For marine organisms to adapt to freshwater, they must overcome a range of challenges, including regulating osmotic pressure and resisting river currents. The progression from sea to freshwater to land represents a transitional step before organisms become fully terrestrial, making the investigation of freshwater adaptation mechanisms a biologically significant undertaking.
Master's Student Satoshi Shimooka and Lecturer Naoto Jimi of the Sugashima Marine Biological Laboratory, Graduate School of Science at Nagoya University, in collaboration with Nagano University, Ryukoku University, Niigata University, and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), have discovered a new species of polychaete worm in the river freshwater zone on Sado Island and named it "kawa-sunagokai." Among polychaetes, few groups have successfully colonized freshwater environments. It is especially rare among the scale worms (family Sigalionidae) to which kawa-sunagokai belongs, making this the first such discovery in Japan and only the second in the world. The findings were published in Zoological Science.
Kawa-sunagokai was found in the gravel of the lower reaches of three rivers where freshwater flows. It was first discovered during the fieldwork of students from Niigata University. Subsequent surveys have also collected specimens from the top of a concrete weir installed in the river and from a rapid-current zone approximately 700 meters upstream.
The family Sigalionidae, to which the newly discovered worm belongs, is highly species-rich, comprising approximately 900 species adapted to diverse environments from shallow to deep seas, yet freshwater-dwelling species within this family are exceptionally rare.
Since no DNA data had previously existed for freshwater scale worms, the researchers extracted DNA from kawa-sunagokai and multiple species of the same genus to construct, for the first time, a phylogenetic tree of scale worms that includes freshwater species. The results revealed that it is a unique species among marine-dwelling close relatives that colonized freshwater.
The species epithet of kawa-sunagokai's scientific name, Pisione mizuchi, is derived from "mizuchi," which refers to a dragon-like creature that inhabits areas along rivers and other water bodies in Japanese and Chinese legend. Reminiscent of the Chinese legend of the "Dragon Gate," where a carp climbs a waterfall and transforms into a dragon, kawa-sunagokai has achieved a dramatic habitat transition from sea to river. This newly found species symbolizes the unique biodiversity of Japan's inland waters and is expected to provide an important clue to understanding the evolutionary process by which marine organisms adapt to new environments.
At present, the mechanisms of freshwater adaptation, including how kawa-sunagokai lives and reproduces in river environments and how it regulates osmotic pressure, remain unknown. It is hoped that more detailed surveys in the future will reveal its ecology and unlock the key to its adaptation. Kawa-sunagokai, which is uniquely adapted to freshwater among many marine-dwelling close relatives, is expected to become a new model organism for investigating the mechanisms by which marine organisms colonize freshwater.
Journal Information
Publication: Zoological Science
Title: Freshwater Colonization by a Scaleless Scale Worm: Pisione mizuchi sp. nov. (Annelida: Sigalionidae), from Sado Island, Japan
DOI: 10.2108/zs250042
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.