2025-12-19 海洋研究開発機構,昭和医科大学,東京大学,北海道大学
図1 リクヒモムシ属の地理的分布と遺伝的分化
ミトコンドリアCOX1遺伝子の部分配列(573 bp)に基づく分子系統解析※4 の結果、小笠原系統(オレンジ)と、バミューダ諸島・キューバ・マルティニーク島・ニューカレドニア・沖縄諸島などに分布する広域分布系統(青)は明瞭に分かれ、国内には両系統が分布していることが明らかになった。 (CC BY-NC-ND 4.0に基づき論文より引用し日本語に改変)
<関連情報>
- https://www.jamstec.go.jp/j/about/press_release/20251219_2/
- https://link.springer.com/article/10.1186/s12862-025-02468-7
ユネスコ世界遺産小笠原諸島における陸生紐状動物の種レベルの多様性がミトゲノミクスによって明らかに Unrecognized species-level diversity of terrestrial nemerteans in the UNESCO world heritage Ogasawara Islands revealed by mitogenomics
Natsumi Hookabe,Shimpei F. Hiruta,Akinori Yabuki,Hiroki Yoshino,Yu Hisasue,Naoto Sawada,Rei Ueshima & Hiroshi Kajihara
BMC Ecology and Evolution Published:19 December 2025
DOI:https://doi.org/10.1186/s12862-025-02468-7
Abstract
Background
The terrestrial ribbon worm Geonemertes pelaensis Semper, 1863 (phylum Nemertea) is widely reported from tropical regions worldwide. In Japan, this species has been recorded from subtropical islands including the Ogasawara Islands, a UNESCO World Heritage Site south of Tokyo recognized for its unique biodiversity, where it has been implicated in the decline of native soil invertebrates. Here, we demonstrate that the nemerteans in the Ogasawara Islands are genetically and morphologically distinct from those found on Yonaguni Island (Okinawa, Japan), indicating the presence of at least two separate species in Japan.
Results
We sequenced the complete mitochondrial genomes of both populations (18,755 bp for Ogasawara; 31,745 bp for Yonaguni), revealing substantial differences in genome size and gene arrangement. The mitochondrial genome of the Yonaguni population is unusually large, exceeding typical sizes reported for metazoans. Uncorrected p-distances in cytochrome c oxidase subunit I (COX1) sequences between the two populations ranged from 6.75 to 8.59%, which is above the widely used threshold for intraspecific variation in nemerteans. Morphological comparisons also support species-level distinction: live specimens from Yonaguni have a pale body with a prominent mid-dorsal stripe (body width-to-stripe ratio: 1:0.078–0.110), whereas individuals from Ogasawara are pale to light brown with a narrower and fading stripe (ratio: 1:0.042–0.050). Moreover, accessory-stylet pouches differ between populations: Yonaguni specimens possess four to five pouches, each containing 3–5 stylets, while Ogasawara specimens have two pouches, each with two stylets. Examination of museum specimens collected in the 1980s from Chichijima showed the extremely similar external morphology as our recent Ogasawara specimens, indicating that this form has been the only Geonemertes species in the Ogasawara Islands for nearly half a century.
Conclusions
Our results indicate the presence of species-level diversity in Japanese terrestrial nemerteans and demonstrate that accurate species identification using molecular barcodes is essential in insular ecosystems. Recognizing cryptic or pseudocryptic lineages is critical for effective biodiversity monitoring and for preventing mismanagement in ecologically sensitive regions such as the Ogasawara Islands.
