The Ogasawara Islands are a textbook example of oceanic island biogeography. Because they have never been connected to any continent, their fauna is relatively poor in species but rich in endemics, drawing from Palaearctic, Oriental and Micronesian biogeographic elements. Yet many terrestrial invertebrate groups remain poorly documented, and millipedes—a major component of soil macrofauna—had been especially neglected. Before this study, only ten identified millipede species had ever been recorded from the archipelago, most sporadically since the 1940s. The new surveys, which involved sifting leaf litter and searching under fallen logs and stones, revealed that the real diversity is considerably higher than previously appreciated.

Prosopodesmus jacobsoni, originally described from Java, Indonesia, is a pantropical species that has spread widely through human activity. It was found on Chichi-jima Island, particularly in the Susaki area, which was reclaimed as a military site in the 1930s and remains a frequently disturbed environment. Males of this species measure between 5.3 and 5.7 millimeters long and possess a head plus 20 body rings, while females range from 5.4 to 5.8 millimeters. The species is yellowish-brown, with a collum bearing 12 lobes on its anterior edge and ozopores on rings 5 and 7 through 19. It was collected from under fallen logs and stones in forests as well as from leaf litter accumulated among rocks near the coast.

Prosopodesmus panporus, by contrast, is far more mysterious. Described in 1980 from a hothouse at the Royal Botanic Gardens, Kew, in London, it was later recorded from the Cape York Peninsula in Australia, which is considered its likely native range. Its discovery on Haha-jima Island represents only the third known locality for this species anywhere in the world. Males are notably smaller than those of P. jacobsoni, measuring 3.5 to 3.7 millimeters, and are exceptional within the genus in having only 19 body rings rather than 20. Females retain 20 rings. The species is pale pinkish-beige and was collected from beneath fallen logs in a forested area near a paved road but far from settlements and the port.

Cryptocorypha ornata, originally described from Hawaii, is a pantropical species that was found on all four surveyed islands, indicating widespread establishment. Males measure 4.2 to 5.0 millimeters and females 4.7 to 6.1 millimeters, both with a head plus 20 rings. The body is considerably slender, with males dull pink and females slightly paler. Notably, the researchers reexamined specimens collected in 1990 that had been previously reported as Prosopodesmus sinuatus and discovered they had been misidentified; they actually belong to C. ornata. The gonopod morphology—quadripartite with a plumose solenomere, a densely spiculate velum, a slender endomere and a curved denticulate exomere—provides decisive diagnostic characters that clearly separate the two species.

Dyskolonius uniramus is perhaps the most remarkable find. Originally described from montane localities in Vietnam at elevations above 800 meters, it had not been recorded again since the original description in 1938. The present study therefore represents the first record of this species in 88 years. In the Ogasawara Islands, it was collected from Ototo-jima Island, which is currently uninhabited, and from relatively undisturbed forests on Haha-jima Island, but not from disturbed habitats such as settlements or agricultural areas. Males measure 4.0 to 4.4 millimeters with a head plus 20 rings, and are pale pinkish-beige to brown, with a collum bearing 10 lobes on its anterior edge.

Solaenaulus butteli, described from Sumatra, Indonesia, has since been reported from a wide range of localities across Southeast Asia. It was found on all four surveyed islands, with collection sites located relatively distant from settlements and the port. Males measure 4.8 to 6.0 millimeters with a head plus 19 rings, while females measure 6.2 to 6.7 millimeters with 20 rings. The species is smoky pink, and the researchers also resolved a long-standing question about a specimen recorded from Chichi-jima Island in 1988 as Solaenaulus sp., confirming it as S. butteli based on detailed sketches of the gonopods and comparison with the original description.

Beyond the taxonomic discoveries, the study provided an unprecedented look at postembryonic development in Prosopodesmus, a genus known for intrageneric polymorphism in the number of body rings in adult males. Millipedes undergo anamorphosis, adding body rings at each moult, so differences in adult ring number directly reflect differences in the timing of sexual maturation. The researchers collected numerous juvenile specimens representing each developmental stage from stadium IV through adulthood, enabling them to document the formation of gonopod primordia on the ventral side of ring 7 in males across all five stadia examined.

In P. jacobsoni, gonopod primordia progress from papillary structures in stadium IV juveniles—slightly smaller than the coxa of the walking leg on ring 7 and with no medial contact—to irregularly circular forms in stadium VI, and then to semicircular primordia in stadium VII that are approximately 1.5 times wider than the leg coxa and divided into anterior and posterior regions by an oblique groove. In adults, the gonopods are semicircular in ventral view, each slightly exceeding twice the diameter of the leg coxa, with a coarse reticulate surface texture on the coxa and a well-developed telopodite bearing dense setae at its base. Males of P. jacobsoni reach adulthood at stadium VIII, consistent with the typical pattern for Polydesmida.

In P. panporus, the developmental schedule is shifted. A male stadium VI juvenile already possessed semicircular gonopod primordia divided into anterior and posterior regions by an oblique groove, with a very short projection on the medial side of the anterior region—features that in P. jacobsoni do not appear until stadium VII. This suggests that gonopod development in P. panporus is accelerated by one stadium, with males attaining adulthood at stadium VII rather than stadium VIII. The most parsimonious explanation is a heterochronic shift in the timing of sexual maturation, whereby males of P. panporus mature one stadium earlier than their congeners. Comparable intrageneric variation in adult male ring number has been reported in other polydesmidan genera, including Eutrichodesmus and Cryptocorypha, suggesting that heterochronic evolution of this kind may have arisen repeatedly within the order. The researchers note that stadium IV and V juveniles of P. panporus were not available in the present study, so further observations will be required to confirm the nature and extent of this developmental acceleration.

The contrasting distribution patterns among the five species may reflect differences in introduction history, dispersal ability or ecological requirements. P. jacobsoni, C. ornata and S. butteli are widely distributed in tropical regions and have previously been suggested to expand their ranges in association with human activities, making unintentional introduction through planting activities or soil transport the most likely explanation for their presence in the Ogasawara Islands. C. ornata was collected in 1990 and S. butteli in 1977, suggesting both had already invaded and become established by that time but were previously overlooked. P. panporus and D. uniramus, however, show markedly different patterns, and while D. uniramus may have occurred naturally given its restriction to undisturbed forests, the possibility of human-mediated introduction cannot be excluded.

With the addition of five species and the removal of the misidentified P. sinuatus, the number of millipede species recorded from the Ogasawara Islands now totals 14. Only a single species, Nedyopus boninensis, is currently considered endemic to the archipelago, and at least about half of the recorded species are likely derived from human-mediated introductions. The discovery of as many as five additional species during a short-term survey strongly suggests that the total number will increase further with continued investigation. The researchers propose new Japanese names for each species and emphasize that molecular phylogenetic approaches—estimating genetic distances among populations from different localities—will be essential to clarify how these millipedes came to occupy one of Japan’s most isolated island chains.

Subject of Research: Five polydesmidan millipede species are newly recorded from the Ogasawara Islands, Japan, including notes on the postembryonic development of Prosopodesmus spp.

Article Title: New records of five polydesmidan millipedes ( Diplopoda , Polydesmida ) from the Ogasawara Islands, Japan, with notes on the postembryonic development of Prosopodesmus spp. ( Haplodesmidae )

Article References: Chiyoda, S., Hisasue, Y., Yoshino, H., Kuwahara, R., & Miura, T. (2026). New records of five polydesmidan millipedes (Diplopoda, Polydesmida) from the Ogasawara Islands, Japan, with notes on the postembryonic development of Prosopodesmus spp. (Haplodesmidae). Biodiversity Data Journal, 14, Article e194219. https://doi.org/10.3897/bdj.14.e194219

Image Credits: AI Generated

DOI: 10.3897/BDJ.14.e194219

Keywords: millipedes, Ogasawara Islands, Polydesmida, Prosopodesmus, postembryonic development, heterochrony, island biogeography, Biodiversity Data Journal, records, five, polydesmidan, Diplopoda