Author(s): Cornelia Jaspers
Recently, both the invasive ctenophore Mnemiopsis leidyi and the arctic Mertensia ovum were discovered in the Baltic Sea but their range expansion remains unclear due to misidentification of their larval stages. Supported by molecular species verification we describe seasonal abundance and distribution of larvae and eggs of these two species. We show that their occurrence is significantly but inversely related to salinity. Mertensia ovum was present year round throughout the brackish Baltic Sea but also occurred in high-saline areas during cold seasons. Larvae of M. leidyi occurred throughout all seasons in high-saline areas but never extended further into the central Baltic. Highest ctenophore egg abundances were observed in high-saline areas during summer along with the first appearance of M. leidyi adults. The M. leidyi population peaked 2 months after the first occurrence of adults in high-saline areas, suggesting these areas as a source for lower saline regions. Low larvae abundances and a reduced transitional-to-adult ratio in the southern Baltic point to reduced or no active recruitment, suggesting that drift of animals from high-saline into lower saline regions sustains the M. leidyi population in the southern Baltic such as the Arkona and Bornholm basins.