\r\n\r\nThe embryo acquires a double rows of cilia, a well-developed pair of lateral tentacles, and a large, apical sense-organ. The entodermal part of the gastro-vascular system consists of 6 lateral diverticula from a central chamber; 2 of these lateral branches lead into the bases of the tentacles and the other 4 lead outward toward the 4 double rows of cilia. The ectodermal buccal pouch or stomodeum has become a long, laterally compressed tube, with its broad axis 90* from the tentacular axis of the animal. Until this time the animal swims about quite freely within the egg-envelope at this stage its cilia may be observed beating in a normal manner and its tentacles to elongate or contract in response to stimuli. Soon after this the larva breaks through the egg-envelope and escapes into the water. Here it passes the development stages which are very similar to those of the young Pleurobrachia.

\r\n\r\nThe tentacles acquire numerous lateral filaments and elongate greatly, as in Pleurobrachia. When the animal is 5mm long, the oral lobes begin to develop as two simple outgrowths on both sides of the mouth in the sagittal plane of the animal. At the time when the oral lobes begin to develop, the meridional ventral canals and the paragastric tubes begin to elongate downward. The former give rise to the characteristic loops in the oral lobes. Four meridional vessels extend downward and fuse with the circum-oral vessel. The primary tentacle-bulbs migrate downward to lie close by the sides of the mouth. The auricles appear last of all, after the lobes have developed to some extent. When attaining 10mm long the animal becomes ellipsoidal in outline. The appearance of its lobes and auricles resembles to that in the adult of Bolinopsis. Afterward the deep, lateral furrows extend upward to the level of the apical sense-organ and the animal acquires the characteristic of Mnemiopsis ( Mayer, 1912 ). The embrional development takes about 20-24 hours in the Black Sea upper water layer by 23 degrees C. The size of hutched larvae is 0.3-0.4mm.

The pelagic ecosystem of the Black Sea was degraded, manifesting as sharply decreased biodiversity, abundance, and biomass of the main components of the pelagic ecosystem-zooplankton (Dumont and Shiganova). Fish stocks in the Black Sea and Sea of Azov have suffered due to predation on eggs and larval stages of food supplies (Shiganova 2003). Effects on the ecosystem in the Caspian Sea were faster and stronger than in the Black Sea. In 2001, repercussions were felt at all trophic levels, including that of the top predator, the Caspian seal (Dumont and Shiganova).

A cascading effect occurred at the higher trophic levels, from a decrease in zooplankton stock and collapsing planktivorous fish, to vanishing predatory fish and dolphins. Similar effects occured at lower trophic levels: from a decrease in zooplankton stock to an increase in phytoplankton, which was released from zooplankton grazing pressure. The majority of these effects were top-down, but a few were also bottom-up. Similar effects, but less pronounced, were recorded in the Sea of Marmara. Effects on Mediterranean food webs have, so far, remained insignificant. Salinity is probably supraoptimal there, and several predators prevent M.leidyi from reaching outbreak levels.

\r\nOne of the factors that provoked high level of population development of M. leidyi in the Black Sea but was not observed within its natural range-estuarial waters of North America was the absence of a predator feeding on M. leidyi and controlling its population size (Purcell et al., 2001). In 1997, another invader, the ctenophore Beroe ovata Mayer 1912, was found in the northeastern Black Sea. It is a predator feeding on planktivorous comb jellies - especially M. leidyi (Konsulov and Kamburskaya, 1998). As with its predecessor, B. ovata arrived with ballast waters from the same coastal waters of North America (Seravin et al., 2002). Development of B. ovata considerably decreased the population of M. leidyi that had deformed the Black Sea ecosystem for over a decade. The reduction of the M. leidyi population limited its influence on the ecosystem and consequently we observed a recovery of the main components of the Black Sea pelagic ecosystem – zooplankton (including meroplankton), phytoplankton, dolphins and fish as well as their eggs and larvae (Shiganova et al.,2000a,b; 2001 c).

\r\nConscious of this, and bearing in mind the devastating impact of M. leidyi on the fisheries in the Black and Azov Seas in the 1990s, we began a number of initiatives in 2001 with a view to take stock of the situation, review and assess remedial measures and take concrete actions. After deliberation, we proposed the introduction of a potential predator of M. leidyi as the only truly viable option. As shown by the example of the Black Sea, the best – and so far only - candidate for this is another ctenophore species, Beroe ovata. After the accidental introduction of Beroe ovata to the Black Sea, the abundance of M. leidyi here immediately dropped to levels so low that no further damage was inflicted. In fact, the ecosystem almost immediately began to recover. It is anticipated that the results of a Beroe ovata introduction in the Caspian will be similar. Summer 2003 is now the target date for the implementation of this plan (Dumont and Shiganova, unpublished).