Study design

This study designed by following process. First of all, distributional data of subgenus Psalidormus species has been organized in Table 1.

Data in Table 1 were used in this research with referring global Biodiversity Information Facility (GBIF) and each specimen holding institutions. The distributional data has been referred to occurrence data of the information uploaded to GBIF, and distributional data of key references stag beetle researchers such as Mizunuma, Nagai, Fujita, Huang, and Chen.

After distribution check, morphological data were analyzed by considering male morphology of each species. Especially head structures of male are distinctly differ in each species. Mandibular structure and shape of clypeus is the key morphology to its taxonomy. In mandibular structures, composition of large inner tooth and apical tooth group are considerable. Clypeus shape is organized in three types which considered as gradual changes in speciation process. Female morphology was not analyzed in here because of morphological difference between each species not remarkable as males.

Also, Bathymetric map under sea surface were drawn by author based on open street map to avoid copyright issues. Three bathymetric elevations which respectively 0 m, –750 m, –1,200 m, were used regard species boundary of distribution.

After all, hypothetical speciation history established based on information as mentioned above.

Materials

Materials used in this research mostly stored in the storage of National Science Museum Korea, National Taiwan Museum, and GBIF occurrence data set ( https://doi.org/10.15468/39omei).

Methods

Habitus photograph were taken by Nikon D7100 Digital Single Lens Reflex Camera (Nikon, Tokyo, Japan) with Tamron 60 mm Macro lens which mounted on Wemacro macro rail set with lighting stands. Photo of each specimen were taken 30 to 40 by each stop and merged together in Zerene Stacker program (Zerene Stacker, Richland, WA, USA) to maximize resolution. Maps of each figure were drawn by author by using PaintTool SAI (Systemax, Port Washington, NY, USA). Male morphological structures labeled as Table 2 for the comparison of each trait. Seven morphological traits have been assessed in the IBM SPSS statistics program (ver. 28.0; IBM Co., Armonk, NY, USA) by hierarchical cluster analysis method.

Taxonomic accounts

Insecta

Scarabaeoidea

Lucanidae Latreille, 1804

Genus Prosopocoilus Westwood, 1845

Subgenus Psalidoremus Motschusky, 1862

Species Prosopocoilus (Psalidoremus) inclinatus (Motschulsky, 1857)

Species Prosopocoilus (Psalidoremus) motschulskii (Waterhouse, 1869)

Species Prosopocoilus (Psalidoremus) dissimilis (Boileau, 1898)

Species Prosopocoilus (Psalidoremus) hachijoensis Nomura, 1960

Species Prosopocoilus (Psalidoremus) pseudodissimilis Kurosawa, 1976

Prosopocoilus (Psalidoremus) inclinatus (Motschusky, 1857) (Fig. 1A)

Body length: male 21-72 mm, female 25-42 mm

Distribution: Korea, Japan, China, Russia

Subspecies: This species consist of seven subspecies. P. inclinatus inclinatus (Motschusky, 1857) P. inclinatus mishimaiouensis Shimizu et Murayama, 1998, P. inclinatus kuchinoerabuensis Shimizu et Murayama, 1998, P. inclinatus kuroshimaensis Shimizu et Murayama, 2004, P. inclinatus miyakejimaensis Adachi, 2009, P. inclinatus mikuraensis Matsuoka et Takatoji, 2010, P. inclinatus yakushimaensis Adachi, 2014.

Prosopocoilus (Psalidoremus) hachijoensis Nomura, 1960 (Fig. 1B)

Body length: male 24-59 mm, female 26-38 mm

Distribution: Japan (Hachijo Island [Is.])

Subspecies: This species does not have subspecies.

Prosopocoilus (Psalidoremus) dissimilis (Boileau, 1898) (Fig. 1C)

Body length: male 26-79 mm, female 25-40 mm

Distribution: Japan (Okinawa Iss., Amami Iss., Tokara Iss.)

Subspecies: This species consist of seven subspecies. P. dissimilis (Boileau, 1898), P. dissimilis elegans (Inahara, 1958), P. dissimilis okinawanus Nomura, 1962, P. dissimilis makinoi Ichikawa et Fujita, 1985, P. dissimilis okinoerabuensis Ichikawa et Fujita, 1985, P. dissimilis kumejimaensis Ichikawa et Fujita, 1985, P. dissimilis hayashii Fujita, 2009.

Prosopocoilus (Psalidoremus) pseudodissimilis Kurosawa, 1976 (Fig. 1D)

Body length: male 28-63 mm, female 25-36 mm

Distribution: Japan (Yaeyama Iss.)

Subspecies: This species does not have subspecies.

Prosopocoilus (Psalidoremus) motschulskii (Waterhouse, 1869) (Fig. 1E)

Body length: male 24-60 mm, female 25-36 mm

Distribution: Taiwan Is.

Subspecies: This species does not have subspecies.

And seven morphological traits in Table 2 coded by binary notation and run through the SPSS hierarchical cluster analysis method and results are displayed in dendrogram in Fig. 2. In this analysis, common ancestor divided into two clades from inclinatus-hachijoensis cluster and others divided for later also divided into dissimilis and motschulskii - pseudodissimlis cluster and for the last motschulskii and pseudodissimlis divided in each area. But morphological characters not strong enough to assume speciation history of this subgenus, so geographical assessment also performed to provide strong hypothesis.

By considering ecological niche of the family Lucanidae, stag beetles not able to move across the sea surface. This provide strong barrier to the gene flow, which enables to proceed to speciation process by allopatric speciation. Also bathymetric elevation maps of distribution range at present sea level in Fig. 3, –750 m under sea surface in Fig. 4, –1,200 m under sea surface were displayed in Fig. 5. This provides strong evidence of the allopatric speciation of the subgenus Psalidoremus. Because of border of species almost identical to the land isolation gap appear on the bathymetric elevation maps in Figs. 3Fig. 4-5.

After all, subgenus Psalidoremus is allopatric group which sharing common ancestor in monophyletic clade. And their speciation scenario could be assumed by referring those morphological characters and bathymetric elevation and result as follows.

Hypothetical speciation scenario

First, common ancestor divided into inclinatus group and dissimilis group by Tokara Gap (marked as B in Fig. 6). Second, dissimilis group divided into two groups by dissimilis and motschulskii - pseudodissimilis cluster by Kerama Gap (marked as C in Fig. 6). Third, inclinatus - hachijo cluster and motschuskii - pseudodissimilis divided into each species level respectively by Hachijo Gap (marked as A in Fig. 6), and Taiwan Gap (marked as D in Fig. 6). This scenario has been pictorialized in Fig. 6 to enhance visibility of concept.