Sex ratio of sub-fossil Ostracoda (Crustacea) from deep lake habitats in northern Poland

The present study examined sex ratios in five lacustrine profundal species displaying sexual dimorphism in the shape and/or size of the carapace valves: Limnocytherina sanctipatricii, Leucocythere mirabilis, Candona neglecta, Fabaeformiscandona levanderi and Fabaeformiscandona protzi. Data derived from a survey of sub-fossil ostracod valves preserved in short (up to 0.5 m) sediment cores collected from the sub-littoral and profundal zones of 12 mainly mesotrophic lakes in northern Poland. Mean sex ratios in all species were female-biased (ranging from 56% to 77% of females in F. protzi and L. mirabilis, respectively) and significantly different from a 1:1 ratio. The goodness of fit to a 2:1 (female:male) sex ratio model, commonly applicable in freshwater ostracods, was also tested and the model was rejected except for F. levanderi. In the remaining four species the differences were still statistically significant, although in F. protzi an excess of males caused the obtained deviation from 2:1. Subsequently, the homogeneity of sex ratios among all samples taken from different sediment layers, cores and lakes was analysed for the five species. For the four species collected in more than one lake (i.e. all but L. mirabilis) sex ratios were highly heterogeneous among all studied samples hence we cannot state that there is one specific sex ratio for each of these species. However, the sex ratios of each species among samples from a particular lake tested separately were homogeneous, i.e. did not show significant changes through time (core depth) in all cases except L. sanctipatricii in one lake. Homogeneity of sex ratios among all species from all samples within a given lake was also examined, whenever more than one species was collected. The sex ratios appeared heterogeneous among species only in two lakes. The test was repeated when the studied species were divided into two taxonomic groups (belonging either to Cytheroidea or to Cypridoidea superfamily) and the result was the same. The obtained results could suggest that the unbalanced sex ratio of some of the studied species could be caused at least partly by environmental conditions.