Four new species of Unixenus Jones, 1944 (Diplopoda: Penicillata: Polyxenidae) from the mid-central coastal region and two islands off the coast of Vietnam

Abstract

Four new species of Unixenus Jones, 1944, U. binhdinhensis sp. nov., U. lysonensis sp. nov., U. phanrangensis sp. nov. and U. phuquyensis sp. nov., are described from grassland habitats in the mid-central coastal region of Vietnam and two offshore islands. All four species share the diagnostic morphological characters typical of Unixenus. Due to their similar body size, overall morphology, and partially overlapping geographic distributions, these taxa are morphologically cryptic and difficult to distinguish using traditional taxonomic characters. Morphological comparisons suggest two closely related species groups with mainland coastal and offshore island distributions. However, molecular phylogeny reveals clear genetic differentiation among all four taxa, supporting their recognition as distinct species. This study also highlights the presence of cryptic diversity within Unixenus and demonstrates the importance of integrative taxonomy, incorporating molecular phylogenetics, for resolving species boundaries in morphologically conservative penicillate millipede lineages from Southeast Asia. A key to the Vietnamese species of Unixenus is also included in this study.