Mountainous millipedes in Vietnam. I. Two new species of the family Paradoxosomatidae from Mount Fansipan (Diplopoda, Polydesmida)

Abstract Two new paradoxosomatid millipede species were discovered at the high elevations of Mount Fansipan in northwestern Vietnam. They are named as Orthomorphoides sapasp. nov. and Hylomus solenophorussp. nov. In addition to their morphological descriptions, sequences from fragments of two mitochondrial genes, COI and 16S rRNA, are also provided for both new species.


Introduction
At least three-quarters of Vietnam's landscape is covered by mountains and hills (Le Ba Thao 2017). Not surprisingly then, this mountainous region is characterised by high biodiversity with many new species and genera being discovered annually (Sterling et al. 2006). Access to the high mountains is, however, very difficult resulting in limited biodiversity investigations, with millipedes from mountainous regions being particularly poorly known (Nguyen et al. 2019).
Among the high mountains in Vietnam, Mount Fansipan is very well-known as Vietnam's highest peak, 3143 m a.s.l. It is located in the Hoang Lien Son Mountain Range, the southeasternmost extension of the Himalaya Range. The biodiversity of this mountain is very rich, containing both temperate and subtropical elements and both low-and highland species (Sterling et al. 2006). Little is known about the millipede fauna of Mount Fansipan. To date, only 23 species have been reported from this mountain and almost all of them are considered to be endemic to the region (Attems 1938(Attems , 1953Golovatch 1984Golovatch , 2009Enghoff 1987;Enghoff 1993, 1994;Nguyen et al. 2005Nguyen et al. , 2019Nguyen 2012) (Table 1). This work will contribute to a better understanding of the millipede fauna of Mount Fansipan by describing two new paradoxosomatid species found there. Both morphological and molecular data are provided for the new species.

Material and methods
Material was collected from high elevations of Mount Fansipan, northwestern Vietnam, and preserved in 90% ethanol. Specimens were observed under an Olympus SZX10 microscope.
All holotypes and paratypes were deposited in the Department of Soil Ecology, Institute of Ecology and Biological Resources, Vietnam Academy of Science and Technology, Hanoi, Vietnam. Diagnosis. The species is easily recognized by having a black sub-moniliform body, small and crest-shaped paraterga, a long and slender gonofemorite, a simple solenophore with neither modifications nor additional processes, a poorly developed lamina medialis, a well-developed lamina lateralis with a tongue-shaped process, and the gonopod tip with a broad apical lobule.

Taxonomy
Description. Holotype body length about 22.1 mm, width of pro-and metazona 1.6 mm and 2.1 mm, respectively.    segments. Waist between pro-and metazonae relatively deep, striolate. Paraterga present, but small, crest-shaped, lying lower than metatergal surface. Pleurosternal carinae present as complete crests on segments 2-3, reduced to caudal teeth on segments 4-7, and missing on subsequent segments. Axial line missing.
DNA barcoding. Fragments of COI and 16S rRNA genes were uploaded to GenBank with the accession numbers: MW647898 and MW648327, respectively. The new species has a close COI gene similarity with Orthomorphoides setosus (KU234720) of 87.17%. It also shares 74.04% and 72.68% of its 16S rRNA gene sequence with Asiomorpha coarctata (KU721885) and Pogonosternum nigrovirgatum (KU745218), respectively.
Etymology. Named after Sapa, the type locality. It is a noun in apposition.
This new species obviously belongs to the genus Orthomorphoides because of its generic characters, such as: long and slender femorite without visible modifications and processes, and the solenomere being sheathed by a solenophore except for the exposed tip. The new species does, however, clearly differ from the two known Orthomorphoides species in body shape and the degree of development of the paraterga. In addition, the solenophore of the new species carries a tuberculiform process laterally while neither O. setosus nor O. exaratus possess additional processes on the solenophore.
The two previously described Orthomorphoides species were found only in high mountains in southcentral Vietnam and Xieng Khouang (>1500 m a.s.l.) (Attems 1953). The discovery of this new species, found in Sapa, may further support the mountainous distribution of this genus, although this must be confirmed with additional field studies and possible discoveries.
Description. Holotype body length about 16.0 mm, width of pro-and metazona 1.2 mm and 2.0 mm, respectively.
Remarks. The genus Hylomus Cook & Loomis, 1924 used to be a synonym of the genus Desmoxytes Chamberlin, 1923 (Golovatch and Enghoff 1994), but it was recently re-evaluated by Srisonchai et al. (2018) and currently comprises 39 species (Sierwald and Spelda 2021). The new species differs distinctly from the majority of known Hylomus species (except for H. spectabilis) in having wing-shaped paraterga (vs. antler-like or spiniform paraterga). As already stated, Hylomus solenophorus sp. nov. is closely similar to H. spectabilis (Attems, 1937) from central Vietnam by having wing- shaped paraterga and the same gonopod conformation. On the other hand, the newly described species differs clearly from H. spectabilis by being smaller in size (length: 16 mm vs. 30 mm; width of metazona: 2.0 mm vs. 6.1 mm) and having darker body coloration (darkish brown or black vs. brown). Furthermore, leg femora of the new species possess no visible modifications, metaterga contain two rows of 2 + 2 and 3 + 3 setiferous knobs and the gonopod has no spine z, but possesses a triangular distoapical process, instead. By comparison, the 7 th femur of the H. spectabilis male has a ventral hump, metaterga are characterised by two transversal rows of 2 + 2 well-developed spines and 1 + 1 smaller spines and the gonopod is characterised by well-developed spine z, but no process k.

Discussion
The Hoang Lien Son Mountain Range with Mt. Fansipan plays a very important role in general geodistribution of animals in Vietnam, particularly of millipedes (Sterling et al. 2006). This mountain range is considered the southeasternmost extension of the Himalaya Range containing both Indian and Chinese zoological features. The Hoang Lien Son granites date from 80 to 29 million years ago, while the uplift of the range by tectonic activity began around 65 million years ago and continues to this day. This is also an interesting place for both ecological and biodiversity studies (Sterling et al. 2006). Despite its important role, the fauna of the mountain, in particular, the soil invertebrates, is still poorly known.
The Paradoxosomatidae is a huge family containing more than 1000 species distributed in about 220 genera (Nguyen and Sierwald 2013). Phylogenetic relationships between the paradoxosomatid genera and species within the family are still questionable. There are, however, some available DNA data for members of this family (183 COI gene records and 53 16S rRNA gene records in GenBank). It is not considered very practical to analyse the relationships between the two new species (belonging to different genera) and other known paradoxosomatid species using two short fragments of COI and 16S rRNA genes. As a result, although the DNA barcoding data are provided here for the two new species, neither the genetic divergence nor phylogenetic relationships are taken into account. The provided DNA barcoding data should be considered an additional characterisation tool for the new species descriptions and identification.

Conclusion
Most of World's high-mountain regions are very difficult to access, but their natural habitats are well conserved. It is, therefore, highly expected that more intensive surveys in those regions will reveal many new taxa, not only of millipedes but also of other invertebrates.