Temporal and spatial variation of sexual size and shape dimorphism of Cyprideis torosa (Ostracoda)

Abstract

Sexual differences in the size and shape of males and females are widespread in the animal kingdom, but research on sexual dimorphism in ostracods has been limited. From 718 individual ostracod valves of the species Cyprideis torosa, we extracted size (length and height) and valve width (from focus-stacked photographs), and we used geometric morphometrics to analyse the shape of valve outlines. Variability of C. torosa was investigated on different geographical (Baltic Sea, Central Germany, and Mediterranean area) and geological (Holocene to living) scales. We found that sexual differences were greater in shape than in size and that populations on our geographical scale showed a differentiation in size. Dimorphism in width was investigated in detail for the first time for ostracods and showed strongest dimorphism within the size traits. The examination of the relationship between size and shape revealed sex-specific, regional, and temporal patterns. All size traits of C. torosa showed allometry consistent with Rensch’s rule. Isolation of the Mansfeld Lakes populations from marginal marine environments might have contributed to morphological differentiation as expression of possible speciation. We suggest that morphological variability in C. torosa results from a combination of sex-specific and scale-dependent geographical phenotypic plasticity and regional sexual and fecundity selection pressures on the sexes.