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, conducted by Ji Hoon Kim, Thodhal Yoganandham Suman, Junsik Woo, and Ihn Sil Kwak of Chonnam National University and the Geosystem Research Corporation in the Republic of Korea, focused on Radix plicatula, a pulmonate snail first described by W. H. Benson in 1842. These animals are ideal candidate biomonitors: they are widespread, sedentary, easy to culture in the laboratory, and, as benthic grazers, they are in constant contact with both dissolved and particulate contaminants in the water column and on submerged surfaces. The team exposed the snails to three different arsenic concentrations over a 21-day period, tracking mortality, the activity of key antioxidant enzymes, and the amount of arsenic that accumulated in snail tissues.

The survival results told a clear and sobering story. In control tanks, 98.3 percent of snails survived the full three weeks. In the highest arsenic treatment, survival fell to 83.3 percent by day 21, with mortality increasing in a manner that was dependent on both the concentration of arsenic and the duration of exposure. This dose- and time-dependent pattern is exactly what toxicologists look for when evaluating whether a contaminant is acting in a predictable, cumulative fashion, and it confirms that even sublethal concentrations of arsenic impose measurable physiological costs on these animals over ecologically relevant timescales.

Perhaps the most biologically revealing findings came from the antioxidant enzyme measurements. When aquatic animals are exposed to arsenic, the metalloid triggers the generation of reactive oxygen species—highly reactive molecules that damage lipids, proteins, and DNA. Cells respond by ramping up enzymatic defenses, chief among them superoxide dismutase (SOD), which converts superoxide radicals into hydrogen peroxide, and catalase (CAT), which then breaks hydrogen peroxide down into water and oxygen. In the exposed snails, these two enzymes displayed what the researchers describe as biphasic responses. Catalase activity peaked early, on day 7, and then declined as the exposure continued. Superoxide dismutase followed a different trajectory, peaking on day 14 at a striking four times the activity measured in control animals before subsiding.

This temporal separation between the two enzymes is more than a curiosity; it offers a mechanistic window into how the snails’ defenses are overwhelmed. The early catalase surge suggests an initial attempt to neutralize peroxide produced by arsenic-induced oxidative stress, while the later SOD peak indicates that the front line of radical detoxification continued to be mobilized well into the exposure period. The eventual decline of both enzymes hints at either regulatory exhaustion or damage to the enzymatic machinery itself as arsenic accumulated—a pattern ecotoxicologists associate with the transition from a compensatory stress response to overt physiological impairment. Staggered enzyme kinetics of this kind, the authors argue, can serve as a sensitive early-warning signal of contaminant stress long before mortality becomes apparent.

The bioaccumulation data were equally telling. Tissue arsenic concentrations in the snails rose in a strongly dose-dependent manner, and the relationship between water concentration and body burden was highly linear, with coefficient of determination values ranging from 0.85 to 0.97. Such tight linearity means that the snails’ tissues effectively function as an integrated chemical record of the arsenic they have encountered, making them a reliable passive archive of contamination history. Even more significant was the effect of time: the biota-sediment accumulation factor, or BSAF, shifted from values at or below 1 at the start of the exposure to values of 2 or greater as the experiment progressed. In practical terms, this means that with prolonged exposure, the snails were concentrating arsenic in their tissues to levels well above those in their surroundings, intensifying the risk that the metalloid will move up the food web.

That last point matters because snails are eaten by fish, birds, and other predators, and arsenic accumulated in gastropod tissue can therefore enter and travel through aquatic food chains. Previous work on other freshwater snails, including the apple snail Pomacea canaliculata and the lymnaeid Lymnaea stagnalis, has documented arsenic and cyanobacterial toxin accumulation and its consequences for reproduction and life history. The new study extends this picture to Radix plicatula and adds quantitative toxicokinetic evidence that bioaccumulation is not a static endpoint but a process that accelerates as exposure lengthens—a critical consideration for ecosystems subjected to chronic, low-level contamination rather than acute spills.

One of the study’s most methodologically interesting components was the use of diffusive gradients in thin films, or DGT, an in situ sampling technique that measures the fraction of a contaminant in water that is truly available for biological uptake rather than simply its total concentration. DGT devices accumulate labile arsenic on a binding layer at a rate governed by diffusion, giving a time-weighted average of bioavailable concentration. In this experiment, DGT measurements correlated strongly with the arsenic concentrations measured in snail tissue, yielding a coefficient of determination of 0.71. That level of agreement, the authors note, confirms DGT as a reliable indicator of arsenic bioavailability—a valuable tool for environmental agencies that need to assess ecological risk from water chemistry data without waiting for biological responses to appear.

Taken together, the findings position Radix plicatula as a sensitive and mechanistically well-characterized bioindicator of arsenic contamination in freshwater ecosystems. A good bioindicator must satisfy several criteria simultaneously: it must accumulate the contaminant predictably, it must show measurable sublethal responses at environmentally relevant exposures, and it must be robust enough to interpret across sites and seasons. This study demonstrates that the snail meets all three, with survival, enzyme activity, and tissue burden each providing complementary information about the severity and duration of contamination. The integrated approach—linking organismal survival, molecular defense responses, and chemical bioaccumulation with an independent bioavailability measurement—represents a template for how modern ecotoxicology can move beyond single-endpoint tests toward a mechanistic understanding of contaminant toxicity.

For regulators and conservation biologists, the implications are concrete. Because antioxidant enzyme responses appeared within the first week of exposure, these biomarkers could be deployed to detect arsenic stress in field populations before population-level declines occur. Because tissue accumulation increased linearly with exposure concentration and intensified over time, snail body burdens could serve as a cost-effective monitoring metric for chronic contamination. And because DGT measurements tracked tissue concentrations closely, water-based monitoring programs could combine chemical and biological indicators in a single assessment framework. As arsenic contamination continues to threaten freshwater resources across South and East Asia and beyond, studies like this one show that some of the most effective sentinels of environmental harm may be quietly grazing on the streambeds beneath our feet.

Subject of Research: Arsenic toxicity, antioxidant responses, and bioaccumulation in the freshwater snail Radix plicatula

Article Title: Survival, antioxidant enzyme activities, and bioaccumulation response in freshwater snail Radix plicatula (W. H. Benson, 1842) to arsenic

Article References: Kim, J. H., Suman, T. Y., Woo, J., & Kwak, I. S. (2026). Survival, antioxidant enzyme activities, and bioaccumulation response in freshwater snail Radix plicatula (W. H. Benson, 1842) to arsenic. Ecotoxicology, 35(8), Article 171. https://doi.org/10.1007/s10646-026-03151-3

Image Credits: AI Generated

DOI: 10.1007/s10646-026-03151-3

Keywords: arsenic, bioaccumulation, freshwater gastropods, antioxidant enzymes, Radix plicatula, environmental biomonitoring, ecotoxicology, oxidative stress, DGT, biota-sediment accumulation factor, water pollution, bioindicator

The research team, led by Žan Cimerman of the University of Ljubljana, took a deliberately realistic approach. Rather than dousing plants with artificially high doses of a single metal, they created a natural concentration gradient by mixing soils collected near a former lead smelter in Žerjav, Slovenia, with clean soil from a geologically similar site in Jezersko. The metalliferous soil contained a striking 9287 milligrams of lead per kilogram, 631 milligrams of zinc and 61.9 milligrams of cadmium, while the control soil held only trace amounts. Both soils, crucially, shared comparable pH, organic matter and nutrient profiles, meaning the only meaningful variable was metal loading.

Clonal gemmae, the tiny lens-shaped reproductive structures the liverwort produces in its distinctive cup-shaped organs, were transplanted onto four soil treatments: pure metalliferous, pure control and two mixtures. Over 35 days, the researchers photographed and measured developing thalli at regular intervals, then subjected the harvested plants to a battery of analyses spanning chlorophyll quantification, antioxidant capacity assays, bulk elemental profiling via ICP-MS, micrometre-scale elemental imaging using micro-PIXE, and transmission electron microscopy of cellular ultrastructure.

The growth penalty was immediate and dramatic. Statistically significant differences in thallus surface area emerged as early as day seven, and by the experiment’s end, plants grown on native metalliferous soil had reached a mean surface area of only 4.00 square centimetres compared with 14.3 square centimetres in controls. Biomass told the same story: control plants averaged 466 milligrams fresh weight and 28.3 milligrams dry weight, while those on the worst soil managed just 86.3 and 7.42 milligrams respectively. Chlorophyll concentrations declined in step, dropping from 24.4 to 17.8 micrograms per millilitre, and total antioxidant capacity peaked at moderate metal levels before falling at the highest doses, hinting at a hormetic response at low exposure and physiological exhaustion at high exposure.

The ionomic story was equally revealing. As soil metal availability rose, concentrations of potassium, manganese and phosphorus in thalli fell steadily, suggesting competitive interference at shared transport pathways. Conversely, calcium, iron, sulphur, zinc, cadmium and lead all accumulated. The kinetics differed by metal: zinc and lead behaved as classic bioindicators, with tissue concentrations rising in near-linear proportion to soil availability, while cadmium showed a sharply different pattern, jumping early and then saturating around 53 milligrams per kilogram in planta. This saturating, accumulator-like response to cadmium had not previously been described in this species, although it fell short of the 100 milligram per kilogram threshold that defines hyperaccumulation in vascular plants.

Micro-PIXE elemental mapping added spatial resolution to the picture. In metal-exposed thalli, zinc, cadmium and lead were concentrated overwhelmingly in the lower parenchyma and ventral epidermis, with mean ventral concentrations reaching 591 milligrams per kilogram for zinc, 399 for cadmium and 1175 for lead. This sequestration strategy physically isolates the toxic load from the photosynthetically active dorsal tissues, a compartmentalisation tactic familiar from studies of other thalloid liverworts such as Conocephalum conicum and Lunularia cruciata.

Transmission electron microscopy revealed the cellular costs of this containment strategy. The ventral cuticle in metal-exposed plants averaged 34.6 nanometres, more than double the 14.4 nanometres seen in controls, suggesting reinforcement of a physical barrier. Chloroplasts showed abnormal enlargement of starch grains and proliferation of plastoglobuli, lipoprotein particles linked to stress responses. Numerous electron-dense cytoplasmic vesicles, possibly peroxisomes, appeared near chloroplasts, consistent with heightened oxidative stress management. In some cells, cytoplasmic deterioration and loss of structural integrity were observed.

Sex differences added another layer of intrigue. Male gametophytes accumulated 1.3 to 1.5 times more cadmium and lead than females, despite being slightly smaller and producing fewer gemmae cups. The researchers hypothesise that females may restrict toxic uptake as an adaptive strategy to protect sporophyte development, though conflicting evidence from other studies on cadmium tolerance in this species means the pattern warrants further investigation.

The findings carry practical implications for biomonitoring. Because zinc and lead concentrations in thalli track soil availability linearly, the species could serve as a reliable indicator of those two metals in soil-based assessments. Cadmium, with its saturating response, cannot be used quantitatively in the same way. The study also effectively debunks an older suggestion that M. polymorpha might be a lead hyperaccumulator under natural soil conditions.

Ultimately, the researchers conclude that M. polymorpha subsp. ruderalis is only weakly tolerant to zinc, cadmium and lead drawn from soil. The species survives and continues to reproduce vegetatively, thanks to a toolkit of phytochelatin-mediated detoxification, tissue-level sequestration and structural reinforcement, but persistence on contaminated ground comes at a measurable fitness cost. As legacy smelter soils and urban metal contamination continue to shape ecosystems worldwide, this small liverwort offers a window into the evolutionary trade-offs that govern survival at the margins of habitability.

Subject of Research: Metal uptake, tolerance mechanisms and fitness costs in the liverwort Marchantia polymorpha grown on zinc, cadmium and lead contaminated soils

Article Title: Distinct accumulation behaviour of Marchantia polymorpha L. grown along a soil metal gradient triggers fitness penalties

Article References: Distinct accumulation behaviour of Marchantia polymorpha L. grown along a soil metal gradient triggers fitness penalties. (n.d.). https://doi.org/10.1007/s11104-026-09105-2

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09105-2

Keywords: Marchantia polymorpha, liverwort, heavy metals, cadmium, lead, zinc, metalliferous soil, bioindicator, micro-PIXE, transmission electron microscopy, phytochelatins, plant stress