Introduction: This review synthesizes approximately 25 years of evidence on M. leidyi in the Caspian Sea, focusing on invasion history, biological characteristics, population dynamics, ecological impacts, environmental controls, and interactions with B. ovata. It addresses how biological characteristics facilitated establishment, how distribution and abundance have changed since introduction, and how environmental change and predator–prey interactions may influence its future trajectory.
Methodology: An integrative narrative review was conducted using scientific literature concerning the biology, invasion history, distribution, reproduction, environmental tolerance, feeding ecology, population dynamics, ecological impacts, and management of M. leidyi. Caspian Sea studies were considered together with comparative evidence from other invaded marine and brackish ecosystems. Relevant literature was identified using combinations of keywords related to M. leidyi, invasive ctenophores, the Caspian Sea, zooplankton, reproduction, temperature, salinity, population dynamics, food webs, Beroe ovata, climate change, and biological invasion
Results: The invasion success of M. leidyi reflects the interaction of several complementary biological traits. Broad tolerance to temperature and salinity allows survival under contrasting environmental conditions, while rapid growth, early maturity, simultaneous hermaphroditism, self-fertilization, and high fecundity facilitate establishment from small founder populations. Its flexible feeding strategy provides an additional advantage. Early developmental stages mainly consume microzooplankton, whereas juveniles and adults exploit mesozooplankton, particularly copepods, as well as crustacean and mollusc larvae and fish eggs and larvae. This trophic flexibility increases resilience to fluctuations in prey availability. Following its introduction in 1999, M. leidyi rapidly expanded throughout the Caspian Sea, with the southern basin becoming the principal population center. The emergence of Beroe ovata represents an important new component of the Caspian food web. This specialized predator feeds primarily on other ctenophores, and its increasing abundance has been associated with reductions in M. leidyi and partial increases in mesozooplankton. However, predator-mediated control alone is unlikely to eliminate M. leidyi or completely restore the pre-invasion food web.
Discussion and conclusion: The 25-year invasion history of M. leidyi demonstrates that its successful establishment and persistence in the Caspian Sea result from interactions between exceptional life-history traits and favorable environmental conditions. Broad physiological tolerance, rapid reproduction, self-fertilization, short generation time, and trophic flexibility facilitated rapid establishment and the initial population outbreak. Subsequent declines indicate increasing regulation by prey availability, environmental variability, water quality, and biological interactions. Future population trajectories will depend on interactions among climate warming, declining water levels, changes in productivity, hydrological variability, water quality, and predator–prey relationships. Future research should integrate long-term monitoring with environmental DNA (eDNA), metabarcoding, satellite observations, ecological modelling, and artificial-intelligence-based forecasting. Such an integrated framework could improve prediction of population outbreaks, identify ecological thresholds, and provide a stronger scientific basis for adaptive management and conservation of the Caspian Sea ecosystem.