A taxonomic review of the Hydraenidae in South Korea (Coleoptera)

Abstract A taxonomic study of the South Korean Hydraenidae is presented. Eight species in two genera are recognized, one of which is reported for the first time in the Korean peninsula, Ochthebius marinus (Paykull). It was also found that Hydraena riparia Kugelann and Ochthebius inermis Sharp previously recorded in South Korea were incorrect identifications of Hydraena puetzi Jäch and Ochthebius lobatus Pu, respectively. Habitus and SEM photographs, line drawings of aedeagus, distribution maps, keys, and redescriptions of the species are provided.


Introduction
The Hydraenidae, relatively small water beetles with unique habitus, are usually found at margins of running water, slowly flowing water, and stagnant water in the vicinity of lotic biotopes (Jäch et al. 2016). Approximately 1600 species in 42 genera have been recorded in the world (Ślipiński et al. 2011); 941 species in nine genera in the Palaearctic (Jäch and Skale 2015); 100 species in seven genera in China, 35 species in three genera in Japan; and 11 species in three genera in the Far East of Russia (Jäch and Skale 2015).
Members of the Hydraenidae are characterized by combination of the following features: separation of the gula and submentum by confluent genae; presence of an anterior plate-like premento-hypopharyngeal extension; large labrum with a deep medial incision in some species; and palpigers connected by a transverse sclerotized bar (Beutel et al. 2003). Kwon and Suh (1986) first recorded Hydraena riparia Kugelann in South Korea. Later, Lee (1995) added two species (Ochthebius inermis Sharp and O. satoi Nakane) with descriptions and habitus illustrations. Recently, Jäch and Delgado (2014) found that a marine littoral species of Neochthebius granulosus (Satô) previously recorded by Park and Ahn (2008) was an incorrect identification of O. ahni Jäch and Delgado and O. parki Jäch and Delgado. Jäch and Skale (2015) added four species [H. miyatakei Satô, H. puetzi Jäch, O. hasegawai Nakane and Matsui, and O. lobatus Pu]. Accordingly, nine species in two genera have been recorded in South Korea.
In this paper, one species is reported for the first time in the Korean peninsula, O. marinus (Paykull). It was also found that H. riparia and O. inermis previously recorded in South Korea were incorrect identifications of H. puetzi and O. lobatus, respectively. Habitus and scanning electron microscopy photographs, keys, redescriptions, and diagnostic characters with illustrations of the species are provided.

Material and methods
To identify South Korean hydraenid species more reliably, they were compared with voucher specimens in the Natural History Museum (NHM, London, United Kingdom), Naturhistorisches Museum (NMW, Wien, Austria) and Ehime University Museum (EUMJ, Matsuyama, Japan). The specimens used in this study are deposited in Chungnam National University Insect Collection (CNUIC, Daejeon, Korea). Habitus and scanning electron microscope (SEM) photographs were prepared based on a former study (Lee and Ahn 2015). See Jäch and Delgado (2014) for detailed descriptions of O. ahni and O. parki. The terms of taxonomic characters and measurements of specimens mainly followed Perkins (2001) and Jäch et al. (2016). The geographical subdivision of China and Russia was based on the standards of Löbl and Löbl (2015).
Remarks. Kwon and Suh (1986) first recorded this species as H. riparia in Korea. After that, many entomologists [Kim et al. (1994); Lee (1995); Han et al. (2007); Han et al. (2008); Cho and Park (2010)] reported the species, only in the local fauna without any taxonomic comments. After studying the specimen (1♂, Samcheok-si, Gagok-stream, 25.VI.1985. SH Lee) previously examined by Kwon and Suh (1986), we found that it had been incorrectly identified and actually is H. puetzi. This species can be distinguished from H. riparia by left paramere (Fig. 7A) parallel at middle, and apical part of right paramere (Fig. 7A) semicircular, with compact long setae on apex.

Ochthebius (O.) ahni Jäch & Delgado, 2014
Ochthebius ( 18) 1.2 times as wide as long and with sparse setae; protruded and widest at anterior third; with a row of long setae on anterior margin. Anterior gena without long setae on posterior part in ventral view. Antenna (Fig. 19) with nine antennomeres; 1 longest, approx. 6.0 times as long as 2, two long setae present on lateral part; 2 widest at base; 3 bulbous at apical part; 4 semicircular; 5-9 clubbed and with pubescence. Maxillary palpomere (Fig. 20) 1 small, approx. 2.0 times as long as wide; 2 bulbous at apical part, 2.0 times as long as 1; 3 largest, bulbous at apical part, slightly longer than 2; 4 slender and paralleled, 0.25 times as long as 3. Pronotum (Fig. 21) reverse trapezoidal, widest at anterior fourth, with indistinct longitudinal groove on medial part, two small oval grooves on anterior part, relatively large oval groove on posterior part; anterior margin bisinuate; antero-medial margin protruded; anterior corner rectangular; lateral margin protruded at anterior third; posterior corner obtuse; postero-medial part rounded. Elytra (Fig. 22) widest at middle, with setae. Mesoventrite (Fig. 23) pentagonal, with T-shaped carina on anterior part; anterior margin transverse. Metaventrite (Fig. 24) with pubescence on medial part. Sternite VIII (Fig. 25) with more or less long setae on posterior part. Male terminal sternite (Fig. 25) semicircular and with a row of compact setae on posterior part. Median lobe of aedeagus (Fig. 51) long, slender, very weakly curved at middle; apical part acute; distal lobe slender, slightly acute apically. Paramere (Fig. 51) very short, with long setae on apical part.

Discussion
This study revises the number and taxonomic status of hydraenid species known to occur in South Korea. In total, eight species are recognized. Two species (O. ahni and O. parki) are apparently endemic to the South Korean fauna (Jäch and Delgado 2014). Of the remaining species, five are widespread in the East Palaearctic region (H. puetzi, H. miyatakei, O. lobatus, O. hasegawai, O. satoi) and one occurs in the Holarctic region (O. marinus).
Diversity of South Korean Hydraenidae is very low compared to other adjacent countries, such as China (82 endemic species out of 100 species), Japan (29 endemic species out of 35 species), and the Far East Russia (no endemic species out of 11 species) (Jäch 2003;Jäch and Díaz 2003;2004;2005;2006;2012;Jäch and Delgado 2014;Jäch and Skale 2015). They show high diversity and endemism because of their small size and limited dispersal abilities (Jäch and Balke 2008). Further collecting efforts in the Korean Peninsula will probably add more hydraenid species to the Korean fauna such as the Palaearctic species, H. riparia Kugelann and O. angusi Jäch, and more species.