Cannibalism makes invasive comb jelly, Mnemiopsis leidyi, resilient to unfavourable conditions

The study addresses how the invasive ctenophore Mnemiopsis leidyi persists through extended periods of low prey availability in newly colonized, high-latitude Eurasian habitats. While invasive success is often attributed to limited predation and opportunistic traits (rapid growth, high fecundity), such traits can increase vulnerability during environmental fluctuations. M. leidyi now occupies regions with longer low-feeding periods than in its native range. Despite massive reproductive investment late in the season and poor larval overwinter survival at northern sites, populations persist. The authors hypothesize that adults meet nutritional needs during prey scarcity by cannibalizing their own larvae and test this with field observations and laboratory stable isotope experiments, aiming to clarify mechanisms enabling bloom maintenance and overwinter survival, and to inform management strategies.

Background literature links M. leidyi to strong impacts on zooplankton communities and fisheries via competition with fish and larval fish, with noted invasion dynamics in the Black Sea and elsewhere. Prior work describes its opportunistic life-history traits, bloom-and-bust dynamics, and high ingestion rates on copepods, but cannibalism as a nutritional strategy in gelatinous zooplankton has been largely overlooked. Management experiences, such as the introduction of Beroe ovata in the Black Sea and the role of overfishing of zooplanktivores, frame the ecological context. The study situates cannibalism within general ecological rules (juveniles consumed more often; occurs when alternative prey declines; density-dependent access to intraspecific prey) and raises evolutionary questions about the ubiquity and origins of cannibalism across metazoans.

Field observations: High-frequency sampling of M. leidyi and its prey in Kiel Fjord (south-western Baltic Sea) from August 12 to October 21, 2008, encompassing pre-bloom to post-bloom phases. Adult and larval abundances were quantified; specimen sizes were measured; mesozooplankton (copepods) and microzooplankton (tintinnids, ciliates, dinoflagellates) were sampled weekly. Temperature and other abiotic variables were recorded. Photos of freshly collected adults (within 30 minutes of capture) were used to detect larvae inside adults during the bloom collapse.

Laboratory experiment: In September 2016, stable isotope labeling experiments incubated adult M. leidyi with 15N-enriched M. leidyi larvae for 36 h. Larvae were enriched via a trophic chain (15N-labeled Rhodomonas sp. → copepods → M. leidyi larvae). Treatments included adults fed labeled larvae and control adults without labeled larvae. After incubation, adults and larvae were analyzed for elemental C and N and atom% 15N using an elemental analyzer–IRMS. An independent samples t-test (n=3 per treatment) compared adult 15N between treatments.

Feeding and energetics: Ingestion rates and daily ration (% body carbon d−1) were estimated from field data using published/allometric equations (as referenced in the text, Eqs. 1–7). Daily ration curves were fitted (inverse cubic polynomials) for predation on copepods and on larvae. Structural Equation Modeling (SEM) evaluated direct and indirect effects of temperature, prey availability (micro- and mesozooplankton), and cannibalism on M. leidyi population dynamics; models used standardized variables, maximum likelihood estimation, chi-square goodness-of-fit, and significance threshold p<0.05.

Reproducibility and data: Statistical analyses were performed in R and AMOS; rations were visualized in Matlab/SigmaPlot. Datasets are deposited in PANGAEA (https://doi.org/10.1594/PANGAEA.893395).

• Field dynamics: Adult and total specimen size of M. leidyi peaked around day 245 (1 September 2008), following the copepod (mesozooplankton) peak and preceding the microzooplankton peak. The relative abundance of larvae rose with microzooplankton availability, but during the collapse of juvenile M. leidyi and zooplankton, adult abundance remained high, indicating a shift in trophic strategy.
• Direct cannibalism evidence: After 36 h, adults exposed to 15N-enriched larvae had significantly higher 15N than controls (t3 = −4.96, P = 0.008). Consumed larvae provided on average 4.1 ± 0.1% of adult carbon and 2.5 ± 0.5% of adult nitrogen (n=3). No larval mortality occurred in larva-only controls over 36 h. Photographs of field-caught adults during bloom collapse showed larvae inside adult oral regions, supporting in situ cannibalism.
• Feeding rations: During copepod bloom peak (day 237), adult daily rations reached up to ~50% of body carbon d−1. Post-bloom, after copepod depletion and shift to larval predation, adult rations decreased but remained ~10–20% body carbon d−1, sufficient to sustain adults for an additional 2–3 weeks.
• Drivers of population growth (SEM): Temperature and microzooplankton abundance had significant positive effects on M. leidyi population growth (standardized path coefficients 0.51 and 0.47, respectively). Bloom decline was associated with negative paths (−0.34 and −0.43). Observations indicate rapid depletion of copepods followed by an adult dietary shift to larvae.

The study provides the first unequivocal demonstration that adult Mnemiopsis leidyi cannibalize their own larvae, revealing a mechanism that enables persistence through prey-scarce periods following blooms. By switching from interspecific prey (copepods) to intraspecific prey (larvae) once the prey field is depleted, adults can continue accumulating nutrients, extending survival and potentially outcompeting intraguild competitors like Pleurobrachia pileus by exploiting a broader prey-size spectrum. Despite vulnerability during post-bloom scarcity, adults can maintain biomass and energy reserves long enough to bridge lean periods, with reserve duration influenced by temperature. This intergenerational strategy effectively converts late-season reproductive output into nutrient reserves for adults, analogous to autophagic or reserve-utilization strategies seen in other taxa. The findings imply that M. leidyi populations should be viewed as integrated adult–larval systems where reproductive investment serves both population growth and adult sustenance under unfavorable conditions. Management implications include considering how cannibalism buffers populations against control measures and prey scarcity; strategies that enhance intraguild competition or reduce late-summer prey may affect bloom dynamics, though historical examples (e.g., Black Sea introduction of Beroe ovata) show mixed ecosystem responses.

Cannibalism in Mnemiopsis leidyi enables adults to persist under unfavorable, prey-scarce conditions by consuming their own larvae after depleting interspecific prey. Field observations, isotope tracer experiments, and energetic estimates collectively demonstrate this dietary shift and its role in sustaining adult biomass post-bloom. This behavior helps explain the species’ resilience and establishment in northern Eurasian habitats with prolonged low-food periods and has important implications for ecosystem impacts and management. Future work should quantify the energetic costs of reproduction (e.g., egg production for field-sized adults), refine larval energy budgets from microzooplankton predation, and assess the prevalence and evolutionary basis of cannibalism across ctenophores to better predict invasiveness and guide control strategies.

• Spatial and temporal scope: Field data derive from a single fjord (Kiel Fjord) over one late-summer to autumn season (Aug–Oct 2008), which may limit generalizability across regions and years.
• Experimental scale and duration: The laboratory isotope experiment had short duration (36 h) and small replication (n=3 per treatment), capturing potential but not long-term rates or behavioral variability.
• Inference on energetics: Daily ration and reserve-duration estimates rely on modeling and allometric equations; uncertainties in parameterization could affect quantitative estimates.
• Mechanistic gaps: The energetic trade-offs between reproductive investment and adult gains from larval cannibalism were not fully quantified; costs of egg production and detailed larval intake/assimilation from microzooplankton remain to be measured.
• Taxonomic breadth: Conclusions about cannibalism as an adaptive trait in ctenophores are based on M. leidyi; broader comparative studies are needed to assess generality.