The Indo-Pacific nudibranch Baeolidia moebii Bergh, 1888 in Greece, with the first documented spawning aggregation in the Mediterranean Sea.
The nudibranch Baeolidia moebii Bergh, 1888 is a sea slug widely distributed in the Indo-Pacific, including the Red Sea, from where it presumably entered the Mediterranean Sea through Lessepsian migration. Although the first record of this species in the invaded area dates back to at least 2007, it is only known so far from the Mediterranean basin based on three scattered records of single individuals, all found in the Levant Sea (Turkey and Cyprus). The present study reports the presence of B. moebii in Kalloni Gulf, a semi-enclosed shallow embayment located in Lesvos Island, a record that widens its known distribution to Greece and the Aegean Sea. Field observations of more than 150 individuals encountered in a single sampling event also allowed observation of intraspecific morphological variation and several specimens laying egg masses, thus first reporting of a spawning aggregation and providing concrete confirmation of its establishment success in the entire basin. Monitoring of such spawning events and aggregations may contribute to a better understanding of the population dynamics of the species. The discontinuous known distribution of B. moebii in the eastern Mediterranean can be attributed to false absence of the species due to its cryptic behaviour, its under-sampled habitat, or to a truly very patchy distribution because of secondary dispersion by currents or vessels, with establishment only in areas of particularly favourable conditions, such as the Kalloni Gulf.