Ecological characteristics of the Mljet Island seawater lakes (South Adriatic Sea) with special reference to their resident populations of medusae

Ecological properties and distribution and abundance of medusae were studie d over an 18-month period in the Mljet Island seawater lakes, south-east Cr oatia. Strong stratification during the summer differentiates these lakes f rom the oligotrophic South Adriatic ecosystem. The lakes are designated as a moderately eutrophicated ecosystem. Very small numbers of hydromedusae we re noted, representing only the Anthomedusae and Leptomedusae. A new specie s of the genus Tima was found in considerable numbers of individuals. High abundance of the scyphomedusa Aurelia sp. was observed throughout the year. This species differs in terms of genetic divergence from Aurelia aurita fo und elsewhere in the Mediterranean and could be attributed to the boreal or igin.