INTRODUCTION

Nannophya is a genus belonging to the family Libellulidae, which includes the world’s smallest dragonflies. They are commonly known as pygmy flies and are found in Asia and Australia. The genus of Nannophya includes the seven species Nannophya australis, N. dalei, N. katrainensis, N. occidentalis, N. paulsoni, N. pygmaea, and N. koreana (Corbet 1999;Bae et al. 2020). Of these, four species occur only in Australia, and one species, N. katrainensis is found in the Himalayas. However, there is limited information on N. katrainensis, and few records, so more research is required. N. pygmaea is found worldwide, and its distribution includes Australia, tropical and subtropical areas of Southeast Asia, China, Japan, India, and Korea, however, N. pygmaea in Korea and Japan recently have been discovered to be N. koreana (Kim et al. 2010; Bae et al. 2020).

N. koreana was first collected from Songnisan (MT.) in Korea by Professor Chang Whan Kim (Korea University, Korea) in 1957 (Kim et al. 2010; Bae et al. 2020). This species was designated as an endangered species, category level II by the Ministry of Environment of Korea in 1988 (Bae et al. 1999) and was listed in the Red Data Book of Korea (National Institute of Biological Resources 2013), but still known as the N. pygmaea. For the last 60 years, this species was known solely as N. pygmaea, and several intensive studies investigated its distribution and habitat, life history and development, and conservation and habitat restoration in Korea and Japan (Bae et al. 2020). The habitats of N. koreana (previously known as the N. pygmaea) are known from more than 20 localities throughout Korea, but now only a few sites are maintaining stable populations (Oh et al. 2017).

Recently, the Nannophya species group in Korea, Japan, and other countries was thoroughly investigated (Bae et al. 2020). Although the Nannophya species in Korea and Japan have a mean genetic distance of 0.88%, this was regarded as an issue at the intraspecific level. As a result, the Nannophya species in Korea was hailed as a new species, Nannophya koreana (Bae et al. 2020).

Therefore, this study was conducted to evaluate all the previous studies, modify the species information, and utilize it as basic data for the preservation and protection of Nannophya species.

MATERIALS AND METHODS

For this study, text mining analysis was performed to identify research relevant to Nannophya species. For the data collection, we used several scientific web data library for broad research fields including taxonomy, ecology, genetic study and so on. The data analysis was carried out based on scientific articles and thesis that are searched for this study.

1. Data collection

In this study, we collected the Korean research related to Nannophya through the Research Information Sharing Services (RISS, http://www.riss.kr) and DBPIA (http:// www.dbpia.co.kr/). In the case of foreign research, we collected the information through the Web of Science (http:// www.webofknowledge.com/), Scopus (http://www.scopus. com/), and Wiley Online Library (http://www.onlinelibrary. wiley.com). The search keywords for the study were ‘TITLE-ABS-KEY (Nannophya).’

2. Data selection

Of the many results, we selected only scientific journals and theses related to the Nannophya species. The tools used to extract keywords were R 3.6.2 ver, and the frequency was analyzed by extracting keywords from the title of each research paper. While analyzing frequency, stop word processing was performed for data purification, and keywords related to Nannophya were extracted.

RESULTS AND DISCUSSION

We extracted 16 studies from Web of Science, 15 studies from Scopus, 18 studies from Wiley online library, 18 studies from RISS, and 18 studies from DBPIA. For data analysis, we removed duplications and selected 31 studies, including four master dissertations and 27 scientific papers finally.

Twenty-one studies were conducted by Korean scientists and 10 by scientists from other countries. Among them, eight were Japanese, and the other two were Malaysian and Australian, respectively. The oldest study was conducted in 1978 and the most recent study in 2020. The largest number of studies were conducted in 2008 and 2010, with one or two studies being conducted each year on average (Fig. 1).

We extracted the 148 keywords through data purification processes such as stop word and misprint (Fig. 2). Species name showed a high frequency and locality, and other subject-related words showed a high frequency. Our results identified the study subject, area, and species. According to the research focus, the studies were divided into seven categories: distribution, behavior, life history and development, conservation and restoration, habitat characteristics, genetic studies, and taxonomy (Fig. 3, Table 1).

1. Distribution

This subject was covered by two studies, in Korea and Japan. Fujita et al. (1978) described the seasonal changes in population size and distribution of N. pygmaea. Bae et al. (1999) also added a new habitat from the southwestern part of Korea. Although Kim (1997) had reported a new habitat of N. pygmaea in Korea, we put this study in the conservation and restoration category because it focused on the protection and conservation of the population.

2. Behavior

In this subject, all studies were conducted by Japanese scientists Tsubaki and Ono (1985, 1986, 1987, 1995) and Tsubaki et al. (1994). They intensively studied the territorial site selection, mating, and guarding behavior after mating. All the studies were based on field observations in natural habitats of N. pygmaea in Japan.

3. Life History and Development

This subject included six studies, most of which, except a study by Siva-Jothy and Tsubaki 1994, were conducted from 2006 to 2010. Siva-Jothy and Tsubaki (1994) studied the sperm competition mechanism and the advantage of sperm re-positioning. Other studies focused on the effect of different temperature conditions on egg development and the growth rate of larvae (Kim et al. 2006;Kim 2008;Kim et al. 2009a, 2009b). Based on these studies, Kim et al. (2010a) investigated aspects of the life history of N. pygmaea in an abandoned paddy field in Korea. Through these studies, they discovered that the Korean population of N. pygmaea were univoltine species.

4. Conservation and Restoration

N. pygmaea was designated an endangered species by the Ministry of Environment of Korea. For these reasons, Korean scientists suggested protection and preservation of N. pygmaea and its natural habitat (Kim 1997;Oh et al. 2017). Furthermore, Park (2008) investigated the habitat preferences of N. pygmaea and, based on the results, suggested the wetland construction model for restoration of N. pygmaea habitat.

5. Habitat characteristics

Habitat was the subject most intensively studied, and most research was carried out from 2008 to 2012. Most studies investigated the landscape properties, habitat size, vegetation types, water environments, water sources, and composition of the flora in the natural habitat of N. pygmaea (Lee et al. 2008;Yoon 2008;Kim et al. 2010b;Yoon et al. 2010, Kim 2011a;Kim 2011b;Cho et al. 2012).

6. Genetic study

Intensive studies on the genetics of N. pygmaea were recently conducted with the use of molecular techniques. Kim et al. (2007) firstly sequenced a portion of the mitochondrial COI gene, corresponding to the DNA barcode region (658 bp) using 40 individuals of N. pygmaea in Korea. Low et al. (2016) assessed the genetic diversity using the DNA barcode data of N. pygmaea. In addition, Wang et al. (2017) sequenced an additional mitochondrial gene (ND5) in the mitochondrial genome of N. pygmaea, and Okude et al. (2017) established an electroporation-mediated RNA interference (RNAi) procedure. Jeong et al. (2018) sequenced the complete 15,112-bp-long mitochondrial genome (mitogenome) of N. pygmaea, and Kim et al. (2018) developed 12 microsatellite markers using the NextSeq 500 platform. These studies have become important evidence for the discovery of a new Korean species of Nannophya.

7. Taxonomy

Before the study of Theischinger (2003), only five Nannophya species, including N. australis, N. dalei, N. katrainensis, N. occidentalis, and N. pygmaea, were known in the world. However, this study described a new Nannophya species from Australia and named it N. paulsoni.

Recently, Bae et al. (2020) described a new species from Korea based on morphology and mitochondrial cytochrome oxidase c subunit I (COI) gene sequences. This new species was named N. koreana.

According to Bae et al. (2020), the male and female adults of N. koreana can be morphologically distinguished from N. pygmaea, by characters such as the lateral synthorax stripe, anal appendage color, superior appendage teeth number, and basal wing pigmentation. Moreover, the body size of N. koreana is generally 1.2-1.4 times larger than that of N. pygmaea. Based on this study, N. pygmaea will have to be treated as a different species to N. koreana in Korea from now on.

Currently, N. pygmaea has stable populations worldwide, having the least concern designation in the IUCN (International Union for Conservation of Nature) red list (Karube 2009). However, the Nannophya species of Korea and Japan now include a new species, which is distinct from the existing N. pygmaea, and the conservation value of the Korean populations has consequently increased. In particular, the Korean population appears to be decreasing because of increasing habitat loss caused by natural succession and climate changes, such as drought (Oh et al. 2017).

In conclusion, most studies on Nannophya species are limited to the N. pygmaea in Korea and Japan. Furthermore, the species was identified to be a new one named as the N. koreana. However, because the Nannophya species was designated as the endangered species in Korea and Japan, collecting its specimen and research is not sufficient. Thus, it would be reasonable to consider that N. koreana and N. pygmaea are existing in both countries although more studies are needed to clarify this ambiguity. Either way, we must strive to preserve and protect both N. koreana and N. pygmaea. In this regard, we suggest that the IUCN red list should be revised to include the species as Endangered (EN) or Critically Endangered (CR) in the threat level.