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

Large herbivores are more than just giants walking the landscape; they act as key ecosystem engineers. Their feeding habits affect vegetation cover, seed dispersal, and nutrient cycling. For millions of years, these dynamics have sustained diverse ecological functions, ensuring the resilience of habitats from dense forests to sprawling grasslands. However, the fossil record chronicles two profound shifts that deeply influenced these communities’ composition and ecological roles—events that underscore the delicate equilibrium between biodiversity and environmental change.

The first significant ecological revolution occurred approximately 21 million years ago. Geological shifts, specifically the closure of the ancient Tethys Sea, forged a land bridge connecting Africa and Eurasia. This corridor unlocked mass migrations of ungulates—hoofed mammals—across continents. The dispersal included ancestors of modern elephants, which had previously been confined to Africa. As they ventured into new territories spanning Europe and Asia, they encountered competing species such as deer, pigs, and rhinos. This faunal interchange rewired food webs and redefined ecological niches on a global scale. Intriguingly, despite the profound rearrangements in species distribution, the overall functional framework of herbivore communities remained robust.

About 10 million years ago, Earth underwent yet another dramatic transformation. A prolonged cooling trend rendered the planet drier, triggering extensive expansion of grasslands at the expense of forests. This climate trajectory favored the evolution and proliferation of grazers adapted to these open habitats—species equipped with specialized dentition to cope with abrasive grasses. Concurrently, forest-dependent herbivores declined, leading to a contraction in the functional diversity of ungulates. The transition marked a critical turning point in the evolutionary saga of these large herbivores, yet, remarkably, ecosystems retained their intrinsic structure even as species composition shifted.

To unravel this complex evolutionary narrative, the international team undertook an unprecedented meta-analysis of fossil data encompassing over 3,000 species of large herbivores. By integrating paleontological records across temporal and spatial scales, the researchers could reconstruct patterns of species turnover and ecological roles through deep time. This approach allowed them to disentangle the interplay between species identity and functional roles within ecosystems, revealing that shifts in species were often compensated by others fulfilling similar ecological functions.

Fernando Blanco, the study’s lead author, emphasized the implications: “We observed that while species came and went over millions of years, the overarching ecological roles within large herbivore communities persisted. The foundational framework of ecosystem function did not collapse, underscoring a resilience that defies the dramatic environmental and faunal upheavals that have punctuated Earth’s history.” This resilience reflects a dynamic balance, where ecological redundancy and evolutionary adaptability safeguard ecosystem integrity.

Ignacio A. Lazagabaster, co-author from Spain’s CENIEH research center, illustrated this concept vividly, likening ecosystems to a football team that substitutes players but retains the same tactical formation. “Even though individual species were replaced, the ecosystems maintained their structural integrity because new species assumed similar ecological roles. This functional continuity is the keystone of resilience.”

However, this ecological robustness faces unprecedented challenges in the contemporary era. The last 129,000 years witnessed the extinction of some of the largest terrestrial mammals—including mammoths and giant rhinos—which, despite their absence, did not immediately dismantle ecosystem structures. Today, though, anthropogenic pressures—habitat destruction, climate change, and accelerated biodiversity loss—are causing species declines at a pace never before witnessed in the fossil record. These rapid transformations could push ecosystems beyond their adaptive thresholds.

Juan L. Cantalapiedra, senior study author from Spain’s MNCN, warned, “Natural ecosystems have evolved with a remarkable capacity to adapt over millions of years. However, the speed and scale of current changes are extraordinary. If we continue on this trajectory, we risk triggering a third global tipping point—a collapse of ecosystem function that could be irreversible.” This sobering perspective calls for urgent conservation efforts that recognize not only species survival but the preservation of ecological functions.

The study’s findings also illuminate fundamental principles in ecology and evolutionary biology. They highlight the concept of functional redundancy—the presence of multiple species that perform similar roles—which acts as an insurance mechanism against environmental variability. Moreover, the research demonstrates how macroevolutionary processes and biogeographical events interplay to shape ecosystems over geological timescales, providing crucial insights into how biodiversity loss today might reverberate through ecological networks.

This expansive research sets a new benchmark for understanding faunal evolution by moving beyond species counts to examine the persistence of ecological roles. It challenges traditional narratives focused solely on extinction, reframing ecosystem change as a dynamic process where function can be maintained through rearrangement and replacement. However, the looming question remains: How long can this resilience endure in the face of accelerated human-driven environmental change?

In conclusion, as we stand at a crossroads in Earth’s ecological history, understanding the legacies of past environmental shifts and their impact on large herbivore ecosystems is more relevant than ever. The interplay between species extinction and ecological functionality as revealed by this study underscores a vital message—preserving biodiversity is not merely about individual species, but about maintaining the complex web of interactions that sustain life on our planet. The next chapters in this saga are unwritten, and humanity’s choices will determine whether large herbivore communities—and the ecosystems they underpin—continue to thrive or unravel.

Subject of Research: Not applicable

Article Title: Two major ecological shifts shaped 60 million years of ungulate faunal evolution

News Publication Date: 5-Jun-2025

Web References: 10.1038/s41467-025-59974-x

Image Credits: Illustration: Fernando Blanco

Keywords: Large herbivores, ecological resilience, functional diversity, faunal evolution, environmental shifts, ungulates, extinction, migration, ecosystem function, biodiversity loss, paleontology, climate change