in developmental rates between the sexes appears unreasonable because the developmental rate for both sexes is low, and the predicted low female life-expectancy is unlikely because the males appear to accumulate in infrapopulations as the females age. Density dependence of female numbers is, however, supported by a significantly lower female recruitment in infrapopulations with old females. We also find that the mean male length is negatively related to measures of crowding, thereby supporting the hypothesis that competition is of importance in E. matsi infrapopulations. A female bias at low intensities of infection, a density dependence in female recruitment and the taxonomic position of E. matsi indicate that sex may be environmentally determined in this nematode.