Rhodoliths may look like ordinary pebbles scattered across the seafloor, but beneath the surface of the ocean off Tanegashima Island in Japan's Kagoshima Prefecture, these living pink rocks are quietly building ecosystems and storing carbon for millennia. Researchers from Hiroshima University have just revealed that the deeper waters surrounding this island—at depths of 35 to 38 meters—harbour an unexpectedly rich and distinct community of these calcifying red algae, including four species entirely new to science.
The discovery matters because rhodolith beds represent some of the largest seaweed habitats in the ocean, supporting marine biodiversity across a vast range of depths while also playing a crucial role in climate mitigation. By creating their calcified structures, rhodoliths lock away carbon in marine sediments, contributing to what scientists call blue carbon—a long-term storage mechanism that helps offset climate change. Yet for all their ecological importance, these hidden ecosystems remain poorly understood.
The research team, led by associate professor Aki Kato at Hiroshima University's Seto Inland Sea Carbon-neutral Research Center, collected samples by dredging the seafloor at two locations approximately 10 kilometers off the western coast of Tanegashima Island. Frozen at minus 20 degrees Celsius and carefully analysed using DNA sequencing alongside microscopy, the specimens revealed a surprising finding: at least 12 coralline algae species from five genera inhabited that small patch of seabed, but only three also appeared in the shallower waters nearby. This suggests the deeper habitat supports an entirely distinct biological community rather than simply extending the shallow ecosystem downward.
The four newly identified species—Orientalilithon compactum, Roseolithon aggregatum, Roseolithon sparsituberculatum, and Sporolithon variotuberculatum—represent the researchers' formal recognition of biodiversity that had remained unknown to science. In describing O. compactum, the team provided the first detailed documentation of both male and female reproductive structures in its genus, adding crucial morphological knowledge to the scientific record.
This discovery reflects the exceptional marine richness of the Tanegashima region. Due to the island's underwater geography and the influence of the warm Kuroshio Current, the mesophotic zone—the twilight depths of 30 to 150 meters where sunlight grows scarce—teems with algal diversity unlike anywhere else in the region. Yet the full extent of this hidden biodiversity has remained largely invisible to science until now.
The research also prompted the team to revise the descriptions of two coralline algal genera, underscoring how much remains to be learned about these ecosystems. Published in the Journal of Phycology, the findings demonstrate that completely different biological communities can thrive at different depths within the same marine area—a lesson that reshapes how scientists understand ocean habitats.
"Taxonomy provides an essential framework for making biodiversity visible," Kato explains, emphasizing that the work goes far beyond simply naming organisms based on genetic sequences. It requires careful examination of shape, structure, and anatomy to understand the biological units that form the foundation of marine life. As climate change reshapes ocean systems worldwide, understanding and protecting these carbon-storing ecosystems has never been more urgent. The living pink rocks off Tanegashima Island may appear unremarkable, but they represent an irreplaceable part of the ocean's hidden architecture.