First record of Mnemiopsis leidyi A. Agassiz 1865 in the Baltic Sea

The invasive ctenophore Mnemiopsis leidyi was first recorded in the Kiel Bight (western Baltic Sea) on 17 October 2006 during a regular weekly sampling program. The M. leidyi abundance gradually increased from 29.5±12.7 ind.m in mid-October to 92.3±22.4 ind.m in late November 2006. The occurrence of M. leidyi in the Baltic Sea is of great concern as this invader has caused negative impacts in the southern seas of Europe.

The ctenophore Mnemiopsis leidyi A. Agassiz 1865 was introduced into the Black Sea in the 1980s (Vinogradov et al. 1989). It showed an explosive mass development there since 1988 and expanded to the Azov, Marmara, and eastern Mediterranean Seas and in 1999 it was introduced into the Caspian Sea likely with ballast water of oil tankers (Studenikina et al. 1991, Shiganova 1993, Shiganova et al. 2001, Ivanov et al. 2000.
The native habitat of M. leidyi includes estuaries and coastal regions along the eastern coast of North and South America (GESAMP 1997). M. leidyi is a polymorphic species with wide tolerance to environmental factors and high phenotypic variability (reviewed in GESAMP 1997). Therefore it could establish in different environmental conditions of the southern Eurasian seas. M. leidyi strongly affected all levels of ecosystems and fishery in the productive Black, Azov and Caspian; however no remarkable effects were recorded in the oligotrophic Aegean Sea (Shiganova et al. 2001, Shiganova et al. 2004a. M. leidyi has not been observed along western and northern European waters until recently, although its wide tolerance to salinity (4-38‰) and temperature (4-32°C) (reviewed in GESAMP, 1997) make most of the Baltic Sea a suitable environment. Low water temperature in winter and a relatively low temperature in summer may negatively impair its reproduction.
On 17 October 2006 J. Javidpour and U. Sommer observed swarms of M. leidyi for the first time in the Kiel Bight (western Baltic Sea). The ctenophores were collected during a regular weekly sampling program in the Kiel Bight, which started in February 2005. The sampling station is located in the Kiel Fjord (54°19.7' N, 10°09.5' E). Vertical hauls from 10 m depth were carried out with a plankton net (opening diameter 80 cm, mesh size 500 µm). The collected animals were determined and counted alive. The preliminary identification of M. leidyi was confirmed by T.A. Shiganova and also DNA sequencing was carried out by the private lab AGOWA (www.agowa.de/struktur/newsbasis.html).
M. leidyi ( Figure 1) can morphologically be distinguished from the native ctenophore Bolinopsis infundibulum. The main difference is the position of the oral lobes. In M. leidyi, the oral lobes originate near the infundibulum, in B. infundibulum the oral lobes originate approx. half-way between the mouth and the infundibulum (Mayer 1912, see also Faasse and Bayha 2006, this  During the first observation more than 80 % of individuals were equal or smaller than 5 mm in total length, the maximal length was 5 cm. The surface water temperature in the Kiel Bight reached 22.6°C in July 2006, which was 1.7 o C higher than in the same period in 2005 ( Figure 4). The water temperature was 5°C higher in October and November 2006 compared to 2005. The salinity ranged from 13.1 to 22.2‰ which is comparable with the Black Sea salinity (14-22‰) (Ovchinnikov and Titov 1990). These conditions are optimal for M. leidyi reproduction (Kremer 1994). In the Black Sea M. leidyi begins to reproduce at water temperatures above 21 o C, reaches a peak at 23 o C and continues until the water temperature drops below 14 o C in autumn (Shiganova et al. 2001).
The main factors to control the M. leidyi population size are temperature and prey availability (Kremer 1982, 1994, Sullivan 2001. The southwestern Baltic is considered as a high productive area during winter and a high abundance of copepods has been investigated in winter months compared to the summer mesozooplankton community (Schneider 1987). It is therefore assumed that the M. leidyi population in the Baltic Sea developed high   Figure 4) and we assume that it will probably not survive such temperatures at the rather low salinities. In contrast, M. leidyi survives winter temperatures <4 o C in its native range if salinities are higher, but it does not survive colder temperatures in low salinity waters of the Black, Azov and the Caspian Seas (Purcell et al. 2001). However M. leidyi may be re-introduced into the Baltic Sea with water currents from the North Sea next year.
The expansion of Mnemiopsis leidyi in the Baltic Sea is of great concern as this aggressive invader has already damaged several productive ecosystems of the southern seas of Europe. Its high density in the Baltic Sea as well as in the North Sea estuaries in 2006 is an indication of a possible M. leidyi establishment in northern European coastal waters.