Craspedacusta sowerbii is an ecological oddity among freshwater cnidarians. Since its first description in 1880, it has spread to every continent except Antarctica, yet it remains one of very few hydrozoans with a pelagic medusa stage in inland waters. Its success owes much to a complex life cycle. A sessile polyp, barely one to two millimeters long, lives attached to rocks and other submerged substrate, reproducing asexually through budding and dispersal via microscopic frustules. Under adverse conditions, polyps can form resistant resting stages called podocysts. The conspicuous medusa stage, which reproduces sexually, typically blooms in summer and early fall when water temperatures are high. Because the polyps are cryptic and the medusae appear only sporadically, most global records of the species are based on medusa sightings alone, leaving the true distribution and genetic diversity of the populations poorly understood.

The ecological stakes are considerable. Medusae are voracious predators of zooplankton and can trigger trophic cascades, altering food web dynamics in invaded systems. Stable isotope studies have shown niche overlap between the medusae and planktivorous fishes, indicating potential competition for prey. In Lake Kinneret, an ecosystem already stressed by climate change, water-level fluctuations, and other invasive species, an established jellyfish population could impose additional pressure on fisheries and ecosystem functioning. The lake is warm, monomictic, and meso-eutrophic, covering 168 square kilometers with a watershed of 2,730 square kilometers, and its main inflow is the Jordan River, which accounts for roughly seventy percent of total inflow.

In September 2023, a research team surveyed seven locations across the Lake Kinneret watershed, focusing on the eastern inflows that originate in the Golan Heights, a volcanic plateau of basalt and paleosols northeast of the lake. The sampling sites included streams and ponds along the Meshushim and Zavitan rivers, both downstream of Iris Pond where medusae had been repeatedly reported from 2003 onwards, a section of the Jordan River, two sites on the lake shore, and two sites in the hydrologically separate Daliyot stream. Medusae were searched for with intensive snorkeling transects in stream rock pools, while potential polyp occurrences were assessed by collecting small rocks nearshore and examining them under a stereomicroscope at 400x magnification. The approach proved decisive: no medusae were detected anywhere, but polyps were found at four of the seven locations, including the Meshushim, the Zavitan, Iris Pond, and one site in the Daliyot stream. Had the survey relied only on pelagic sampling, the species would have gone entirely undetected.

Genetic analyses of 66 successfully sequenced polyps revealed three distinct cytochrome c oxidase subunit I, or COI, haplotypes, corresponding to lineages previously described from European and Asian populations. The specimens fell into the two well-known major mitochondrial clades of the species. One clade, designated C2, is cosmopolitan, with records from Europe, Asia, and both North and South America. The other, C1, is most often detected in the Mediterranean region and Central Europe, with rare reports from Africa and Asia, and had never been recorded in the Americas. Notably, the study provides the first record of COI haplotype 1.2 specimens outside Europe, while haplotype 1.1, the most abundant among European medusae records, was not detected among the Israeli polyps.

The spatial pattern of haplotypes was strikingly heterogeneous. Iris Pond contained only haplotype 2.2, the Zavitan sites contained haplotypes 1.2 and 2.2, and the single Daliyot stream specimen belonged to haplotype 1.2. Two haplotypes, 1.2 and 2.1, co-occurred at a single site, the Meshushim Pond, despite the small sample size there. At other localities, only one haplotype per site was detected: all twelve specimens from Zavitan Pond 1 were haplotype 1.2, whereas all fifteen specimens from Zavitan Pond 2, roughly 280 meters downstream, were haplotype 2.2. This near-complete lack of overlap between nearby sites is consistent with separate location-specific founder events and limited short-distance dispersal, pointing to multiple independent introduction events rather than expansion from a single founder population. The haplotypes identified in Israel were identical to those previously described from Europe, suggesting that the watershed populations may represent secondary introductions rather than direct transfers from the native range in China.

How the jellyfish reached the Golan Heights remains an open question. Natural short-distance dispersal via water flow is possible but appears limited, given the strong local founder effects within streams. Human-mediated vectors such as recreation, fisheries, or aquaculture are likely to facilitate long-distance dispersal. Zoochory by migratory birds could also play a role, as Israel lies along the main flyway between Europe and Africa, and the nearby Hula Valley serves as a major stopover site. Podocysts, which can function as dispersal propagules, may be transported externally or internally by waterfowl, although their desiccation tolerance remains debated. The Zavitan and Meshushim streams, situated within the Yehudiya Forest Nature Reserve, may serve as corridors for spreading between otherwise isolated habitats.

Because colonization of Lake Kinneret itself appears likely in the future, the team turned to numerical modeling to predict where medusae would accumulate if they entered the lake. The researchers employed the Massachusetts Institute of Technology General Circulation Model, MITgcm, configured in its hydrostatic, free-surface, partial-cell mode with a horizontal resolution of 400 by 400 meters and 47 vertical layers ranging from 0.25 meters near the surface to 1 meter at 43 meters depth. The model was forced with hourly air temperature, humidity, precipitation, and wind fields from the COSMO model covering Israel at 2.5 kilometers resolution, together with observed radiation data from the Kinneret Limnological Institute Meteorological Station at Ginosar. Embedded within the runs, the Particle Tracking and Analysis Toolbox, PaTATO, for MATLAB computed Lagrangian trajectories representing medusae with relatively low motility, tracked for 72 hours under hourly varying flow fields for representative days in January and June.

The simulations revealed a pronounced seasonal pattern in how medusae would penetrate the lake. In January, when the water column is unstratified and stream discharge can reach 30 to 120 cubic meters per second, particles entering from the Jordan River may either penetrate deeper layers, if the river water is colder than the lake, or be transported far from the inlet along the surface, if the river water is in thermal equilibrium with the lake. In June, stream discharges are weak and the lake is strongly stratified, so particles remain confined along the northern coast within the upper ten meters. Monthly accumulation maps built from particle releases every six hours along the entire littoral zone identified a permanent hotspot along the western coast near Tiberias, a seasonal hotspot in the northern part of the lake near Capernaum during July and October through December, and a weaker seasonal accumulation zone along the southern edge during the winter months. These simulated coastal hotspots could facilitate sexual reproduction if male and female medusae co-occur at high densities, and they provide concrete focal sites for future monitoring campaigns.

The researchers caution that their particle dynamics tacitly assumed low medusae motility, whereas Craspedacusta medusae are relatively good swimmers with a strong ability for vertical self-propulsion, potentially able to move vertically even in stably stratified waters. A detailed active matter agent-based model embedded in the lake hydrodynamics would be needed to simulate the generation and maintenance of small-scale blooms, a task outside the scope of the current research. The team also notes that medusa budding is typically reported at temperatures above 20 degrees Celsius, but medusae have been found in December and January at similar latitudes in Nevada, Japan, Morocco, and Croatia, and can persist well below 20 degrees. Recent warming in Lake Kinneret has extended stratification periods and maintained temperatures above 20 degrees into late December, increasing the likelihood of winter persistence. Because stream discharge into the lake is higher during winter, the hydrological regime in the Golan Heights streams may permit propagation of medusae only during those months, even though medusae production peaks in summer and fall when low precipitation could impede transport between ponds. The study underscores that intensified monitoring, especially at stream inflows and the Jordan River delta, combined with experimental studies on the tolerance of polyps, medusae, and podocysts to local abiotic stressors such as the lake’s higher salinity and elevated nitrate levels, is strongly recommended to evaluate invasion dynamics and potential impacts on this ecologically and economically critical lake.

Subject of Research: Invasion genetics and hydrodynamic modeling of the freshwater jellyfish Craspedacusta sowerbii in the Lake Kinneret watershed

Article Title: Short-range diversity and invasion dynamics of the freshwater jellyfish Craspedacusta sowerbii in Lake Kinneret, Israel, and its watershed: field sampling combined with hydrodynamical simulations

Article References: Dehos, S., Amitai, Y., Kuplik, Z., Sussholz, N., Gießler, S., Stibor, H., & Heifetz, E. (2026). Short-range diversity and invasion dynamics of the freshwater jellyfish Craspedacusta sowerbii in Lake Kinneret, Israel, and its watershed: field sampling combined with hydrodynamical simulations. NeoBiota, 107, 309-329. https://doi.org/10.3897/neobiota.107.179109

Image Credits: AI Generated

DOI: 10.3897/neobiota.107.179109

Keywords: invasive species, freshwater jellyfish, Craspedacusta sowerbii, Lake Kinneret, Golan Heights, COI haplotypes, hydrodynamic modeling, Lagrangian particle tracking, NeoBiota, Short-range, diversity, invasion