Naha ¨C At low tide, the sea withdraws from Okinawa¡¯s coast, exposing a landscape suspended between shore and ocean. Shallow pools shimmer in the morning light. Patches of seagrass sway beneath the surface. Across the reef flats, harvesters in wetsuits move slowly through knee-deep water, dragging blue mesh bags behind them and gathering tangled strands of brown seaweed from the seabed.
The work follows rhythms that have changed little over generations. Guided by tides and weather, harvesters collect mozuku, the slippery brown seaweed that has become one of Okinawa¡¯s most recognizable foods and one of its most important aquaculture products.
Hours later, some of that same seaweed will appear in bowls of mozuku udon. Elsewhere, it will be served chilled in vinegar or alongside bright green clusters of umibudo, or sea grapes, another Okinawa specialty. For Okinawans, it is an everyday ingredient. For visitors, it is often their first taste of the islands¡¯ marine culture.
Yet behind these familiar scenes, a quiet transformation is underway.
Rising ocean temperatures are placing increasing pressure on the seaweed industry, while the coral reefs that support Okinawa¡¯s coastal ecosystems are experiencing repeated bleaching events. In response, scientists are turning to genomics, fisheries cooperatives are investing in reef restoration, and both are searching for ways to protect a tradition whose future is no longer guaranteed.
More than a local delicacy
Mozuku occupies a unique place in Okinawa¡¯s economy and identity. More than 99% of Japan¡¯s mozuku production comes from the prefecture, and roughly 90% is cultivated through aquaculture.
Yet those figures conceal growing volatility. After producing more than 22,000 tons in 2007, Okinawa¡¯s harvest plunged to just over 8,000 tons in 2010 following unusually warm waters linked to the El Nino weather phenomenon. Although production later recovered, fluctuations have become increasingly common, fueling concern among those whose livelihoods depend on the sea.
¡°Twenty years ago, we could predict the season much more easily. These days, everything depends on the weather and the water temperature,¡± said Ken Mori, a local mozuku harvester. ¡°Some years are good, some years are not, and people worry about what things will be like for the next generation.¡±
For consumers, mozuku may simply be a familiar side dish or an ingredient in soups and noodle dishes. But the seaweed¡¯s importance extends well beyond the table. The species cultivated in Okinawa, Cladosiphon okamuranus, contains unusually high concentrations of fucoidan, a sulfated polysaccharide found only in brown algae. Responsible for the seaweed¡¯s characteristic slippery texture, fucoidan has attracted growing interest from the health food and cosmetics industries because of its reported antioxidant and antimicrobial properties.
Mozuku also plays an ecological role. Brown algae create habitats for fish and invertebrates, forming part of the web of life that sustains Okinawa¡¯s coastal waters. Yet despite its economic and ecological importance, surprisingly little was known about the seaweed¡¯s biology when Koki Nishitsuji first began studying it at the Okinawa Institute of Science and Technology.
¡°When I surveyed the field of algal research, I found that most studies focused on morphology, taxonomy and population genetics, while genomic research remained largely unexplored,¡± said Nishitsuji, now an associate professor at Fukui Prefectural University. ¡°At that time, only two brown algal species had been sequenced worldwide.¡±
The realization led him to launch a project that would eventually place Okinawa at the forefront of brown algae genomics.
¡°I realized that decoding the mozuku genome would allow us to lead global algal genomics research, provide valuable information for addressing future climate change-related challenges and raise international awareness of this unique Okinawan resource,¡± he said.
The reef beneath the harvest
The future of mozuku is intertwined with that of another marine species that is suffering from the effects of climate change: coral.
Mozuku grows on shallow reef flats where healthy coral ecosystems help shape local environmental conditions. Reefs reduce wave energy, stabilize sediments and support biodiversity throughout coastal habitats.
When those reefs wither, the effects ripple through the ecosystem.
Across Okinawa, repeated marine heat waves have triggered widespread coral bleaching.
¡°Okinawa¡¯s coral reefs have been affected by both climate change and local pressures such as coastal development,¡± said Parviz Tavakoli-Kolour, a coral eco-physiologist at the University of the Ryukyus. ¡°The most severe bleaching event I have observed occurred in 2024, resulting in ¡ high coral mortality across many reefs.¡±
The relationship between reefs and seaweed is one reason why fisheries cooperatives have become active participants in coral conservation. In the village of Onna, one of Okinawa¡¯s most important mozuku-producing regions, proceeds from seaweed production have helped fund coral cultivation and transplantation projects.
The logic is straightforward. If reefs help sustain the coastal environment on which seaweed farming depends, protecting reefs becomes an investment in the future of the harvest itself.
At the same time, researchers have noticed signs that some corals may be adapting to climate change.
¡°Some coral species and genera appeared more tolerant during the 2024 bleaching event compared to previous events, suggesting a degree of acclimation or adaptation,¡± Tavakoli-Kolour said.
Whether such adaptation can keep pace as the planet warms further remains uncertain. For seaweed researchers, that uncertainty has prompted a different strategy.
A race against warming seas
Nishitsuji and his team drove efforts to understand mozuku at the genetic level, hoping the genome could reveal traits linked to growth, reproduction and resilience to environmental change. Instead, the work opened a window onto a surprisingly complex organism whose biology had long remained poorly understood.
¡°One important discovery was that the germling stage could be propagated clonally, which made large-scale mozuku aquaculture possible in Okinawa,¡± Nishitsuji said.
The finding explained why an industry that now produces thousands of tons of seaweed each year could exist in the first place. By reproducing genetically identical germlings, farmers were able to maintain desirable strains and supply cultivation grounds on a commercial scale.
Yet the same life cycle that made large-scale farming possible also frustrated efforts to improve the crop. Mozuku undergoes a process known as heteromorphic alternation of generations, in which different stages of its life cycle look so dissimilar that they could easily be mistaken for separate species. Male and female germlings, meanwhile, are virtually indistinguishable, depriving researchers of one of the most basic tools available to plant breeders.
Selective breeding programs had identified strains with differences in texture, growth rates and yield, but systematic crossbreeding remained largely beyond reach. Unlike rice or other terrestrial crops, where breeders can easily control pollination, researchers studying brown algae lacked a reliable way to distinguish the sexes. Even the basic foundations needed to breed heat-tolerant varieties had yet to be established.
That changed in 2022, when scientists developed a method based on a polymerase chain reaction (PCR) that is capable of identifying male and female germlings. The advance, made possible by earlier genome sequencing work, gave researchers a way to deliberately cross strains with desirable characteristics and laid the foundation for breeding programs aimed at helping mozuku withstand a warming ocean.
The advance has opened the door to the development of heat-tolerant varieties that may be better equipped to survive future ocean warming.
Researchers are now working with Okinawan authorities and fisheries organizations to evaluate strains with different characteristics and identify combinations that could improve resilience.
Commercially viable heat-tolerant varieties have not yet emerged. But scientists say the foundation for climate-adapted breeding now exists.
Preserving the future
The search for climate resilience has also changed how researchers think about biodiversity.
Not every strain of mozuku is commercially attractive. Some grow slowly. Others produce lower yields or possess characteristics that farmers currently regard as undesirable.
Yet those same traits may become valuable under future environmental conditions.
¡°Some of these strains may appear economically unattractive,¡± Nishitsuji said. ¡°However, these seemingly low-value strains may hold tremendous scientific and genetic value.¡±
For scientists, preserving genetic diversity is increasingly viewed as a form of insurance against an uncertain future.
¡°Protecting this diversity today will provide the biological resources needed to address the challenges of tomorrow,¡± Nishitsuji said.
Back on Okinawa¡¯s reef flats, the tide is beginning to return. Harvesters haul bags of seaweed toward shore as water slowly covers the seabed once again.
To most observers, the scene appears timeless.
Yet hidden beneath this familiar harvest are two parallel efforts to adapt to a changing ocean. Offshore, coral restoration projects seek to rebuild ecosystems damaged by warming seas. In laboratories, scientists search for genetic traits that might help mozuku withstand the environmental conditions of the future.
Both are attempts to preserve something larger than a crop.
They are efforts to protect a coastal tradition whose survival now depends on forces far beyond the reef flats where each day¡¯s harvest begins.
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