Mnemiopsis leidyi is a ctenophore that is native to the western Atlantic, but by the late 1980s was established as an invasive exotic in the Black Sea, presumably after crossing the Atlantic in ship ballast water (it has subsequently appeared in the Caspian, Aegean, Azov, Marmara, North, Baltic, Skagerrak, and Mediterranean Seas). It reached very large numbers and depleted stocks of zooplankton as well as fish eggs and larvae, triggering the crash of several fisheries. In 1997, however, another ctenophore native to the western Atlantic, Beroe ovata, was discovered in the northeastern Black Sea. Beroe ovata is known to feed on planktivorous ctenophores and, in particular, on M. leidyi. The arrival of B. ovata appears to have stabilized the Black Sea ecosystem, leading to a reduction in M. leidyi populations and subsequent recovery of plankton and fish populations. (Shiganova et al. 2003 and references therein)
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