species diversity of aquatic hemiptera and coleoptera in japan

Japanese Journal of Systematic Entomology, 26 (2): 191–200. December 30, 2020.

Ⓒ Japanese Society of Systematic Entomology

REVIEW ARTICLE

Species Diversity of Aquatic Hemiptera and Coleoptera in Japan

Masakazu HAYASHI1), Jun NAKAJIMA2), Kazuo ISHIDA3), Tadashi KITANO4), Hiroyuki YOSHITOMI5)

1) Hoshizaki Green Foundation, Sono, Izumo, 691-0076 Japan2) Fukuoka Institute of Health and Environmental Sciences, Fukuoka, 818-0135 Japan

3) Narashino, Chiba, 275-0012 Japan4) Department of Human Development, School of Humanities and Culture, Tokai University, Hiratsuka, 259-1292 Japan

5) Ehime University Museum, Matsuyama, 790-8577 Japan

Abstract The species diversity of aquatic Hemiptera and Coleoptera in Japan on the water surface and in the water column during both larval and adult stages was reviewed. The list was based on the latest taxonomic knowledge, and summarized species recorded in Japan at the end of 2019. Hemiptera included 118 species in 13 families, among which 22 species (18.6%) were endemic to Japan. Coleoptera included 358 species in 12 families, among which 156 species (43.6%) were endemic to Japan. A total of 29 taxa of Hemiptera and 109 taxa of Coleoptera listed in the Red List of Japan were published in 2019. The total number of Coleoptera taxa increased from 41 to 109 from 2007 to 2019.Keywords: aquatic beetles, aquatic true bugs, biodiversity of Japan

IntroductionThe Japanese archipelago is located in the ranges of

latitude 20–46° N and longitude 122–153° E, and contains 6852 large and small islands on the east side of Eurasia (Geospatial Information Authority of Japan, 2020). The Kuril Islands, the four main islands (Hokkaido, Honshu, Shikoku, and Kyushu), and the Ryukyu Islands form island arcs. Geologically, the archipelago is dominated by continental islands formed by the separation of the continental crust into the Pacific Ocean during the Miocene (Otofuji & Mastuda, 1987; Barnes, 2003). The island arcs are in the subduction zone of the oceanic plate, and crustal deformation is active. The main island has mountain ranges that are 2,000–3,000 m above sea level and contain several volcanic fronts. In the North Pacific Ocean, the Daito, Ryukyu, and Ogasawara Islands are oceanic islands.

The climate of Japan is warm and humid. From the Japan Meteorological Agency (2020) statistics, the average temperature and precipitation values for the 30 years from 1981 to 2010 were 15.4 °C and 1528.8 mm in Tokyo, and 8.9 °C and 1106.5 mm in Sapporo, Hokkaido, respectively. The average annual temperature in Naha in Okinawa Prefecture is 23.8 °C, and the annual precipitation is 2099.3 mm. Surface water is abundant due to high precipitation. Paddy fields are mainly used as farmlands in the plains, and valleys in low mountainous areas are often used as paddy fields because rice is the primary agricultural crop. Many agricultural reservoirs are created in areas with relatively low precipitation, and there are approximately 96,000 locations nationwide (Ministry of Agriculture, Forestry and Fisheries, 2020). Therefore, both lotic and lentic water environments are abundant in Japan. In the mid-latitudes of the Northern Hemisphere, surface water-rich areas are limited. Owing to the abundance of surface water, a variety of aquatic insects adapt to various inland water environments.

This review covered species of aquatic insects that live on

the water surface and in the water column during both their larval and adult stages. In Coleoptera, these are defined as “true water beetles” (Jäch, 1998); however, there is a similarly definable species in Hemiptera. The authors published a bibliographical picture book summarizing these Japanese species (Nakajima et al., 2020). During the preparation of this book, the current situation of Japanese species was reexamined. No data have been compiled on species diversity to date; therefore, the number of known species, endemic taxa, endangered species, and genetic diversity studies are discussed in the present review. In particular, the increase in endangered species of aquatic Coleoptera is a significant problem.

MethodsThe species list (Nakajima et al., 2020) is mainly based on

Kawai & Tanida (2018) and incorporates the latest taxonomic knowledge and summarizes species recorded in Japan at the end of 2019 (Kamite & Hayashi, 2019; Minoshima & Inahata, 2019; Nakajima, 2020). Counting was performed at the species level, and taxa that were divided into multiple subspecies were grouped into species.

Endangered species are based on the Red List 2019 published by the Ministry of the Environment, Japan (MOE, 2019a). The list is based on species and subspecies ranks, and does not represent the exact number of species.

The distribution was divided into seven regions (Fig. 1): Hokkaido, Honshu, Shikoku, Kyushu, Nansei Islands (Ryukyu Islands), Tsushima, and Ogasawara Islands. Of these, the three major islands of Honshu, Shikoku, and Kyushu are often classified as an integrated area in terms of biogeography (e.g., Millien-Parra & Jaeger, 1999).

Non-native species were not included in the species count, which was only applicable to Rhagadotarsus kraepelini Breddin, 1905 (Hemiptera: Gerridae).

192 Hayashi, M., J. Nakajima, K. Ishida, T. Kitano and H. Yoshitomi

December 30, 2020, JJSE 26 (2)

Habitat of aquatic speciesNakajima et al. (2020) classified the habitat of aquatic

insects into ponds, rivers, puddles, paddy fields, wet rocks, sea, and groundwater.

Ponds include natural waters such as ponds, lakes, marshes, and agricultural reservoirs.

Rivers include running water environments such as rivers and streams.

Puddles are temporary small water bodies or very small water bodies recharged by springs.

Paddy fields are farmland where rice is grown; dry rice fields are dry from autumn to winter, and wet rice fields are damp all year.

Wet rocks, also known as hygropetric environments, are environments in which the surface of the rocks is constantly moist due to currents and splashes flowing through the rock.

Sea includes intertidal zones such as tidal flats and rocky reefs, inner bays, and coastal waters.

Groundwater is a water environment that exists in underground gravel deposits and fractured zones.

HemipteraThe taxa of Japanese species that inhabit the water surface

and water column during their larval and adult stages include

the families Nepidae, Belostomatidae, Corixidae, Naucoridae, Aphelocheiridae, Notonectidae, Pleidae, Helotrephidae, Mesoveliidae, Hydrometridae, Veliidae, Gerridae, and Hermatobatidae. The families of other groups are semi-aquatic, for example, Hebridae, Saldidae, and Ochteridae.

Taxonomic historyThe first species of aquatic Hemiptera described in Japan

was Appasus japonicus Vuillefroy, 1864. Then, Ilyocoris cimicoides exclamationis (Scott, 1874), Hydrometra albolineata (Scott, 1874), Laccotrephes japonensis Scott, 1874, Ranatra unicolor Scott, 1874, and Microvelia douglasi Scott, 1874 were described as Japanese species of Hemiptera by Scott (1874).

According to Hayashi & Miyamoto (2018), the study of Hemiptera was begun by a Japanese taxonomist after 1900. Shônen Matsumura described Aphelocheirus vittatus Matsumura, 1905, Anisops ogasawarensis Matsumura, 1915, and 10 other species (Matsumura, 1905, 1915). During the 1950s and 1960s, 21 species were described by Teiso Esaki and Syôiti Miyamoto (Esaki & Miyamoto, 1955; Miyamoto, 1958).

While the valid species and subspecies were accrued each year, the increase was remarkable during the 1950s–1960s (Fig. 2), which was due to the research undertaken by Esaki and Miyamoto. However, since the 1980s, the increase in the

Hokkaido

Honshu

ShikokuKyushu

Tsushima

Nansei Isls Ogasawara Isls

25°N

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125°E 130° 135° 140° 145°

500250

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Fig. 1. Biogeographical division in this study.

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1750 1800 1850 1900 1950 2000

Coleoptera

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Fig. 2. Cumulative number of described species and subspecies by year. Only valid names are counted.

Miyamoto, 21

Esaki, 15

Matsumura, 12

Horváth, 10

Fabricius, 6Fieber, 5Scott, 5

Others, 59

Sharp , 64

M.Satô , 36

Nomura , 31

Jäch , 28

Nakane , 21

Díaz , 17

Kamite , 15

Fabricius , 12Kamiya , 11Matsui , 11

S.Uéno , 11

Regimbart, 11

Clark, 10

Others, 163

Hemiptera Coleoptera

Fig. 3. Number of described species/subspecies per author. Only valid names are counted.

193Review: Species Diversity of Aquatic Hemiptera and Coleoptera in Japan


number of species described has not occurred, yet the number of species in Japan has increased. This is because species described overseas are often found in Japan. Comparing the number of authors for species/subspecies, Miyamoto, Esaki, and Matsumura accounted for approximately one-third of the total (Fig. 3).

Known speciesThe total number of Japanese species in the 13 families

is 118 (Table 1; Fig. 4). In descending order of the number of species, there are 29 species of Corixidae, 28 species

of Gerridae, 19 species of Veliidae, and 11 species of Notonectidae. Corixidae includes 12 species of Sigara, and 9 species of Micronecta.

The total number of Japanese endemic species is 22, which is 18.6% of the total number of species. The taxa including endemic species are Corixidae, Aphelocheiridae, Helotrephidae, Mesoveliidae, Veliidae, and Gerridae. Veliidae contains 12 endemic species.

Counting common species in the surrounding area, there were 85 species from East Asia; 40 species from Siberia, Sakhalin, and Kuril; 35 species from southeast Asia; 10

Fig. 4. Known species number for each family of aquatic Hemiptera.

Family Japan Hokkaido Honshu Shikoku Kyushu Nansei Isles Tsushima Ogasawara Isles HSK

Nepidae 7(0) 2(0) 4(0) 4(0) 3(0) 6(0) 2(0) 0(0) 4(0)Belostomatidae 5(0) 3(0) 3(0) 3(0) 3(0) 3(0) 2(0) 0(0) 3(0)Corixidae 29(3) 10(1) 20(0) 15(0) 17(0) 11(1) 4(0) 0(0) 21(0)Naucoridae 1(0) 0(0) 1(0) 0(0) 1(0) 0(0) 0(0) 0(0) 1(0)Aphelocheiridae 3(1) 0(0) 3(1) 1(0) 2(0) 0(0) 0(0) 0(0) 3(1)Notonectidae 11(0) 2(0) 4(0) 3(0) 4(0) 9(0) 2(0) 1(0) 5(0)Pleidae 3(0) 0(0) 2(0) 2(0) 2(0) 2(0) 1(0) 0(0) 2(0)Helotrephidae 1(1) 0(0) 0(0) 0(0) 0(0) 1(1) 0(0) 0(0) 0(0)Mesoveliidae 6(1) 4(0) 6(0) 5(0) 4(0) 3(0) 1(0) 1(0) 6(0)Hydrometridae 5(0) 2(0) 4(0) 3(0) 3(0) 3(0) 3(0) 0(0) 4(0)Veliidae 19(12) 2(0) 9(1) 7(0) 8(0) 15(7) 6(0) 2(1) 9(1)Gerridae 27(3) 11(0) 16(0) 10(0) 14(1) 13(0) 9(0) 1(1) 17(1)

Hermatobatidae 1(1) 0(0) 0(0) 0(0) 0(0) 1(1) 0(0) 0(0) 0(0)

Total 118(22) 36(1) 72(2) 53(0) 61(1) 67(10) 30(0) 5(2) 75(3)

Table 1. Species number for each area of aquatic Hemiptera, and endemic species number in parentheses. HSK shows major three islands, Honshu, Shikoku and Kyushu.



species from Europe; and one species from North America (Table 2). There is a high degree of commonality with species distributed in the Oriental Region and the eastern part of the Palaearctic Region.

Species in each areaThe number of species in each region is shown in Table

1. Hokkaido accounts for 30.5% of all species, with the three major islands (Honshu, Shikoku, and Kyushu) accounting for 63.6%, and the Nansei Islands accounting for 56.8%. The Ogasawara Islands contained only Notonectidae,

Region All species inJapan Endemic species E Asia SE Asia Oceania Siberia, Sakhalin

& Kuril Europe N America

Hemiptera 118 22 85 35 10 40 10 1

Coleoptera 358 156 169 69 13 96 29 11

Table 2. Distribution pattern of aquatic Hemiptera and Coleoptera. Total number of common species each distribution area and Japan.

Family Ponds Rivers Puddles Paddies Wet rocks Sea Ground-water

Nepidae 7 2 2 5

Belostomatidae 5 1 5

Corixidae 23 9 13 16

Naucoridae 1

Aphelocheiridae 3

Notonectidae 11 2 7 4

Pleidae 3 1 1

Helotrephidae 1

Mesoveliidae 5 2 1 2

Hydrometridae 5 1 1 4

Veliidae 10 13 10 5 1

Gerridae 17 10 6 4 7

Hermatobatidae 1

Total 87 45 39 45 0 11 0

Ratio % 38.3 19.8 17.2 19.8 0 4.8 0

Table 3. Habitats for each species of aquatic Hemiptera. Data from Nakajima et al. (2020).

Family EX CR EN VU NT DD Total

Nepidae 0 0 0 0 2 0 2

Belostomatidae 0 2 0 1 1 0 4

Corixidae 0 0 0 0 5 2 7

Naucoridae 0 0 1 0 0 0 1

Aphelocheiridae 0 1 0 1 0 0 2

Notonectidae 0 0 0 0 1 0 1

Pleidae 0 0 0 0 0 0 0

Helotrephidae 0 0 0 1 0 0 1

Mesoveliidae 0 0 0 0 0 0 0

Hydrometridae 0 0 0 1 0 0 1

Veliidae 0 0 0 1 1 0 2

Gerridae 0 0 0 3 4 0 7

Hermatobatidae 0 0 0 0 1 0 1

Total (2019RL) 0 3 1 8 15 2 29

Total (2007RL) 0 2(CR+EN) 9 14 4 29

Table 4. Endangered species of aquatic Hemiptera and Coleoptera listed in Red List 2019 by MOE, Japan.



Mesoveliidae, Veliidae, and Gerridae, with a total of five species, and a percentage of only 3.4%. These latter islands are oceanic islands; therefore, the aquatic Hemiptera has limited opportunities for dispersal and settlement.

Only a few Japanese endemic species are further limited to each region. Sigara toyohirae (Matsumura, 1905) was recorded only from Hokkaido. There were four species in the three major islands, including Aphelocheirus kawamurae Matsumura, 1915 and Xiphovelia japonica Esaki & Miyamoto, 1959. Xiphovelia boninensis Esaki & Miyamoto, 1959 and Neogerris boninensis Matsumura, 1913 are endemic to the Ogasawara Islands, whereas 10 endemic species to Nansei Islands have been recorded, including Micronecta japonica Chen, 1960, Heterotrephes admorsus Esaki & Miyamoto, 1959, and Rhagovelia esakii Lundblad, 1937. The percentage of endemic Hemiptera species is lower than that of Coleoptera because many Hemiptera species have high dispersal ability. Hemiptera has many wing polymorphisms, with macroptery enabling flying and thus dispersal (Fujisaki, 1994).

HabitatsNakajima et al. (2020) classified the main habitats and

listed each species, which were in descending order of 38.3% in ponds, 19.8% in rivers and paddy fields, 17.2% in puddles, 4.8% in the sea, and 0% in wet rocks and groundwater (Table 3). Hemiptera, which inhabits Japanese wet rock habitats, is semi-aquatic and was excluded from the list. Overseas, Naucoridae living on wet rocks has been reported (Sites, 2015; Sites & Vitheepradit, 2011).

Endangered taxaIn the Red List published by the MOE (2019a), zero

species are classified as Extinct, three species as Critically Endangered, one species as Endangered, eight species as Vulnerable, fifteen species as Near Threatened, and two species as Data Deficient (Table 4), with a total of 29 species. Lethocerus indicus (Lepeletier & Serville, 1775) and Aphelocheirus kawamurae Matsumura, 1915 have not been recorded recently.

Compared to the Red List 2007 (MOE, 2007), the total number of species is 29. The total number of endangered species (Critically Endangered, Endangered, and Vulnerable ) has increased slightly from 11 to 12 (Table 4).

Genetic diversityFew species or genera have been studied for genetic

diversity. Appasus has been examined most closely (Suzuki et al., 2013, 2014). Kirkaldyia deyrolli (Vuillefroy, 1864) has been reported to be no different from its counterparts on the Korean Peninsula (Sareein et al., 2019); however, the sample size was small and more detailed studies are required.

ColeopteraThe taxa of Japanese species that inhabit the water

surface and water column during both larval and adult stages include families Haliplidae, Noteridae, Dytiscidae, Gyrinidae, Torridincolidae, Hydraenidae, Hydrochidae, Helophoridae, Hydrophilidae, Spercheidae, Dryopidae, and Elmidae. Hydrophilidae contains terrestrial species, and this review excludes such species. Other aquatic species such as Psephenidae, Scirtidae, and Ptilodactylidae live in water only during the larval stage; therefore, these families were also not included.

Taxonomic historyThe taxonomic study of aquatic Coleoptera in Japan began

with David Sharp. The first report, Sharp (1873), described 26 species of Dytiscidae, 2 species of Gyrinidae, and 21 species of Hydrophilidae (including terrestrial species). Then, 37 species were described by Sharp (1882, 1884). Most of the genus he described have been used after the original description, and their classification accuracy is noteworthy. The work of Japanese taxonomists, Kazuo Kamiya, Takehiko Nakane, and Masataka Satô was developed based on Sharp’s work and helped understand the species diversity.

The valid species and subspecies have accrued each year, with increases being remarkable in the 1870s–1880s and 1950s–1960s (Fig. 2), with Sharp greatly contributing to these increases and him describing the largest number of species to date (Fig. 6). In a particular family, Shizumu Nomura described 31 species of Elmidae in the 1950s–1960s, and recently nine species were described by Yuuki Kamite. Manfred A. Jäch has described 29 Hydraenidae species since 1988.

Known speciesA total of 358 Japanese species are included in the 12

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EX CR & EN VU NT DD Total

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2007 2019

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Fig. 5. Species number listed in Red Lists 2007 and 2019 by MOE, Japan.



families (Table 6; Fig. 6). There are 128 species of Dytiscidae, 76 species of Hydrophilidae, 57 species of Elmidae, and 39 species of Hydraenidae.

The total number of Japanese endemic species is 156, accounting for 43.6% of the total Japanese species. The only taxa that do not contain endemic species are the families Helophoridae and Spercheidae.

Counting common species in the surrounding area, there were 169 species from East Asia; 96 species from Siberia, Sakhalin, and Kuril; 69 species from southeast Asia; 29 species from Europe; and 11 species from North America (Table 2).

Fig. 6. Known species number for each family of aquatic Coleoptera.

Family Ponds Rivers Puddles Paddies Wet rocks Sea Ground-water

Haliplidae 11 2 6

Noteridae 9 3 7

Dytiscidae 96 26 33 57 1 5

Gyrinidae 9 14 1 4

Torridincolidae 1

Hydraenidae 2 32 6 1 4 9

Hydrochidae 4 1 1

Helophoridae 5 3 1

Hydrophilidae 49 25 20 32 6 3

Spercheidae 1

Dryopidae 1 4

Elmidae 2 51 1

Total 189 154 64 105 11 13 13

Ratio % 34.4 28.1 11.7 19.1 2 2.4 2.4

Table 5. Habitats for each species of aquatic Coleoptera. Data from Nakajima et al. (2020).



Species in each areaThe species numbers in each region are shown in Table

6. Hokkaido accounts for 31.8% of all species, with the three major islands (Honshu, Shikoku, and Kyushu) contributing 67.3% and Nansei Islands contributing 43.3%. The Ogasawara Islands contain only Dytiscidae, Hydraenidae, and Hydrophilidae, with a total of nine species, and a percentage of only 2.5%. The Ogasawara Islands are oceanic islands; therefore, the aquatic Coleoptera have limited opportunities for dispersal and settlement.

The distribution of endemic species in each region is shown in Table 6. Many endemic Japanese species are further restricted to each region. Hokkaido contains only three endemic species, including Hydroporus ijimai Nilsson & Nakane, 1993 and Ochthebius hokkaidensis Jäch, 1998. The lack of endemic species in Hokkaido is due to the high

commonality between Sakhalin and Siberian species. There are 76 endemic species in the three major islands including Haliplus kamiyai Nakane, 1963, Copelatus kammuriensis Tamu & Tsukamoto, 1955, Orectochilus agilis Sharp, 1884, Ochthebius japonicus Jäch, 1998, Orientelmis parvula (Nomura & Baba, 1961), and Zaitzeviaria sotai Hayashi & Yoshitomi, 2015. Another characteristic of this area is the presence of groundwater species such as Phreatodytes relictus S. Uéno, 1957 and Morimotoa phreatica S. Uéno, 1957. Copelatus ogasawarensis Kamiya, 1932 and Ochthebius sasakii Yoshitomi, Karube & Hayashi, 2019 are endemic to the Ogasawara Islands. There are 43 species endemic to the Nansei Islands, including Allopachria bimaculate (M. Satô, 1972), Gyrinus ryukyuensis M. Satô, 1971, Hydraena okinawaensis Jäch & Díaz, 1999, and Ochthebius amami Yoshitomi & M. Satô, 2001. There are two species endemic

Family Japan Hokkaido Honshu Shikoku Kyushu Nansei Isles Tsushima Ogasawara Isles HSK

Haliplidae 11(2) 5(0) 8(1) 6(0) 6(0) 5(0) 5(0) 0(0) 8(1)Noteridae 16(8) 3(0) 8(3) 7(3) 5(1) 6(0) 1(0) 0(0) 12(7)Dytiscidae 128(32) 57(2) 78(5) 47(3) 52(0) 53(4) 15(0) 4(1) 83(19)Gyrinidae 16(6) 6(0) 10(1) 7(0) 8(1) 6(2) 2(0) 0(0) 11(3)Torridincolidae 1(1) 0(0) 1(0) 1(0) 0(0) 1(0) 0(0) 0(0) 1(0)Hydraenidae 39(31) 7(1) 24(12) 7(0) 8(0) 14(10) 1(1) 1(1) 25(16)Hydrochidae 4(2) 1(0) 3(1) 2(0) 2(0) 1(0) 0(0) 0(0) 3(2)Helophoridae 5(0) 4(0) 3(0) 0(0) 0(0) 0(0) 1(0) 0(0) 3(0)

Hydrophilidae1) 76(22) 23(0) 51(2) 34(0) 40(0) 43(9) 9(0) 4(0) 54(5)Spercheidae 1(0) 0(0) 0(0) 0(0) 0(0) 1(0) 0(0) 0(0) 0(0)Dryopidae 4(1) 1(0) 3(0) 1(0) 1(0) 2(1) 1(0) 0(0) 3(0)

Elmidae 57(51) 7(0) 33(7) 22(2) 26(2) 23(17) 4(1) 0(0) 38(24)

Total 358(156) 114(3) 222(32) 134(8) 148(4) 155(43) 39(2) 9(2) 241(76)1)Except for terrestrial species

Table 6. Species number for each area of aquatic Coleoptera, and endemic species number in parentheses. HSK shows major three islands, Honshu, Shikoku and Kyushu.

Table 7. Species number of aquatic Coleoptera listed in Red List 2007 & 2019 by MOE, Japan

Family EX CR EN VU NT DD Total

Haliplidae 0 0 1 3 1 1 6

Noteridae 0 0 4 2 0 3 9

Dytiscidae1) 1 6 5 11 20 8 51

Gyrinidae 0 1 2 3 4 2 12

Torridincolidae 0 0 0 0 0 0 0

Hydraenidae 0 0 0 0 3 2 5

Hydrochidae 0 0 1 1 1 1 4

Helophoridae 0 0 1 1 0 0 2

Hydrophilidae2) 0 0 1 1 7 2 11

Spercheidae 0 0 0 0 0 0 0

Dryopidae 0 0 0 0 0 0 0

Elmidae 0 0 3 4 0 2 9

Total (2019RL) 1 7 18 26 36 21 109

Total (2007RL) 0 13(CR+EN) 8 15 5 411)Includes subspecies number2)Except for terrestrial species



to Tsushima: Hydraena tsushimaensis Jäch & Díaz, 2012 and Zaitzeviaria kuriharai Kamite, Ogata & M. Satô, 2006.

HabitatsNakajima et al. (2020) classified the main habitats and

listed each species, in descending order of 34.4% in ponds, 28.1% in rivers, 19.1% in paddies, 11.7% in puddles, 2.4% in the sea and groundwater, and 2.0% in wet rocks (Table 5). Compared to Hemiptera, Coleoptera is characterized by a higher proportion found in rivers and the presence of 11 species in the groundwater.

Endangered taxaThe Red List published by the MOE (2019a) lists one

species as Extinct, 7 taxa as Critically Endangered, 18 taxa as Endangered, 26 taxa as Vulnerable, 36 taxa as Near Threatened, and 21 taxa as Data Deficient, for a total of 109 taxa (Table 7; Fig. 5). Hydaticus satoi Wewalka, 1975 has been found to be extinct (Fig. 7). Orectochilus teranishii Kamiya, 1933 has not been recorded since it was originally described and Hydaticus thermonectoides Sharp, 1884 has not been confirmed in recent years.

Several species have been rediscovered because increasing number of researchers have been investigating aquatic Coleoptera. Examples include Copelatus parallelus Zimmermann, 1920, Dytiscus sharpi Wehncke, 1875, Gyrinus niponensis Brinck, 1941, and Orientelmis parvula (Nomura & Baba, 1961).

Compared to the Red List 2007 (MOE, 2007), the total number of taxa increased from 41 to 109 by 2019. The total number of endangered species (Critically Endangered, Endangered, and Vulnerable) has more than doubled from 21 species to 52 species (Table 7). In Japan, the number of species of Gyrinidae and Dytiscidae has been decreasing since the beginning of the 2000s, and the number of Red List species has been increasing.

Genetic diversityFew species or genera have been studied for genetic

diversity. Dytiscus sharpi Wehncke, 1875 has been studied in

regional populations (Nagata et al., 2018).In beetle species, intraspecific mutations of Horelophopsis

hanseni (Minoshima et al. 2013) and Enochrus vilis (Sharp, 1884) have been reported (Minoshima, 2019).

Although the phylogenetic relationships of Elmidae have been investigated (Hayashi et al., 2013, 2016, 2019), no consideration has been given to regional populations.

DiscussionSpecies diversity

Tojo et al. (2017) stated seven factors for insect diversity in Japan: 1) the extended latitudinal range from north to south, 2) the rugged terrain and geography, 3) the humid climate and abundant precipitation resulting from the Asian monsoon climate, 4) the complex formation of the Japanese islands and its geological history, 5) dispersal via land bridges, 6) dispersal of long distances by aerial flight and oceanic floating, and 7) climatic changes in glacial and interglacial cycles. All of these factors contribute to the diversity of aquatic Hemiptera and Coleoptera in Japan. The climate is particularly important; however, the abundance of endemic species is greatly influenced by other factors. Molecular phylogeographic studies on aquatic insects have been conducted on two species of Appasus (Belostomatidae), and their divergence between the continental and Japanese clades was estimated (Suzuki et al., 2013, 2014). Hydraena (Hydraenidae), which describes many species and contains several undescribed species, is expected to be a useful taxa for molecular phylogeographic studies. In Coleoptera, Quaternary fossil record have accumulated, and it is known that two species of Dytiscidae, Ilybius apicalis Sharp, 1873 and Ilybius nakanei Nilsson, 1994, were distributed in Honshu during the last glacial period (Hayashi, 2007). These two species are thought to have become extinct in Honshu due to the warming at the end of the glacial period. Fossils are the most reliable record of how regional differences in species composition have emerged within the Japanese archipelago.

Focusing on aquatic Coleoptera, Short (2018) summarized the described species for each family worldwide. When limited to families distributed in Japan, the total number of described species worldwide is 12,485. There are 358 Japanese varieties, totaling at different times, but the Japanese species numbers are estimated to be less than 3%. Given that the land area of the Japanese archipelago is only 0.25% of the world and it is a mid-latitude region, the number of species is large. Bilton et al. (2019) reviewed the ‘Models in Ecology and Evolution’ using aquatic beetles and introduced some important areas of historical biogeography, such as Gondwana, but did not mention East Asia, including the Japanese islands. This is because few studies have been conducted on aquatic beetles. The Japanese islands were formed by a complex geological history, and are among the most tectonically active regions worldwide. Thus, further studies are required for the speciation of many endemic species in Japan.

ConservationMany aquatic insects are considered endangered in

Japan. In Hemiptera, of the 29 species on the Red List

Fig. 7. Hydaticus satoi Wewalka, 1975. Collected in Mastuyama, Ehime Prefecture, Shikoku, Japan (3 Oct, 1935; collected by Mutsuo Miyatake).



(MOE, 2019a), four species (13.8%) live mainly in the lotic environment and three species (10.3%) live in the sea, with the remaining 22 species found in the lentic environment. In Coleoptera, of the 109 species on the Red List, 22 species (20.2%) live mainly in the lotic environment and six species (5.5%) live in the groundwater, with the remaining 80 species found in the lentic environment. Anthropogenic modification of the river environment is a significant factor in the decline of species inhabiting the lotic environment (Kamite, 2016; Nishihara, 2016). In Japan, rivers have had increasing water quality since the 1970s and, as of 2017, the achievement rate of water quality standards was 94% (MOE, 2019b). Therefore, the habitat status of endangered species could be improved by changing the way rivers are utilized.

Nishihara (2016) cited large-scale development, land development, pesticide use, field maintenance, reservoir renovation, the abandonment of cultivation, the introduction of invasive alien species, and the collection of species as factors contributing to the decline of species since 2000, with climate change also being a factor. In reservoirs, native aquatic insects have been heavily affected by non-native species, Micropterus salmoides Lacépède, 1802, Lepomis macrochirus Rafinesque, 1819, Procambarus clarkii (Girard, 1852), and Lithobates catesbeianus (Shaw, 1802). Common species such as Gyrinus japonicus Sharp, 1873, Hyphydrus japonicus Sharp, 1873, and Laccophilus difficilis Sharp, 1873, which were common on the main islands of Japan until 1990, are found in paddy fields and reservoirs, and have been rapidly declining since 2000 (Ichikawa, 2016). The area of paddy fields in Japan has been declining since its peak in the 1960s (MAFF, 2019). The impact of the decline is pronounced, particularly in the Nansei Islands, because of the limited cultivable paddy fields on the narrow islands, and the breeding sites being greatly reduced (Nishihara, 2016).

In Japan, M. salmoides, L. macrochirus and Lithobates catesbeianus have been designated as invasive species since the Alien Species Act was enforced in 2004. L. catesbeianus can move over land by walking, and M. salmoides and L. macrochirus are transported by humans. In addition, Procambarus clarkii is widely established in Japan, but has not been designated as a specified invasive organism. However, P. clarkii has significant effects on aquatic insects and plants (Anastacio et al., 2005; Nishijima et al., 2017). They live in shallow water and burrow deep into the mud; therefore, they are difficult to eradicate. This species should be designated by the act before further expansion of its distribition.

The Species Protection Act in Japan was enacted in 1993 and, as at the end of 2019, Dytiscus sharpi Wehncke, 1875, Acilius kishii Nakane, 1963, Cybister lewisianus Sharp, 1873, Cybister limbatus (Fabricius, 1775), and H. thermonectoides Sharp, 1884 were designated (EOE, 2019c). Among these, specific conservation plans have been implemented for two species, A. kishii and D. sharpi (EOE, 2019c). The designation prohibits collection; therefore, there is a certain conservation effect. Depending on the legal basis, there are examples of cooperation between the government and private organizations regarding conservation. Habitat restoration and monitoring

is being undertaken by the Conservation Society for D. sharpi in Chiba Prefecture (Nishihara, 2019). However, the situation has not improved for some species due to ongoing habitat degradation and dispersion by invasive species. In this context, university laboratories and insect breeding facilities collaborated to conserve habitat (Kitano & Watanabe, 2016; Nishihara, 2019). Conservation both inside and outside habitats requires the elucidation of the life histories of each target species and the establishment and coordination of organizations to carry out this work on an ongoing basis. The establishment of efficient breeding techniques and the creation of manuals are also important for ex situ conservation. The ultimate goal of conservation is to recover as many endangered species as possible.

AcknowledgementsWe are indebted to Mr. Tomohiro Nakamura (Bun-ichi

Sogo Shuppan, Tokyo) for his help in preparing our book, ‘Aquatic Coleoptera and Hemiptera of Japan,’ on which this review was based.

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[Received: October 5, 2020; accepted: October 31, 2020]

species diversity of aquatic hemiptera and coleoptera in japan

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