Development of Sympetrum striolatum and S. vulgatum (Odonata: Libellulidae) in brackish water

Sympetrum striolatum (Charpentier, 1840) and S. vulgatum (Linnaeus, 1758) are two closely related Libellulidae that are widespread and common in Central Europe. The idea for this research originates from normally using saltwater shrimps for rearing young larvae, the observations of Sympetrum species laying eggs in seawater and the suggested ability of S. striolatum to colonize brackish water habitats. This topic will also be of rising relevance for dragonfly populations as in the future due to climatic changes or anthropogenic activity the salinization of freshwaters will likely increase. The experiments presented in this study served to find out whether eggs and larvae of both species can develop in brackish water. For this purpose, eggs of both species were kept at four different salinities from 0.5–1.5% and the development duration, hatching curves and growth rates as well as mortality were recorded and compared to respective data from an earlier experiment conducted in tap water. It was possible to investigate whether embryonic development, the hatching behaviour and larval growth are disturbed by different salinity levels compared to rearing in freshwater. We found for both species that the eggs can develop at different salt concentrations up to 1.5% and the larvae survive and grow in the brackish water. Especially for S. striolatum a slightly increased salinity even seems to be advantageous compared to rearing in tap water shown by high hatching and survival rates. The results of this study add some knowledge about the influencing effects of salt on both species. It seems that low salt concentrations seem to be well tolerated by both species or become even beneficial for S. striolatum. Furthermore, the results provide methodological aspects about the rearing of young dragonfly larvae.