Status of alien species in the Mediterranean and Black Sea

15
This species comes originally from the Atlantic coasts of North America. Mnemiopsis is found in
coastal waters of North America from Cape Cod southwards to Carolina. It is abundant in ports and
harbors of the above areas and can be pumped (presumably as larvae or small juveniles) or gravitated
(as adults as well) with ballast water into cargo ships. While sufficient zooplankton may be available
to sustain this comb jelly in ballast water on a voyage lasting 20 or more days from the Americas to
the Black Sea, food resources are not necessary, since Mnemiopsis can live for three or more weeks
without food, reducing body size at the same time (Reeve et al., 1989). Like other ctenophores,
Mnemiopsis is a simultaneous hermaphrodite. This means, in theory, that a single animal could
successfully invade a new area.
The first record of Mnemiopsis appearance in the coastal water of the Black Sea goes back to 1982
(Pereladov, 1988). The first registration of this species in open water was made in winter 1986–1987
(Zaitsev et al., 1988). The massive growth of the Black Sea population started in 1988 and at first
covered only bays, gulfs and coastal waters. Its abundance reached 10–12 kg·m -2 in several coastal
areas (e.g., Anapa, the southwestern Bulgarian coast) although it did not exceed 1.5–3 kg·m -2 in the
open sea (Shushkina and Vinogradov, 1991). Maximal development of this species was registered in
1989 and 1990 (about 1 200 g·m -3 ), but then its abundance started to decrease (Vinogradov et al.,
2000). For example, the average biomass of Mnemiopsis during 1991–1994 in the Romanian littoral
zone was 2.2–3.5 g·m -3 and decreased to 0.2 g·m -3 in 1995 (Radu et al., 1996–1997). The same
quantitative distribution was investigated in the Dnieper River influence zone of the Black Sea.
Average biomass of Mnemiopsis between 1993–1997 was 3.2–5.1 g·m -3 . Its population density during
these years stabilized at 300 to 800 g·m -2 in the Black Sea and at 500 to 600 g·m -2 in the Sea of Azov
(GESAMP, 1997).
Meanwhile, another ctenophore species, Beroe ovata, which is a predator of Mnemiopsis, was
introduced to the Black Sea presumably by ballast water and observed in 1997. As explained in the
below section for Beroe, this introduction helped mitigate the Mnemiopsis outburst.
The distribution maps of Mnemiopsis in the Black Sea have been prepared for different years and
seasons. To make a generalized map, the model description of this comb jelly was used. This
description was based on peculiarity of its biology (reproduction, growth, mortality) and water mass
transportation in the Black Sea (Lebedeva, 1998). The map showing the generalized distribution of M.
leidyi was made for the water layer of 0–30 m (typical inhabited layer for this species) for September
1998 (Figure 5).
Mnemiopsis is usually found close to shore, in bays and estuaries, although they have been collected
occasionally several hundred kilometers offshore. They are able to tolerate a wide range of salinity and
temperature, and can live and reproduce in temperatures ranging between 1.3 o C and 32 o C and in
salinities ranging between 3.4 and 75 ‰. They survive well in oxygen-poor environments. They are
most abundant in brackish waters with high levels of suepended materials and appear to be little
affected by contaminants. The only factors which appear to restrict their rapid population growth are
the temperature, the availability of food and the presence of predators (GESAMP, 1997).