A new species of the genus Ochthebius (Coleoptera, Hydraenidae) from the Ogasawara Islands, Japan, with a description of the larva

Abstract A new species of the genus Ochthebius, O. (O.) sasakiisp. nov., is described from the Ogasawara Islands, Japan, with a description of the larva. This record is the first of the family Hydraenidae from the Ogasawara Islands. This species belongs to the punctatus species group and is similar to two Japanese species, O. (O.) inermis Sharp, 1884 and O. (O.) danjo Nakane, 1990, but it differs from them in both adult and larval characters. The larva of O. (O.) inermis is also described for comparison.


Introduction
The Ogasawara Islands (= Bonin Islands) are oceanic islands that were never connected to the continental mainland (Japan Wildlife Research Center 2010); therefore, the islands contain many endemic species within a small geographic area and are registered as a UNESCO World Heritage Site (WHS). The fauna of the islands is comparatively well studied (Ohbayashi et al. 2004), and the coleopteran fauna was reviewed by Kuro-sawa (1976a, b). A few aquatic beetles were recorded from the islands (Ohbayashi et al. 2004); the only known endemic species is Copelatus ogasawarensis Kamiya, 1932 (Dytiscidae, Copelatinae).
The genus Ochthebius Leach, 1815 (Ochthebiinae, Ochthebiini) is distributed in the Palaearctic, Oriental, Nearctic, Neotropical, Afrotropical, and Australian regions, and includes 540 species within 10 subgenera (Villastrigo et al. 2019). Fourteen species of the genus Ochthebius are recorded from Japan under two subgenera and five species groups (Yoshitomi and Satô 2001;Villastrigo et al. 2019), but no species are recorded from the Ogasawara Islands.
In the present paper, we describe a new species of the genus Ochthebius from the Ogasawara Islands, with a description of the larva. This record is the first of the family Hydraenidae from the Ogasawara Islands. The larva of O. inermis Sharp, 1884 is also described for comparison with the larvae of the new species and O. danjo Nakane, 1990.

Material and methods
The material examined in this paper is preserved in the Ehime University Museum, Matsuyama, Japan (EUMJ); Kanagawa Prefectural Museum of Natural History, Odawara (KPMNH); and Naturhistorisches Museum Wien (NMW).
General observations, dissections, and microstructures of dissected parts were made under a Leica MZ95. After observation, the dissected parts were mounted on the same card with the specimen. Photographs were taken under a Leica MZ95.
Morphological abbreviations used in this study are as follows: EL elytral length from anterior margin to elytral apex; EW maximum elytral width; HL head length; PL pronotal length in median line; PW maximum width of pronotum; TL total length (PL + EL + HL).
The average is given in parentheses after the range.
Diagnosis. Ochthebius (Ochthebius) sasakii belongs to the punctatus species group (sensu Villastrigo et al. 2019) and is similar to two Japanese species, O. inermis and O. danjo. The adult of O. sasakii differs from the two Japanese species in having a smaller body size (see Table 1), anterior margin of labrum not shallowly concave, median groove of pronotum shallow, and short elytral setae. The larva of this species differs in the following characteristics: mandibles strongly curved inwardly in the apical part (gently curved in O. inermis and O. danjo); labrum arcuate in anterior margin (projecting apically in O. inermis); and presence of subprimary setae on pronotum and mesonotum (see Table 2).
Biological notes. All the specimens (both adults and larvae) were collected from the surface of littoral rocks covered with a film of sheeting fresh water (depth ca 1-2 mm; Fig. 5). All habitats are situated on the seashore (the nearest point from the edge of the water was ca 3 m); we could not find any habitats in inland areas.
The fauna of the Ogasawara Islands was seriously affected by a long drought in 2016-2017. In addition, a serious drought occurred from autumn to winter 2018/2019. In February 2018, HK found many individuals of O. sasakii sp. n. at the Ani-jima site (Fig. 5E) and collected some specimens as the type series. However, in February 2019, HK could not find this species at this site because sheeting fresh water had completely dried up. At the Chichi-jima site, HK found many individuals of this species in both 2018 and 2019, but the area of sheeting fresh water was markedly reduced.

Discussion
The new species is closely related to two Japanese species, viz., O. inermis distributed in Japan (Honshu, Shikoku, Kyushu), Kunashir, and Taiwan, and O. danjo distributed in southern Kyushu (including Danjo Islands and Yakushima). The former species inhabits mainly stagnant or flowing water along rivers, usually associated with filamentous green algae (Jäch 1998), and is sometimes collected from hygropetric microhabitats in mountainous areas (e.g., Yoshitomi 2001) or rocky seashores (e.g., Sugaya 2009). The latter species was found in marine rock pools (Delgado and Matsui 2000;Hayashi 2008b). Ochthebius sasakii sp. n. and O. inermis live in fresh water, whereas O. danjo lives in brackish water.