Population development of the invader ctenophore Mnemiopsis leidyi, in theBlack Sea and in other seas of the Mediterranean basin

In the last two decades of the twentieth century, the ctenophore Mnemiopsis leidyi (A. Agassiz) has invaded the Black, Azov, Marmara and Aegean Seas, and, recently, the Caspian Sea. Here, we compare its spatial and temporal d istribution, seasonal dynamics and the time and duration of reproduction. W e also discuss factors that control its abundance throughout its invasive r ange and its effect on ecosystems. Observations are based on the long-term field data collected by three research institutes. An analysis of the effec ts of temperature, salinity, prey (zoo- and ichthyoplankton) availability a nd predation (by ctenophores of the genus Beroe) on M. leidyi population si ze, and the effects of M. leidyi on zoo- and ichthyoplankton, and on fish p opulations in the Black and Azov Seas is also provided. With the Black Sea current, M. leidyi spreads to the upper layers of the Sea of Marmara, where it now occurs around the year. At regular intervals, the Black Sea current also takes it to the northern Aegean Sea. In contrast, it has to re-invade the Sea of Azov every spring or summer, dying out during winter when the t emperature drops below 4 degreesC. The warm summer and mild winter temperat ures, relatively low salinity and abundance of prey in the Black Sea are cl ose to optimal for M. leidyi, while they are suboptimal in the northern Aeg ean Sea, where salinity and temperature are often too high. In the Black Se a the absence of gelatinous and other predators led to an enormous ctenopho re abundance for a decade, but with the appearance of Beroe oi,ata in 1999, M. leidyi abundance greatly decreased. Analysis of seasonal dynamics of M. leidyi in the Black Sea and in other seas of the Mediterranean basin indic ates similarities in the timing of maximum abundance and biomass, in spite of some differences in the initiation and duration of reproduction. A peak biomass and density occurred in 1989 in the Black and Azov Seas and in 1990 in the other seas. The M. leidyi invasion negatively affected the ecosyste ms of the Black Sea and the Sea of Azov. The zooplankton, ichthyoplankton a nd zooplanktivorous fish stocks all underwent profound changes. Similar eff ects, but less pronounced, were recorded in the Sea of Marmara. Effects on Mediterranean food chains have, so far, remained insignificant. Salinity is probably supraoptimal here, and several predators prevent M. leidyi from r eaching outbreak levels.