Hidden Split: Japan’s Alpine Beetle Nebria taketoi Revealed as Two Species

Deep in the high mountains of central Honshu, a small ground beetle long assumed to be a single rare species has just been revealed as two. A taxonomic revision published in Alpine Entomology shows that specimens historically filed under the name Nebria (Falcinebria) taketoi Habu, 1962 actually belong to at least two distinct species: the true N. taketoi, confined so far as confirmed to the northern Hida Mountains, and an entirely new species, N. kobushicola sp. nov., described from the Okuchichibu and Yatsugatake mountains. The finding, by entomologist Kôji Sasakawa, matters far beyond the naming conventions of beetle specialists. It reshapes our understanding of how alpine insects are distributed across Japan’s sky islands and hints that more cryptic species may still be hiding in museum drawers.

N. taketoi has always been an enigma. Endemic to high-elevation zones on Honshu, it is one of the least-studied Japanese members of the genus Nebria, largely because it is so rarely collected. Only a handful of field records exist, and public collections hold limited material. Compounding the problem, the species carries a tangled nomenclatural history. Two of the three earlier taxonomic papers on it were written in Japanese, and researchers outside Japan repeatedly misread the record. Some catalogs even placed the type locality in the Yatsugatake Mountains, when Habu’s original 1962 description clearly designated Mikurigaike on Mount Tateyama in Toyama Prefecture. An older name, N. nakanei, proposed by Uéno in 1953 for specimens from the Azusagawa River area, was never validly published and remains unavailable under the International Code of Zoological Nomenclature, leaving N. taketoi as the correct name.

To untangle this confusion, the study deployed two analytical approaches that had proven their worth in related Nebria taxa but had never been applied to N. taketoi. The first is comparative morphology of the endophallus, the membranous inner sac of the male genitalia that, when fully everted and inflated, displays complex lobes and protrusions whose shapes carry rich taxonomic signal. The second is geometric morphometrics, a quantitative technique that captures subtle shape differences invisible to traditional measurements such as simple aspect ratios. Both methods had already demonstrated their utility in species-level and supra-species-level taxonomy of Nebria, including N. reflexa and its relatives, which belong to the same subgenus and are closely allied to N. taketoi.

The material examined came from museum and private collections across Japan, including the Gifu Prefectural Museum, the Hokkaido University Museum, the National Agriculture and Food Research Organization, the Tateyama Caldera Sabo Museum, and the collection of Kenta Sawada. In total, 10 male and 17 female specimens from 14 sites were analyzed, spanning a remarkable geographic range from the Iide Mountains in the north to Mount Kitadake in the Akaishi Mountains in the south, and west to Mount Ontakesan. Murôdodaira, only 70 meters from the type locality at Mikurigaike, was treated as effectively the same site as the type locality, anchoring the study to Habu’s original concept of the species.

The endophallus work required delicate preparation. The membranous sac was everted and fully inflated by injecting toothpaste through the base of the aedeagus, a technique that allows the internal structures to be examined in their expanded state. In some specimens, the gonopore protrusion and certain lobes resisted full eversion, a difficulty also encountered in N. reflexa and allied species. To avoid damaging these fragile structures, the researcher observed the endophallus with those parts not fully everted, relying on the shapes of the lobes that could be inflated. The homology and terminology of the endophallus structures followed established conventions from earlier work on the group.

For the geometric morphometrics, the dorsal outline of the pronotum, the shield-like plate behind the head, was quantified. Four landmarks were plotted at anatomically defined points, and 42 semi-landmarks were distributed at regular intervals along the contour between them. Raw coordinates were converted into Procrustes coordinates, removing variation due to rotation, position, and size, with semi-landmarks slid along the contours to minimize bending energy. Relative warp analysis then distilled the shape variation into interpretable axes. The first relative warp, accounting for 32.8 percent of total variance, captured whether the pronotum was square or cordate in outline. The second, accounting for 20.5 percent, captured the shape of the anterior angles, ranging from narrowly and strongly protruding to widely rounded and barely protruding.

The results were unambiguous. In the scatterplot of the first two relative warps, the areas occupied by N. taketoi and the newly described N. kobushicola did not overlap at all. Strikingly, none of the specimens whose species identity could not be determined fell within either species’ area, with the sole exception of a single male from Kitazawatôge Pass that landed just barely inside the male N. taketoi zone. This clean separation, achieved purely from pronotum shape, independently corroborated the species split suggested by the endophallus morphology, demonstrating how a taxonomically useful external character can reinforce conclusions drawn from genital anatomy.

The two species differ in consistent ways. N. taketoi has less protruded anterior angles on the pronotum, more densely punctate margins on the pronotum and on the ventral sides of some notal and abdominal segments, and lateroapical and dorsoapical lobes of the endophallus whose apices are not bifurcated. N. kobushicola, by contrast, shows more protruded anterior angles, sparser punctation on those surfaces, and, most tellingly, bifurcated apices on the lateroapical and dorsoapical lobes. Body lengths overlap broadly, with N. taketoi males measuring 10.29 to 10.94 millimeters and N. kobushicola males 10.78 to 11.08 millimeters, so external size alone would never have separated them. The absence of setigerous punctures on elytral stria 1 is a character found only in N. taketoi among the specimens previously treated under that name, though individual variation means it cannot serve as a definitive single-specimen diagnosis.

Beyond the species split, the study sheds light on the evolutionary history of the subgenus Falcinebria itself. The gonopore protrusion, a structure around the opening of the endophallus, appears to be a feature unique to N. reflexa and its relatives, and the study’s evidence suggests it is shared by both N. taketoi and N. kobushicola, making it a likely autapomorphy defining the subgenus. More provocatively, N. kobushicola shares two putative apomorphic endophallus characters with N. niohozana and N. dichotoma, species previously treated as N. reflexa, that N. taketoi lacks: T-shaped laterobasal lobes and a largely bifurcated apex of the dorsoapical lobe. One interpretation is that these similarities reflect true phylogenetic affinity, implying that the ancestor of the N. taketoi species complex evolved from a clade containing N. niohozana and N. dichotoma, and that N. kobushicola retains the ancestral condition. The alternative is convergent evolution driven by sexual selection, if those genital shapes confer reproductive advantages. Molecular phylogenetic work on the group will be needed to settle the question.

Perhaps the most tantalizing implication concerns the specimens whose identities remain unresolved. Nine females and two males from sites including the Iide Mountains, Mount Ontakesan, and Mount Kitadake could not be assigned because no suitable male material was available for endophallus examination. Some of these sites lie far from both the Hida Mountains and the Okuchichibu and Yatsugatake ranges; the Iide Mountains localities are more than 200 kilometers from the nearest confirmed populations. The morphometric analysis placed none of these specimens squarely within either known species’ shape space. That pattern raises the real possibility that additional cryptic species of alpine Nebria are waiting to be formally recognized, each confined to its own mountaintop refuge. As alpine habitats face mounting pressure from climate change, clarifying these hidden divisions is not just an academic exercise but a prerequisite for understanding what biodiversity Japan’s highest peaks actually harbor.

Subject of Research: Taxonomic revision of the alpine carabid beetle Nebria taketoi in Japan using endophallus morphology and geometric morphometrics

Article Title: Taxonomic study of the alpine carabid beetle Nebria ( Falcinebria ) taketoi Habu, 1962 ( Coleoptera , Carabidae )

Article References: Sasakawa, K. (2023). Taxonomic study of the alpine carabid beetle Nebria (Falcinebria) taketoi Habu, 1962 (Coleoptera, Carabidae). Alpine Entomology, 7, 185-194. https://doi.org/10.3897/alpento.7.109855

Image Credits: AI Generated

DOI: 10.3897/alpento.7.109855

Keywords: Nebria taketoi, Carabidae, ground beetles, taxonomy, geometric morphometrics, endophallus, Japan, alpine entomology, new species, Honshu mountains, Falcinebria, species delimitation