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 species at the center of the experiment is Neohelice granulata, the granulated crab, a burrowing decapod whose engineering prowess ranks among the highest documented for any organism. Its relentless digging aerates the sediments of Argentina’s coastal lagoons, modifies nutrient flows, and creates the structural foundation on which entire salt marsh communities depend. Like many marine invertebrates, N. granulata times its reproduction to the lunar cycle, with females releasing larvae on nocturnal ebb tides. After roughly twenty days of development through four or five zoeal stages and a megalopa stage, the larvae return to the intertidal sediment to settle. That settlement window, when a microscopic megalopa commits to a patch of mud, is one of the most consequential moments in the animal’s life, and it is precisely the moment that artificial light appears to hijack.

The research team, led by Jesús Darío Nuñez and colleagues at the Instituto de Investigaciones Marinas y Costeras of the Universidad Nacional de Mar del Plata-CONICET, designed a field experiment to ask a deceptively simple question: does ALAN change how many larvae settle, and does the answer depend on the phase of the moon? Working in midsummer 2022 within the UNESCO Man and Biosphere Reserve, they deployed 30-watt white LED lamps with a 6000 K cool daylight spectrum on three-meter stainless-steel poles along the shoreline. The lamps sat two meters above the sediment, remained above the waterline at all tides, and were powered by solar panels that switched them on at dusk and off at dawn. Unlit control sites were established midway between adjacent lights to capture the natural baseline.

At each lit and unlit site, the researchers pressed four settlement mats, each ten by ten centimeters, into the mid-intertidal sediment in areas dense with juvenile and adult crabs. The mats were deployed at low tide, left in place for seven days, then collected and replaced with fresh mats the following week. Repeating this cycle across four consecutive weeks allowed the team to sample all four lunar phases: new, waxing, full, and waning. In total, 32 mats were deployed per lunar phase across the two treatments. Light measurements confirmed a stark contrast between conditions: ALAN sites averaged 11 lux on the ground, comparable to the glow near ports, harbors, and streetlights, while unlit sites averaged just 0.6 lux, with natural darkness dipping below 0.1 lux. Because moonlight itself ranges from roughly 0.1 to 0.3 lux, the lit sites were bathed in illumination tens of times brighter than the full moon.

Back in the laboratory, the researchers counted two distinct settlement stages under a microscope. Early settlers were megalopae, the final larval stage, while late settlers were early juvenile crabs. Densities were standardized per square centimeter of mat, and the resulting counts were analyzed with generalized linear mixed-effects models using Poisson error distributions, with lighting treatment as a fixed effect and settlement mats nested within sites as a random effect. Because each lunar phase was represented by only a single temporal instance, the team took a deliberately conservative approach, analyzing each phase independently rather than pooling across the cycle. Residual diagnostics were validated with simulation-based checks from the DHARMa package, significance was assessed with Wald chi-squared tests, and post hoc power analyses using the mixedpower package confirmed that the design had adequate sensitivity, with power consistently above 0.6 and exceeding 0.8 for the strongest effects.

The headline result is striking. During the full moon, early settlers were six times more abundant at lit sites than at unlit ones, with mean densities of 3.37 individuals per square centimeter under ALAN compared with 0.56 in the dark, a difference the models registered as highly significant. The statistical effect of ALAN on early settlers was confined to this phase; during the new, waxing, and waning moons, no significant treatment differences emerged. Yet the raw data told an even more intriguing story. During the waning and full moon phases, unlit mats contained no early settlers at all, while lit mats recorded presence in 33 percent of cases during the waning moon and 25 percent during the new moon. In other words, artificial light appeared to reveal settlement during periods when larvae were present at vanishingly low, otherwise undetectable abundances, suggesting a powerful phototactic attraction that can override the natural lunar cues that normally govern the process.

Late settlers, the early juvenile crabs, told a different and more muted story. Their densities nearly tripled under ALAN during the waning moon, rising from 0.62 to 1.55 individuals per square centimeter, and this was the only phase in which the effect reached statistical significance. The authors attribute this reduced sensitivity to ontogenetic shifts in sensory reliance: older, more competent larvae depend less on external cues than freshly arriving megalopae. Comparable stage-dependent responses to ALAN have been documented in terrestrial insects such as crickets, whose circadian rhythms are disrupted differently across developmental stages. Notably, few studies have examined late-stage responses to light pollution at all, making this one of the first datasets to show how artificial lighting differentially affects crustacean settlement across a life history transition.

Perhaps the most scientifically interesting aspect of the findings is what they contradict. Previous work on intertidal barnacles in the Southeast Pacific and Atlantic Canada found that ALAN consistently reduced settlement, particularly among late-stage larvae. The granulated crab responds in the opposite direction, at least during certain phases. The authors point out that phototactic behavior in crustacean larvae is notoriously inconsistent across taxa: blue crab larvae of Callinectes sapidus swim toward light, while Florida stone crab larvae of Menippe mercenaria avoid it, and even within a species the response can flip with developmental stage. Similar dual roles of ALAN, sometimes facilitative and sometimes neutral, have been observed in the sandhopper Talitrus saltator, whose migration routes shift under artificial lighting without any clear loss of abundance. The emerging picture is that light pollution does not simply disrupt ecological processes; it interacts with species-specific traits, developmental stages, and environmental cycles to produce complex, context-dependent outcomes that defy easy generalization.

The authors are careful to acknowledge the limits of their design. With only one temporal replication per lunar phase, they cannot conclusively separate true lunar effects from other seasonally covarying factors such as water temperature, wave action, or planktonic food availability. They also distinguish between settlement, the initial establishment of larvae in the habitat, and recruitment, the successful incorporation of those individuals into the adult population. Their study measured only the former, and processes such as post-settlement mortality, cannibalism, which earlier work suggests is actually intensified by ALAN in this species, or predation may erase any apparent settlement advantage before it translates into population growth. A small amount of light spillover, measured at 0.6 lux, may also have reached control stations, potentially attracting some larvae across treatments and highlighting how difficult it is to isolate experimental light fields in open coastal habitats.

Even with those caveats, the implications are considerable. Roughly 60 percent of major urban centers lie within 100 kilometers of a coastline, exposing more than 20 percent of coastal regions, including salt marshes near cities, to artificial light at night. As LED infrastructure proliferates globally, the kind of light field simulated in this experiment, 11 lux of cool white glow, is becoming the norm rather than the exception along the world’s urban shores. For an ecosystem engineer whose burrows oxygenate sediments, modulate nutrient loads, and structure entire marsh food webs, even subtle shifts in where and when larvae settle could ripple through the ecosystem services these wetlands provide, from carbon sequestration to nursery habitat for commercially important fish. The study’s authors argue that light pollution must now be treated as a genuine ecological variable in the assessment and management of salt marsh ecosystems, particularly across the southwest Atlantic where N. granulata anchors the marsh community. What a single field season in an Argentine lagoon reveals is that the night sky, increasingly rewritten by human hands, is no longer just a backdrop to coastal ecology but an active participant in it, and the crabs, it turns out, are reading every word of the new script.

Subject of Research: Effects of artificial light at night on lunar-phase-dependent larval settlement of the salt marsh crab Neohelice granulata

Article Title: The effect of artificial light at night on settlement patterns of a salt marshes crab

Article References: Nuñez, J. D., Pérez García, M., Merodio, C. B., & Luppi, T. A. (2025). The effect of artificial light at night on settlement patterns of a salt marshes crab. BMC Environmental Science, 2(1), Article 23. https://doi.org/10.1186/s44329-025-00038-2

Image Credits: AI Generated

DOI: 10.1186/s44329-025-00038-2

Keywords: artificial light at night, light pollution, Neohelice granulata, salt marsh, lunar cycles, crab settlement, larval ecology, marine invertebrates, ecosystem engineer, LED lighting, coastal ecology, Argentina