Hidden Species Diversity Revealed by Mitochondrial Genome Analysis
-Land Nemerteans of the Ogasawara Islands Represent a Distinct Lineage, Not the Widespread Species-

1. Key Points

• Land nemerteans inhabiting the Ogasawara Islands have long been regarded as identical to a widespread species considered invasive and harmful to native island fauna. However, recent ecological studies suggested differences in prey preference between Ogasawara populations and the widespread lineage.
• In this study, researchers discovered that two genetically distinct lineages of land nemerteans occur in Japan: one in the Ogasawara Islands and another representing the widespread Indo-Pacific/Atlantic lineage (Fig. 1). The Ogasawara lineage differs from the widespread lineage in external morphology, mitochondrial genome structure, and gene order. Re-examination of museum specimens collected in the 1980s further confirmed the long-term presence of this lineage in Ogasawara.
• These findings demonstrate that hidden species diversity exists even among small, inconspicuous invertebrates, and highlight the importance of DNA-based species identification for island ecosystem conservation.

Figure 1. Geographic distribution and genetic divergence of Geonemertes
Molecular phylogenetic analysis based on partial mitochondrial COX1 sequences (573 bp) clearly separated the Ogasawara lineage (orange) from the widespread lineage (blue), which occurs in Bermuda, Cuba, Martinique, New Caledonia, and the Okinawa Islands. The results show that both lineages occur in Japan. (Adapted and translated from the original publication under CC BY-NC-ND 4.0.)

2. Overview

A research team led by Dr. Natsumi Hookabe at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), together with collaborators from Showa University, the University of Tokyo, and Hokkaido University, conducted a comprehensive analysis combining detailed morphological observations with mitochondrial genome-based phylogenetic methods to determine the species identity of land nemerteans in Japan.

Land nemerteans from the Ogasawara Islands have long been identified as Geonemertes pelaensis Semper, 1863-a species widely distributed across tropical and subtropical islands from the Pacific to the Atlantic. However, recent studies showed that individuals from Ogasawara exhibit prey preferences different from those of other regions, raising concerns about their ecological impact on the islands' native terrestrial fauna.

Because many nemerteans are difficult to distinguish based solely on external morphology, traditional morphological identification methods have been insufficient. Moreover, no DNA data had previously been obtained for Ogasawara populations.

To address this gap, the research team analyzed specimens from the Ogasawara Islands and from Yonaguni Island (Okinawa), sequencing their mitochondrial genomes and comparing both external and internal morphological characters.

The analyses revealed that Japan harbors two distinct species-level lineages of land nemerteans (Fig. 1):
- The Ogasawara lineage, which is genetically and morphologically unique
- The widespread lineage, including individuals from Yonaguni, found across islands in both the Pacific and Atlantic

The Ogasawara lineage differs markedly in mitochondrial genome size, gene arrangement, and external and internal morphology, indicating that it is not conspecific with G. pelaensis.

In addition, re-examination of museum specimens collected in the 1980s and preserved at the Naturalis Biodiversity Center in the Netherlands revealed morphological features identical to those of specimens collected in this study. This indicates that the same lineage has been present in the Ogasawara Islands for at least half a century.

This study reveals previously overlooked diversity among small terrestrial invertebrates and emphasizes the importance of long-term monitoring that integrates field surveys, museum collections, and molecular data-particularly in fragile island ecosystems where effective conservation management depends on accurate species identification.

The findings were published in BMC Ecology and Evolution on December 19 (JST).