The research, published in the journal Discover Ecology, provides the first seasonally explicit quantitative assessment of Cassiopea populations in a natural Caribbean mangrove system. Led by Ramón D. Morejón-Arrojo of the Universidade de São Paulo, together with Marta Mammone of the University of Galway, André C. Morandini, and Leandro Rodríguez-Viera of the University of Cadiz, the team conducted quadrat-based surveys during the dry season in December and the rainy season in July across two consecutive years, from 2023 to 2024. In total, the researchers counted 3,441 individual jellyfish across 190 one-square-meter quadrats positioned in the shallow subtidal zone of the mangrove lagoon.

The contrast between seasons was striking. In December 2023, mean jellyfish density reached 35.64 individuals per square meter, nearly 3.8 times higher than the 9.28 individuals per square meter recorded the previous July. Mean bell diameter told a similar story: jellyfish in December 2023 averaged 8.87 centimeters across the bell, about 29 percent larger than the 6.94-centimeter average measured in July 2023. The authors describe this as a “winter giants, summer dwarfs” pattern, in which both the number and the size of medusae peak during the cooler, clearer dry season.

Yet the pattern proved anything but fixed. In 2024, the seasonal contrast in bell diameter all but vanished, with July and December populations averaging 10.66 and 10.59 centimeters respectively — a difference so small that a Tukey-adjusted statistical comparison returned a p-value of 0.9997, indicating no meaningful seasonal divergence. Overall, jellyfish were larger in 2024 than in 2023 regardless of season. Statistical modeling captured this nuance precisely: negative-binomial and Gamma generalized linear models detected significant effects of both season and year on density and size, along with a highly significant season-by-year interaction, confirming that the strength of seasonal demographic responses shifted between the two years.

To make sense of these fluctuations, the researchers turned to the jellyfish’s unusual life cycle. Like other scyphozoans, Cassiopea alternates between a bottom-dwelling polyp stage and free-swimming medusae produced through strobilation, a process in which polyps sequentially bud off tiny juvenile jellyfish called ephyrae. Laboratory studies have shown that strobilation is typically triggered above 28 degrees Celsius, yet the field temperatures in Bajos de Santa Ana hovered between 28 and 31 degrees Celsius year-round. This suggests that in natural mangroves, the timing of recruitment may depend less on absolute temperature and more on the stabilization of environmental conditions — the return of clear water and steady salinity after the stressful rainy season. The dry-season peak in December 2023, with its broad size range of 2.4 to 19 centimeters, is consistent with a cohort produced by a successful strobilation pulse several months earlier.

The rainy season, by contrast, appears to impose a multi-stressor bottleneck. Satellite-derived environmental data compiled from Landsat 9, the CHIRPS precipitation dataset, and NOAA sea surface temperature products revealed pronounced wet-season spikes in turbidity, with near-infrared turbidity values peaking in June 2024. Elevated turbidity reduces the penetration of photosynthetically active radiation into shallow waters, potentially starving the jellyfish’s symbiotic dinoflagellates of light. Because these Symbiodiniaceae algae can supply up to 70 percent of the host’s basal energy requirements through photosynthesis, shading could force individuals to rely more heavily on capturing plankton and dissolved organic matter. Heavy rainfall events, some exceeding 75 millimeters per day, may also produce transient drops in salinity that are particularly lethal to vulnerable ephyrae and juvenile medusae, even though spot measurements of salinity remained within a tolerable range of roughly 28 to 31 practical salinity units.

The jellyfish’s seasonal swings also track changes in the wider benthic community. Principal component analysis of centered log-ratio transformed cover data separated the sampling periods clearly along environmental gradients, explaining nearly 65 percent of compositional variance in the first two axes. Cassiopea abundance was negatively associated with unvegetated substrate and macroalgal cover, and bell diameter declined significantly as macroalgal cover increased, a correlation the authors suggest might reflect competition for space or the physical impedance of macroalgal mats to the pulsation-driven feeding currents the jellyfish generate. Seagrass cover followed an inverse seasonal pattern relative to jellyfish density, peaking in July 2023 precisely when jellyfish numbers were lowest. Ten fish species, including the ubiquitous yellowfin mojarra and schoolmaster snapper, were recorded in association with the jellyfish aggregations, hinting at the habitat-shaping influence these animals exert on their neighbors.

That influence is far from trivial. Dense Cassiopea beds are known ecosystem engineers: their pulsation can turn over the entire water column every 15 minutes, they pump nutrient-rich porewater out of the sediment, and they can shift lagoon sediments from net oxygen consumption to net oxygen production. Because these effects scale non-linearly with both body size and density, the 3.8-fold dry-season increase in abundance, combined with larger individuals, implies that the winter population exerts a substantially greater biogeochemical footprint than its summer counterpart. The authors caution, however, that they did not directly measure nutrient fluxes at the site, so these functional consequences remain extrapolations grounded in prior experimental literature rather than observations from Bajos de Santa Ana itself.

Perhaps the most provocative comparison emerges when the mangrove populations are set against those in human-modified habitats. In Brazilian shrimp farms, Cassiopea andromeda maintains stable year-round populations and grows to nearly three times the size of mangrove conspecifics. In Cuba’s tourism-heavy Jardines de la Reina National Park, the largest jellyfish occur in the most heavily visited zones. The new findings suggest a gradient from maximally seasonal natural systems to environmentally buffered aquaculture ponds, along which Cassiopea‘s demographic plasticity is progressively expressed — positioning the genus as a potential bioindicator of how much humans have stabilized coastal environments. Because the release of stinging mucus structures called cassiosomes poses documented risks to bathers, the timing of dry-season blooms also carries practical relevance for coastal management.

The authors are careful to frame their results as a baseline rather than a rule. Four sampling events across two years cannot capture the full spectrum of interannual variability, and the researchers call for multi-year monitoring, high-frequency environmental measurements, and direct studies of polyp and ephyra dynamics using tools such as environmental DNA and settlement collectors. Still, the study fills a genuine gap in Caribbean ecology, documenting that Cassiopea populations in natural mangroves are profoundly shaped by seasonal environmental forcing — light, salinity, and resource availability interacting with a life cycle finely tuned to environmental windows. As climate change alters rainfall regimes, turbidity, and hydrology across tropical coastlines, these gelatinous barometers may soon tell scientists a great deal about how quickly those windows are shifting.

Subject of Research: Seasonal population dynamics of Cassiopea upside-down jellyfish in a Caribbean mangrove ecosystem

Article Title: Seasonal dynamics of Cassiopea spp. jellyfish species in a Caribbean mangrove system

Article References: Morejón-Arrojo, R. D., Mammone, M., Morandini, A. C., & Rodríguez-Viera, L. (2026). Seasonal dynamics of Cassiopea spp. jellyfish species in a Caribbean mangrove system. Discover Ecology, 2(1), Article 16. https://doi.org/10.1007/s44396-026-00034-z

Image Credits: AI Generated

DOI: 10.1007/s44396-026-00034-z

Keywords: Cassiopea, upside-down jellyfish, mangrove ecosystem, seasonal dynamics, Caribbean, Cuba, population ecology, benthic communities, symbiosis, strobilation, turbidity, ecosystem engineering