Morphological study identifies three Japanese Pterostichus species and maps regional diversity

A morphology-based revision of the endemic Japanese Pterostichus macrogenys species group identified three previously unknown species, extended the known distributions of three named species and produced the first broad morphological phylogenetic analysis of nearly the entire group. The study, based on 41 of the group’s 42 members and specimens collected in northern Tohoku, also documented a new sympatric pair in Akita Prefecture. Its geographic analysis found the greatest species diversity in Yamagata and Niigata prefectures and generally higher diversity on the Sea of Japan side of the range than on the Pacific side. The authors described the evolutionary interpretation as preliminary because many nodes in the strict consensus tree remained unresolved and many bootstrap values were low.

The macrogenys group belongs to the subgenus Nialoe and comprises medium to large carabid beetles measuring 12–20 millimeters. Members have large heads, long mandibles, flattened bodies, relatively simple secondary sexual characteristics on the male last abdominal segment and completely atrophied hind wings. Because these wingless beetles can be very similar externally, researchers primarily identify males by the form of the everted and inflated endophallus, the membranous internal portion of the aedeagus. Females are usually assigned to species by matching them to identified males collected at the same site because female external morphology alone generally does not permit reliable identification.

Most of the newly collected specimens came from subterranean baited traps using silkworm pupa powder as an attractant, while one specimen was hand-collected from soil. The authors examined the external anatomy of adults and the male and female genital structures, recording 12 characters for the phylogenetic analysis: ten male genital characters, one female genital character and one female external character. Character states were coded as unordered and equally weighted. The analysis was conducted in TNT version 1.6 using maximum parsimony, Wagner starting trees, 100 tree searches and tree bisection and reconnection, with 1,000 bootstrap replicates. Pterostichus micropoides was selected as the outgroup, while P. awashimaensis was excluded because no male specimen was available.

Three species were formally described from northern Tohoku. Pterostichus namahage was established from a male holotype collected at Haraikawa in Funagawaminato on the Oga Peninsula, Akita Prefecture. It has a glossy reddish-brown body, a large head, long hooked mandibles, weakly convex elytra and atrophied hind wings. Its species-specific endophallic features include a short, unbent left apical lobe with a bifurcated tip, a protruding basal part of the left pre-apical lobe and a short, nearly straight right paramere. The specific name refers to the Namahage folk event on the Oga Peninsula. Because no female was available, the description of P. namahage is based on a single male specimen.

Pterostichus kamurosanus was described from a male holotype and six female paratypes collected around Ariya in Kaneyama, Yamagata Prefecture, at elevations of about 403–416 meters. Male body length, measured from the mandible apices to the elytral end, ranged from 14.46 to 15.97 millimeters; female measurements ranged from 14.19 to 15.08 millimeters. Its male endophallus has a short left apical lobe, an apically projecting left pre-apical lobe and a weakly sclerotized left pigmented band. The seventh abdominal sternum is only shallowly concave in males, while the female’s innermost vaginal region is pigmented. The name kamurosanus refers to Mount Kamuro, where the type specimens were collected.

The third new species, Pterostichus atsumidakensis, was represented by a male holotype and one female paratype from Take-no-koshi near Mount Atsumi in Tsuruoka, Yamagata Prefecture. The holotype measured 16.54 millimeters from the mandible apices to the elytral end, with a clypeal-apex-to-elytral-end measurement of 14.36 millimeters. Diagnostic male structures include a short, stout right pre-apical lobe with a hooked apex, a slender and slightly bent left apical lobe with a weakly sclerotized surface, and a right paramere bent at an acute angle near its apical third. Unlike the female of P. kamurosanus, the female paratype of P. atsumidakensis lacks pigmentation in the innermost part of the vagina. Its name derives from Mount Atsumi and the locality suffix -ensis.

The collection records altered the known ranges of three established species. Pterostichus asahinus was recorded from Akita Prefecture for the first time and at its northernmost known locality. P. chokaisanus was found at the southernmost known locality, Zaō-chūō-kōgen in Yamagata Prefecture, following a previous record in Akita. At Momoyake, at an elevation of 1,030 meters in Akita Prefecture, one P. asahinus male and three P. chokaisanus specimens—one male and two females—were collected together. This was the first documented sympatric occurrence of the species pair. P. asahinus was larger in the available comparison, but that comparison involved only one male from each species, so the authors treated the observation as evidence consistent with reproductive isolation rather than a comprehensive test of that isolation.

Pterostichus kitakamisanus was also documented at its southernmost known locality, at Ōdateminami in Ayukawahama, Ishinomaki, Miyagi Prefecture. Its identification was based on comparison with the male holotype, including a left pre-apical lobe directed laterally, a character state found only in P. kitakamisanus among the known members of the macrogenys group. The new Miyagi record and the locations of previously collected specimens suggest that the species may be broadly distributed in the Kitakami Mountains, although the study did not survey the full extent of that range. Together, the new species and range records filled a reporting gap in northern Tohoku, where the group had received no published species or distribution records since 2009.

Maximum-parsimony analysis produced 80 most-parsimonious trees, each with a length of 40, from which the authors derived a strict consensus tree. One recovered clade united P. takadateyamanus, P. atsumidakensis, P. shinbodakensis and P. asahinus. Within it, P. shinbodakensis and P. asahinus were sister taxa, supported by one unambiguous and one possible synapomorphy, while P. atsumidakensis and P. takadateyamanus occupied basal positions in a polytomy. P. chokaisanus and P. tateishiyamanus were also recovered as sister taxa. A further clade contained P. ohsawacavus, P. sumondakensis, P. gassanus and P. macrogenys, and an 18-taxon clade comprised species distributed mainly in western areas of the group’s range. Many other relationships remained unresolved, and low bootstrap support in parts of the tree constrained the conclusions that could be drawn.

The regional compilation counted nine species in each of Yamagata and Niigata prefectures, six each in Akita, Nagano and Gifu, four each in Fukushima and Tochigi, and three in Miyagi. Other prefectures contained one or two species. In general, the Sea of Japan side of the distribution contained more species than the Pacific side, and the central part of the range contained more than its northern and southern portions. These are geographic contrasts in recorded diversity, not evidence of a decline through time. Counts for Nagano and Gifu require particular caution because some closely distributed, morphologically similar species have not had their endophallic structures compared, a critical step in confirming their status as distinct species.

The authors proposed that initial differentiation of several clades occurred on the Sea of Japan side, probably around Yamagata and Niigata, followed by dispersal into other regions and further geographic differentiation. This interpretation was based partly on the narrower distributions of basal species and the wider ranges of species bearing more apomorphic traits. A related hypothesis was that the formation of the Sea of Japan may have been associated with the group’s origin, given that many species in the sister opacipennis group occur in north-eastern China, northern Korea and Russian Primorye, across the sea from the proposed area of differentiation. The study also found that most known sympatric species were not close relatives. The authors interpreted this pattern as suggesting that body-size differences contributing to coexistence may be ancient, but the preliminary tree, limited sympatric records and small available samples leave that evolutionary interpretation open to revision.

Subject of Research: Morphological revision and phylogenetic analysis of the Japanese Pterostichus macrogenys beetle group

Article Title: A morphology-based revision and phylogenetic analysis of the Pterostichus macrogenys species group ( Coleoptera , Carabidae ) and implications for differentiation of the species group

Article References: Sasakawa, K., & Mitsuduka, Y. (2023). A morphology-based revision and phylogenetic analysis of the Pterostichus macrogenys species group (Coleoptera, Carabidae) and implications for differentiation of the species group. Deutsche Entomologische Zeitschrift, 70(2), 291-310. https://doi.org/10.3897/dez.70.107322

Image Credits: AI Generated

DOI: 10.3897/dez.70.107322

Keywords: Pterostichus beetles, taxonomic revision, species discovery, morphological phylogeny, Japanese biodiversity, ground beetles, Carabidae, endophallus morphology, sympatric species, regional diversity, morphology-based, revision