World's first mass collapse of Jeju's protected soft corals documented

A mass collapse of soft coral colonies — natural monuments that give Jeju's waters their vivid colors — was documented for the first time last summer, researchers announced Tuesday. The research team said the event, likely driven by a combination of abnormally high water temperatures and prolonged low salinity, serves as a warning signal of climate-driven disruption to marine ecosystems.

The Korea Institute of Ocean Science and Technology (KIOST) said a research team led by Dr. Kim Tae-hoon was the first in the world to observe and record the physical collapse of soft coral colonies along the Jeju coast, formally naming the phenomenon "slumping" in academic literature.

The study began after Paran, a Jeju-based environmental group, filmed abnormal changes in soft corals in the waters off Seogwipo last summer and shared the footage with the research team.

The study area, the Seogwipo marine zone, is home to tropical and subtropical species and is protected as a natural monument — the Jeju Coastal Soft Coral Colony — recognized as rare even by international standards.

According to the researchers, slumping follows a distinct progression: the soft coral first swells abnormally, then its stalk droops and its body inverts and hangs downward, before the entire structure deflates like a punctured balloon and ultimately collapses.

Researchers pointed to exceptional ocean conditions recorded in the waters south of Jeju last year as the likely cause. In 2024, the area simultaneously recorded its highest average water temperature and lowest average salinity in a decade. The team concluded that soft corals already under physiological stress from elevated temperatures were then exposed to low-salinity conditions for more than 50 days due to freshwater inflows from the Yangtze River, disrupting their ability to regulate osmotic pressure.

The research team also developed a new index — the DFW, or Degree Freshening Week — that captures both the duration and intensity of low-salinity exposure. Analysis showed that 2024 was the year soft corals experienced the most severe low-salinity stress in the past decade. The team said the index is expected to help provide early diagnosis of low-salinity risk for soft corals and other marine organisms.

The findings were published in the international journal Scientific Reports.

"This research is the first in the world to observe and document the collapse of soft corals and to propose a cause," Kim said. "The loss of Jeju's soft coral colonies could affect not only the marine ecosystem but also the broader community, including fisheries and tourism." He added that building an AI-based marine ecosystem monitoring system was necessary, rather than simply confirming abnormal ecological events after the fact.