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Background <p>The terrestrial ribbon worm <i>Geonemertes pelaensis</i> 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.</p> Results <p>We sequenced the complete mitochondrial genomes of both populations (18,755&#xa0;bp for Ogasawara; 31,745&#xa0;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 <i>p</i>-distances in cytochrome <i>c</i> oxidase subunit I (<i>COX1</i>) 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 <i>Geonemertes</i> species in the Ogasawara Islands for nearly half a century.</p> Conclusions <p>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.</p>

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Unrecognized species-level diversity of terrestrial nemerteans in the UNESCO world heritage Ogasawara Islands revealed by mitogenomics

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  • 发表日期：

    2025年12月19日

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  • Natsumi Hookabe,
  • Shimpei F. Hiruta,
  • Akinori Yabuki,
  • Hiroki Yoshino,
  • Yu Hisasue,
  • Naoto Sawada,
  • Rei Ueshima,
  • Hiroshi Kajihara

摘要

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.