VOL.209 NOVEMBER 2025
ENJOYING JAPAN’S MARKETS
[Science & Technology―The Next Generation of Young Leaders] Understanding Changes in Marine Ecosystems to Promote Coexistence with Nature
A vibrant coral reef ecosystem. Coral reefs are home to a diverse array of marine life.
Photo:YASUDA Nina
The ocean, which covers about 70% of the Earth’s surface, is essential not only for human life but also for countless other species. Yet for those of us living on land, there are few opportunities to directly observe how the ocean is being affected by climate change or how marine ecosystems are evolving. We spoke with YASUDA Nina, a researcher studying marine life, to shed light on the changes currently occurring beneath the waves and to discuss the importance of monitoring and protecting these vital ocean resources.
The ocean plays a crucial role in mitigating climate change and is a treasure trove of both unknown biodiversity and marine resources. Among these, coral reefs are particularly important, hosting roughly 30% of all known marine species, says Professor YASUDA Nina, who conducts research on marine life at the Graduate School of Agricultural and Life Sciences, The University of Tokyo. Focusing on the crown-of-thorns starfish,1 YASUDA investigates the current state of marine organisms inhabiting coral reefs. Her work, “Research on the Conservation of Marine Ecosystems, in Particular Coral Reef Ecosystems, using an Integrated Approach,” has earned her high recognition, including the 21st (FY2024) JSPS Prize (see box for details).
According to YASUDA, the world’s coral reefs have declined by half since the 1950s, with the primary biological driver being coral predators such as the crown-of-thorns starfish. In an effort to unravel the mystery of how outbreaks of these starfish spread rapidly across different regions, YASUDA became the first in the world to collect and identify2 crown-of-thorns starfish larvae in the wild. She also analyzed the genetic profiles of crown-of-thorns starfish populations throughout the Indo-Pacific, including the Nansei Islands, revealing that in areas influenced by western boundary currents such as the Kuroshio Current,3 outbreaks are more likely to propagate through larval dispersal.
Photo: YASUDA Nina
Analysis of historical outbreak patterns in Japan over the past century has revealed that crown-of-thorns starfish larvae can disperse up to 100 km via ocean currents, leading to secondary outbreaks. This genetic analysis technique has also been applied to investigate reef-building corals4 such as Acropora and precious coral.5 Among corals that are morphologically very similar and inhabit the same regions, it has been found that many are in fact distinct species, each with different reproductive timings. These findings have shed light on how coral species have diverged over time.
According to a study by YAMANO Hiroya and colleagues at the National Institute for Environmental Studies, an examination of coral distribution in the western Pacific over the past 80 years suggests that some coral species are shifting toward higher latitudes (northeast and northwest). Before World War II, the northern limit for many coral species was considered to be off the coast of Wakayama Prefecture, Japan. In recent years, however, corals have been observed as far northeast as off the coasts of Kanagawa and Chiba Prefectures, and northwest to the Goto Islands and Tsushima (see Figure 1). Rising sea temperatures caused by global warming are believed to be one factor driving this northward movement. Similar northward shifts have also been observed in not only corals but also fish, other echinoderms, and crown-of-thorns starfish, which prey on corals.
“Ecosystems, not just in the ocean, are constantly changing; they are never static. When corals die due to stress exceeding a certain threshold, the foundational species of the ecosystem itself can be drastically altered, making it impossible for the ecosystem to fully recover even if the stress is later reduced. However, corals may be attempting to survive by moving their offspring into temperate regions. In Japan’s coastal waters, coral bleaching6 is reportedly occurring at twice the normal rate, driven not only by global warming but also by marine pollution caused by human activity. Regular underwater surveys are essential to accurately understand and communicate these changes in the marine environment. It is also crucial to continually consider how we can protect marine resources and adapt to coexist with the evolving ocean ecosystems,” YASUDA explains.
What happens in the ocean is never someone else’s concern. To continue coexisting with the sea—a resource that provides countless benefits to our daily lives—we need to carefully consider how our everyday actions impact the ocean and act accordingly.
An examination of the distribution of corals known as Acropora spp. shows that, over the years since the 1980s, their presence has been confirmed progressively farther north. Yellow circles indicate locations newly confirmed in the northwest, and red circles indicate those in the northeast. Today, corals have been observed as far north as off the coasts of Kanagawa and Chiba Prefectures.
Figure 1: Changes in the distribution of corals (Acropora spp.) in Japanese coastal waters
Created based on materials provided by YASUDA.
Figure 2: Distribution of coral genotypes in Japanese coastal waters
Created based on materials provided by YASUDA.
The JSPS Prize
In order to raise the level of scientific research in Japan to the world’s highest standard, it is important to recognize and support young researchers with rich creativity and superlative research ability at an early stage in their careers. To sustain the zeal of such researchers while offering them support in advancing their work, the Japan Society for the Promotion of Science7 established the JSPS Prize in FY2004. For more details, see the website of the Japan Society for the Promotion of Science.
- 1. A species of echinoderm with spiny surfaces, the crown-of-thorns starfish is a large starfish measuring approximately 15-30 cm in diameter. It inhabits coral reefs, with larvae feeding on phytoplankton and adults preying on coral. Outbreaks can occur every few years and are considered a major factor in coral decline.
- 2. The process of comparing previously unknown plants, animals, or microorganisms with existing references to determine and identify their species.
- 3. A warm current flowing northeast along Japan’s coast, also known as the Japan Current.
- 4. Reef-building corals with hard calcium carbonate skeletons grow relatively quickly and form dense colonies, constructing coral reefs. They typically inhabit shallow waters up to around 30 m deep.
- 5. Precious corals have very hard red or white skeletons and are found in deep waters over 100 m. True to their name, they are used in jewelry and other ornamental applications.
- 6. Corals normally live in symbiosis with algae called zooxanthellae, which provide mutual nutritional benefits. When sea temperatures rise or nutrient-rich runoff increases, the algae produce excessive reactive oxygen species, causing the corals to expel them. Without zooxanthellae, corals turn white, and if this condition persists, they can die from malnutrition.
- 7. Established in 1932 with an endowment from Emperor Showa, JSPS is today Japan’s only independent funding agency with a mission to advance science. JSPS carries out a wide variety of programs in pursuit of this mission. They include programs to fund scientific research, foster researchers, promote international scientific exchange, and reform university systems.
By FUKUDA Mitsuhiro
Photo: YASUDA Nina