The review, led by Ramón Damián Morejón-Arrojo of the Institute of Marine Science in Havana together with colleagues in the United States and Spain, synthesizes decades of scattered research into a single framework for understanding the genus. Cassiopea comprises roughly twelve confirmed species distributed across the tropical and subtropical waters of the Atlantic, Indian and Pacific Oceans, inhabiting coral reefs, mangroves, seagrass meadows and coastal lagoons. Yet the authors found that field-based studies make up only about fourteen percent of the roughly 195 papers published on the genus between 1774 and 2023, and most of that fieldwork has been concentrated in the Western Atlantic, leaving the global ecology of these animals poorly understood.

The key to Cassiopea’s success lies in its biology. Unlike most jellyfish, which drift through the water column, Cassiopea medusae rest bell-down on the substrate with their frilly oral arms pointing upward, maximizing the sunlight that reaches the millions of dinoflagellate algae living inside their tissues. These symbionts, members of the family Symbiodiniaceae, supply up to seventy percent of the jellyfish’s basal energy requirements through photosynthesis. The partnership is so essential that the algae are required for strobilation, the process by which a sessile polyp transforms into a free-swimming juvenile medusa. Without the symbionts, the life cycle simply cannot complete in nature.

That dependence shapes every stage of development. Cassiopea medusae are usually gonochoric, meaning they have separate sexes, and fertilization occurs inside the female’s gastrovascular system. Embryos develop on the oral disc, become ciliated within about forty-eight hours, and by ninety-six hours form swimming planula larvae that eventually settle and metamorphose into polyps. Polyps reproduce asexually by releasing planuloid buds at a rate of two to three per day, and they acquire their algal symbionts only after settling. Larval settlement itself is triggered by specific bacteria such as Pseudoalteromonas and Vibrio and by compounds released from degrading red mangrove leaves, including a peptide of roughly 5.8 kilodaltons. Strobilation typically begins about three weeks after settlement and infection, generally at temperatures above twenty-eight degrees Celsius, and does not occur at twenty degrees or below even when food is abundant.

As adults, the jellyfish are mixotrophs, combining photosynthetic nutrition with active predation. Their bell pulses up to fifty times per minute, drawing water and prey into the space between the subumbrella and the oral arms, where finger-like structures called digitata armed with nematocysts intercept small animals. Gut-content analyses across Panama, Florida, Cuba, the Philippines, Italy and Australia show a remarkably consistent diet dominated by epibenthic crustaceans, especially harpacticoid copepods, along with ostracods, tanaids, mysids, nematodes and other small invertebrates. Larger medusae consume more prey, and the same pulsing that captures food also serves respiration, waste removal, gamete dispersal and the mobilization of nutrients from sediment pore waters, which in turn may boost the productivity of the algal symbionts.

It is this relentless pumping that gives Cassiopea its reputation as an ecosystem engineer. In habitats with little water movement, such as mangrove systems, the jellyfish act as living pumps that mix the water column and draw nutrient-rich interstitial water out of the sediment. They function as hotspots of nutrient excretion and oxygen consumption in the dark, and as oxygen producers and nutrient assimilators in the light, with effects on ammonium regeneration and oxygen concentrations that follow the daily rhythm of their symbionts’ photosynthesis. Their pulsations also release copious mucus loaded with dissolved organic matter, including proteins, lipids and carbohydrates, which fuels microbial growth and shapes biogeochemical cycling in the surrounding water.

The same sensitivity to its environment that makes Cassiopea an engineer also makes it a sentinel. The jellyfish respond rapidly to elevated metals and nutrients, and their tissues can accumulate trace metals such as lithium, copper, manganese and zinc at concentrations up to two hundred times those found in seawater. They have been used as models in ecotoxicological studies of herbicides such as atrazine and hexazinone, and microplastics have been detected in specimens from Florida estuaries, apparently trapped in their oral arms rather than ingested. Remarkably, in polluted habitats the jellyfish appear to tolerate contamination by shifting toward anaerobic metabolism, with elevated activity of glycolytic enzymes such as pyruvate kinase and lactate dehydrogenase allowing energy production even under low-oxygen stress, an adaptation that lets them thrive where many other invertebrates decline.

That tolerance helps explain a troubling pattern: Cassiopea blooms are increasingly associated with human-impacted coasts. Nutrient enrichment from agricultural runoff and coastal development correlates with high densities, with the highest reported abundance reaching about 106 individuals per square meter in Codrington, Barbuda, compared with as little as 0.06 per square meter in Lake Macquarie, Australia. Densities this high can alter benthic invertebrate communities, reduce seagrass cover, and compete with vegetation for light, since the jellyfish often rest directly on top of seagrass shoots. The review also documents an inverse relationship between population density and average bell diameter, likely reflecting competition for prey and nutrients at high densities, although nutrient loading can partially offset this constraint by boosting individual growth.

The genus is also expanding its range, and human activity appears to be a major driver. Invasions have been recorded in Australia, India, Vietnam, the Cape Verde and Canary Islands, the Mediterranean, Hawaii, Brazil and the Caribbean, and one hypothesis links the Indo-Pacific spread to the movement of United States naval vessels during the Second World War. Modified coastal habitats such as marinas and shrimp farms act as springboards for establishment, offering stable food supplies and the higher lipid reserves that support survival during unfavorable conditions. The jellyfish’s environmental tolerance is impressive: they survive temperatures up to thirty-six degrees Celsius, salinities near forty-seven practical salinity units, and pH as low as seven, and they can shrink under starvation or low light and regrow when conditions improve.

Climate change, however, tests even this resilience. Because Cassiopea depends on dinoflagellate symbionts, it is vulnerable to the same thermal bleaching that devastates coral reefs. Bleached individuals have been recorded in Singapore, in a Saudi Arabian lagoon during a historical temperature record of forty-four degrees Celsius, and on the north coast of Havana in 2023, when an asymmetric bleaching event affected 0.44 percent of jellyfish during a heatwave. Recent physiological work shows that bleaching compromises the expression of glucose and glycerol transporters, limiting energy availability during recovery, while symbiont genotype matters: jellyfish hosting Durusdinium species tolerate heat better but receive less carbon than those hosting Symbiodinium, a trade-off between survival and growth. Even pigmentation appears to matter, as specimens with blue appendages showed higher survival under acute heat stress.

The Western Atlantic remains the best-studied region, encompassing both native and invasive populations across the Bahamas, Barbuda, Cuba, Mexico, the Florida Keys and Puerto Rico, and the review’s authors argue that this region offers the richest dataset for understanding the genus’s ecological roles. But they emphasize that the field’s future lies elsewhere. Only by extending research beyond the Western Atlantic, integrating ecological, physiological and molecular approaches, and monitoring the growing frequency of blooms can scientists predict how these inverted jellyfish will reshape tropical and subtropical ecosystems in a rapidly changing ocean. For now, the animal lying quietly on the lagoon floor, pumping water, feeding microbes and accumulating pollutants in its tissues, is revealing itself as one of the most multifaceted small players in the coastal sea.

Subject of Research: Ecological roles of the upside-down jellyfish genus Cassiopea in tropical marine ecosystems

Article Title: Life upside-down: review of ecological roles of Cassiopea (Cnidaria: Scyphozoa) in marine ecosystems—a look at the Western Atlantic

Article References: Life upside-down: review of ecological roles of Cassiopea (Cnidaria: Scyphozoa) in marine ecosystems—a look at the Western Atlantic. (n.d.). https://doi.org/10.1007/s44396-025-00006-9

Image Credits: AI Generated

DOI: 10.1007/s44396-025-00006-9

Keywords: Cassiopea, upside-down jellyfish, Scyphozoa, Symbiodiniaceae, mixotrophy, symbiosis, ecosystem engineer, bioindicator, jellyfish blooms, Western Atlantic, climate change, marine ecosystems

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