Sex ratio of Stygiopontius senokuchiae (Dirivultidae, Copepoda), an endemic copepod species at deep hydrothermal vent sites, is biased to males

Stygiopontius copepods (Dirivultidae, Siphonostomatoida, Crustacea) are among the most successful meiobenthic organisms at deep-sea hydrothermal vents. Most of their ecology is not yet known, including the spatiotemporal differences in their sex ratios and their controlling factors. We investigated spatial variation in the sex ratio of adult Stygiopontius senokuchiae and its association with environmental parameters, including food quality at hydrothermal vent chimney structures in the calderas of three neighboring sea knolls (Bayonnaise Knoll, Myojin Knoll, and Myojin-sho Caldera) in the western North Pacific Ocean. Their sex ratio was significantly biased to males from 1:1, which was different from some of the other Stygiopontius species. The ratios did not show a significant correlation with the density of total adults. While previous studies have shown that the abundance of S. senokuchiae is positively associated with δ 13 C values of detritus on active chimneys, multivariate analyses in this study did not detect any significant association between their sex ratio and any investigated parameters including δ 13 C. These findings suggest that neither population density nor chemoautotrophic food availability drives the sexual difference in the spatial distribution around vents among the adults of the species.