The ctenophore Mnemiopsis in native and exotic habitats: US estuaries versus the Black Sea basin

The native habitats of the ctenophore, Mnemiopsis, are temperate to subtrop ical estuaries along the Atlantic coast of North and South America, where i t is found in an extremely wide range of environmental conditions (winter l ow and summer high temperatures of 2 and 32 degreesC, respectively, and sal inities of <2-38). In the early 1980s, it was accidentally introduced to th e Black Sea, where it flourished and expanded into the Azov, Marmara, Medit erranean and Caspian Seas. We compile data showing that Mnemiopsis has high potentials of growth, reproduction and feeding that enable this species to be a predominant zooplanktivore in a wide variety of habitats; review the population distributions and dynamics of Mnemiopsis in U.S. waters and in t he Black Sea region; and examine the effects of temperature and salinity, z ooplankton availability and predator abundance on Mnemiopsis population siz e in both regions, and the effects of Mnemiopsis on zooplankton, ichthyopla nkton and fish populations, focusing on Chesapeake Bay and the Black Sea. I n both regions, Mnemiopsis populations are restricted by low winter tempera tures (<2 degreesC). In native habitats, predators of Mnemiopsis often limi t their populations, and zooplanktivorous fish are abundant and may compete with the ctenophores for food. By contrast, in the Black Sea region, no ob vious predators of Mnemiopsis were present during the decade following intr oduction when the ctenophore populations flourished. Additionally, zooplank tivorous fish populations had been severely reduced by over fishing prior t o the ctenophore outbreak. Thus, small populations of potential predators a nd competitors for food enabled Mnemiopsis populations to swell in the new habitats. In Chesapeake Bay, Mnemiopsis consumes substantial proportions of zooplankton daily, but may only noticeably reduce zooplankton populations when predators of Mnemiopsis are uncommon. Mnemiopsis also is an important predator of fish eggs in both locations. In the Black Sea, reductions in zo oplankton, ichthyoplankton and zooplanktivorous fish populations have been attributed to Mnemiopsis. We conclude that the enormous impact of Mnemiopsi s on the Black Sea ecosystem occurred because of the shortage of predators and competitors in the late 1980s and early 1990s. The appearance of the ct enophore, Beroe ovata, may promote the recovery of the Black Sea ecosystem from the effects of the Mnemiopsis invasion.