Introduction
The oribatid mites belonging to the family Achipteriidae Thor, 1929 occur frequently, even sometimes with high numbers, in forest soils, litters, meadow soils, liverworts, bogs and at edges of lakes with mosses, but rarely found in arboreal habitats. Representatives of this family are diverse in both northern and southern hemispheres, but in the tropics, achipteriid species are mainly found at high elevations, for example, in cloud forest litter. Achipteriid species whose feeding habits have been studied are saprophages and mycophages that apparently feed opportunistically on available resources of fungi, algae and decaying plant material (Root et al. 2007, Seniczak and Seniczak 2007, Lindo et al. 2008, Norton and Behan-Pelletier 2009).
Some species of Achipteriidae are sensitive to environmental changes, including pollutants, and therefore, they may indicate changes in habitats. Several species of this family serve as intermediate hosts of tapeworms of the superfamily Anoplocephalata, which parasitize on wild and domestic animals (Rajski 1959, Denegri 1993, Seniczak and Seniczak 2007). The family is known from the Holarctic, Oriental and Neotropical regions with most species described from the North America, Europe, Central America and East Asia.
Currently, the family Achipteriidae Thor, 1929 includes seven genera, three subgenera, 90 species and four subspecies (Subías 2004, 2015). Among the genera, Achipteria Berlese, 1885 is largest in terms of species richness, and it includes two subgenera and 35 species (including two subspecies). Most known species belong to the nomino-typical subgenus Achipteria (31 species, two subspecies).
Balogh and Mahunka (1979) proposed Izuachipteria and Hokkachipteria as new genera based on the character states of interlamellar setae, but Subías (2004) considered these as a subgenus of Achipteria. The main difference between subgenera Achipteria and Izuachipteria is size of interlamellar setae, which are long and thick, extending beyond basis of lamellar cusps in A. (Achipteria), in contrast very short and slender interlamellar setae (or it is completely absent) in A. (Izuachipteria). Only two species have hitherto been grouped into A. (Izuachipteria), namely A. (I.) imperfecta (Suzuki, 1972) and A. (I.) alpestris (Aoki, 1973).
Eleven species of Achipteriidae have been recorded previously from Japan (Aoki 1959, 1961, 1970, 1973, 1976, Suzuki 1972, Fujikawa et al. 1993, Hirauchi and Aoki 1997, Maruyama 2003, Ohkubo et al. 2015).
The aim of the present work is to redescribe the morphology of a little known species, A. setulosa Golosova, 1981, which is found for the first time in Japan. This species has character states of the subgenus Achipteria (Izuachipteria), therefore, we combine this species in the latter subgenus. Proposing the species grouping of the known species of Achipteriasensu lato along with review of the composition of the family Achipteriidae in Japan, with remarks on their biogeography, habitat ecology, and construction of an identification key to all known species from this country are the other goal of this study.
Discussion
In the comprehensive checklist of oribatid mites of Japan, Fujikawa et al. (1993) presented eight species of Achipteriidae belonging to five genera, namely Achipteria, Anachipteria, Parachipteria, Hokkachipteria and Izuachipteria, but the two latter taxa are now considered as subgenera of Achipteria.
Most of achipteriid species found in Japan are known to be widely distributed in vast areas of the northern hemisphere. Thus, Achipteria coleoptrata (Linnaeus, 1758), A. curta Aoki, 1970, A. nitens (Nicolet, 1855), Anachipteria achipteroides (Ewing, 1913) and Parachipteria punctata (Nicolet, 1855) are widely distributed through Holarctic region. Some of these species were also recorded from the other biogeographic regions, e.g. in addition to their common distributions in Europe (everywhere), North America (USA and Canada), and Asia (Russian Far East, Siberia, Kazakhstan, Mongolia and Japan), A. coleoptrata, A. curta and P. punctata were reported from India, Vietnam, subtropical part of China and Santa Helena islands (Wallwork 1977, Haq and Sumangala 2003, Wang et al. 2003, Chen et al. 2010). Two other species, such as Anachipteria grandis Aoki, 1966 and Parachipteria distincta (Aoki, 1959) have also fairly wide distributions in the Palaearctic region. Only four species, A. (I.) alpestris, A. (I.) imperfecta, A. (I.) setulosa and Parachipteria truncata Aoki, 1970 have restricted distributions mainly in Japan, but two of these, (A. (I.) imperfecta and A. (I.) setulosa) have extended distributions in Taiwan and the Russian Far East (Aoki 1991, Ryabinin and Pan’kov 2002, Ohkubo et al. 2015, Subías 2015).
Among these species, P. distincta is most common species in Japan, which is ubiquitous in this country. Some other species, such as A. curta, A. (I.) alpestris, A. (I.) imperfecta and An. grandis are rather common, especially in its northern and central regions of the country. The other species (A. coleoptrata, A. nitens, A. serrata, An. achipteroides, P. truncata, P. punctata) are relatively rare, and known to be distributed only in one prefecture each. Most species of Achipteriidae in Japan are the inhabitants of the litter of various forests, such as natural broad leaved forests in high mountainous areas, soils of grasslands, wetlands and mosses growing on rocks.
As mentioned above, Achipteriasensu lato is the largest genus of Achipteriidae, and it encompasses diverse species in terms of morphological characters. Balogh and Mahunka (1979) attempted to classify species of Achipteria using the size of the interlamellar setae, but this proposal was not broadly accepted. In this sense, validity of the subgenus Achipteria (Izuachipteria) might not acceptable, but further detailed studies are required on the morphology of both adults and immature stages to clarify the status of this subgenus, which is beyond the scope of the present work.
The structure of lamellar complex is quite diverse in various species of Achipteria, e.g. some species have anteriorly narrowed, elongate triangular lamellae pointed distally with sharp lateral cusps, which is a typical lamellar complex for Achipteriidae and an apomorphic character, according to Weigmann (2010). The other species have very broad lamellae distally with large cusps, which is a plesiomorphic character, according to the above-mentioned author. In case of the latter lamellar complex, the distal ends of lamellar cusps are mostly bent downwards, but in various species, these bending cusps are being either dentate or evenly rounded distally. Based on these different characters, it might be possible to establish at least two subgenera within the genus Achipteria. However, we do not do so, because of the below given reason.
As stated by Weigmann (2010) there are many genera of oribatid mites, creation of which were based upon single conspicuous character or some combination of characters, whose value for assessing phylogenetic relations is questionable. Moreover, Behan-Pelletier (2001) and Lindo et al. (2008) declared that the shapes of the lamellae vary extensively not only within the family Achipteriidae, but even among different families of poronotic Brachypylina, and the polarity of these variations is unclear. They justified that proposing a separate generic taxon based on character of the lamellae is not appropriate.
Although it is not preferable to establish new subgeneric level taxa based on the characters of lamellar complex, it is suggested to classify the known species of Achipteria into three species-groups. The first species-group, which we call the coleoptrata-group, has lamellar complex with anteriorly narrowed, elongate triangular lamellae pointed distally with sharp lateral cusps, but without medial cusps. Besides the type species, A. coleoptrata, this species-group includes such species as A. bicarinata Moskacheva, 1973, A. borealis (Banks, 1889), A. cucullata Moskacheva, 1973, A. elegans Schweizer, 1956, A. holomonensis Cancela da Fonseca & Stamou, 1987, A. italica (Oudemans, 1914), A. oregonensis Ewing, 1918, A. quadridentata (Willmann, 1951) and A. sumatrensis Willmann, 1931.
The second species-group, the serrata-group, has very broad lamellar complex, and the cusps are distally serrated with various dens or teeth. Achipteria serrata has strong serration on the distal end of lamellar cusps, and some other species with same character could be included in this group, e.g. A. (I.) alpestris, A. curta, A. (I.) setulosa and A. catskllensis Nevin, 1977.
The third group, the nitens-group, has similar structures of the lamellar complex to the serrata-group, but the distal end of lamellar cusps are not serrated, i.e. bluntly rounded or sometimes with pointed lateral tooth. This species-group includes A. baleensis Ermilov, Rybalov & Kemal, 2011, A. clarencei Nevin, 1977, A. hasticeps (Hull, 1914), A. (I.) imperfecta, A. nitens (Nicolet, 1855), A. longesensillus Schweizer, 1956, A. longisetosa Weigmann & Murvanidze, 2003 and A. verrucosa Rjabinin, 1974.
This grouping might be useful for further classification of Achipteria species, and it should mentioned here that we do not include some hitherto known species of Achipteria (e.g. A. armata (Banks, 1895), A. hasticeps (Hull, 1914), A. languida (Nicolet, 1855), A. minuta (Ewing, 1909), A. moderatior Berlese, 1923 etc.) into any species-group, due to their unclear diagnostic characters.
It is evident that the large lamellar complex is for protecting the dorsal, lateral and anterior parts of the prodorsum and especially the anterior legs in redrawn position, but in some species of Achipteria the lamellar complex became distinctly smaller; the structure and function of different lamellar complexes are the interesting topics of the future studies.
In conclusion, the following key can be used to identify the adults of all known species of Achipteriidae in Japan.
A key to adults of known species of Achipteriidae in Japan
1 |
Octotaxic system expressed as four pairs of notogastral porose areas |
2
|
– |
Octotaxic system expressed as four pairs of notogastral saccules instead of poros areas (Achipteriasensu lato) |
3
|
2 |
A knife-like humeral projection of pteromorphs lacking (Anachipteria) |
9
|
– |
Pteromorphs with a knife-like humeral projection (Parachipteria) |
10
|
3 |
Lamellar and interlamellar setae long, setae le extending beyond anterior tip of lamellar cusps; setae in not extending far beyond basis of lamellar cusps (Achipteria (Achipteria)) |
4
|
– |
Lamellar and interlamellar setae short, thin, sometimes setae in absent; setae le not reaching anterior tip of lamellar cusps; setae in not reaching basis of lamellar cusps Achipteria (Izuachipteria) |
7
|
4 |
Lamellar cusps rounded or with large lateral dens; sensilli long |
5
|
– |
Lamellar cusps without lateral dens, but medially with 3-4 small dens; sensilli short |
Achipteria (Achipteria) curta Aoki, 1970
|
5 |
Notogastral setae well developed; lamellar cusps broad distally, concave medially, with few serrations or rounded distally |
6
|
– |
Notogastral setae minute or represented by their alveoli; lamellar cusps with large, elongate-triangular lateral dens |
A. (A.) coleoptrata (Linnaeus, 1758)
|
6 |
Notogastral setae long, especially setae c and la very long; lamellar cusps with small lateral dens or blunt at tip; interlamellar setae extending beyond lamellar cusps; sensilli long, slender |
A. (A.) nitens (Nicolet, 1855)
|
– |
Notogastral setae c and la medium long, other setae very short; lamellar cusps with large lateral dens, concave medially and with few serrations; interlamellar setae not reaching tip of lamellar cusps; sensilli short, club-shaped |
A. (A.) serrata Hirauchi & Aoki, 1997
|
7 |
Interlamellar setae short, thin, but conspicuously developed; anterior margin of lamellar cusps distinctly serrated |
8
|
– |
Interlamellar setae absent; anterior margin of lamellar cusps not serrated, but bluntly rounded |
Achipteria (Izuachipteria) imperfecta (Suzuki, 1972)
|
8 |
Rostrum with strongly-developed median horn-like projection; lamellar setae long, thick, reaching anterior end of cusps; body size relatively small (550-610 μm) |
A. (I.) alpestris (Aoki, 1973)
|
– |
Rostrum rounded, without median horn-like projection; lamellar setae short, thin, not reaching anterior end of cusps; body size large (718–796 μm) |
A. (I.) setulosa (Golosova, 1981)
|
9 |
Sensilli fusiform, long, extending far anterior to pedotecta I; lamellar cusps without medial dens; lamellar setae smooth |
Anachipteria achipteroides (Ewing, 1913)
|
– |
Sensilli club-shaped, short, not reaching level of the anterior end of pedotecta I; lamellar cusps with distinct medial dens; lamellar setae barbed |
A. grandis Aoki, 1966
|
10 |
Relatively small species with body length less than 450 μm; notogastral porose areas large; notogaster without granular punctuations |
11
|
– |
Relatively large species with body length greater than 550 μm; notogastral porose areas small; notogaster with large granular punctuations |
P. punctata (Nicolet, 1855)
|
11 |
Lamellar cusps with blunt, but distinct medial dens, lateral dens large; region between medial and lateral dens of lamellar cusps deeply concaved; interlamellar setae extending beyond anterior end of lamellae |
P. distincta (Aoki, 1959)
|
– |
Lamellar cusps truncate, without medial dens; end of lamellar cusps not concaved, but convex, with few small teeth; interlamellar setae not reaching anterior end of lamellae |
P. truncata Aoki, 1976
|