1✉ c/o Institute of Zoology, University of Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria.
2021 - Volume: 61 Issue: 2 pages: 365-379https://doi.org/10.24349/acarologia/20214436
Prior to the present study 174 species of the oribatid family Brachychthoniidae Thor, 1934 (including 12 spp. inquir., Schatz et al. 2011, updated) were described and grouped into 11 genera using morphological characters of the adult. Brachychthoniidae occur in all terrestrial zoogeographic regions. About two third of all species are recorded from the Holarctic region (Subías 2004, electronic update 2020), with 39 species known from Austria (Krisper et al. 2017). They are common inhabitants of forest litter and moss, but occur also in xeric grassland or in tundra with lichens, as well as in raw humus and soil, and in some localities, they were found with considerable abundances (Schweizer 1956, Moritz 1976a, b, Leonov 2020). Brachychthoniidae are among the smallest oribatid mites, the length of most adults varies between 120-250 µm. They are frequently overlooked and apparently underrepresented in several faunistic and ecological investigations.
The late Konrad Thaler, Innsbruck, forwarded me collections of oribatid mites from high mountains of the Eastern Alps, in particular from the summit of the Festkogel mountain. This material contains numerous remarkable findings which will be discussed elsewhere (Schatz in prep.). In the present paper a new species is presented and described. Its generic position is discussed, which leads to the conclusion that this species represents a new genus.
The Festkogel (3035 m a.s.l.), situated southwest of the locality Obergurgl, is the highest summit on the mountain ridge that descends from the Granatenkogel, between the Gaisberg valley and Ferwall valley. The summit area is on the upper edge of the alpine grassland belt (mainly formed by loose stands of Carex curvula) with the beginning small cushions of subnival vegetation (e.g. Androsace, Cerastium), lichens and mosses.
The specimens were mounted in lactic acid on temporary cavity slides for illustration, and subsequently preserved in ethanol. Drawings were made with a camera lucida attached to a transmission light microscope ''Nikon Eclipse E400''. Each parameter was measured from the particular optimal parallax free perspective. Measurements of total body length was done in lateral view, from tip of the rostrum to the posterior edge of the notogaster, body width as maximal width of notogaster from dorsal view. Measurements indicated are means, with range in parentheses; all measurements are given in micrometers (μm). The terminology of morphological features used in this paper follows that of Moritz (1976a) and Grandjean (summarized by Travé and Vachon 1975, Travé et al. 1996, Norton and Behan-Pelletier 2009). Since the new genus is currently monotypic, its description also serves as the diagnosis of the type species.
Following abbreviations are used: L—length; W—width; RO—rostrum; NA, NM, NP—anterior, median, posterior notogastral sclerites; AD, AN, PR—adanal, anal, peranal sclerites; SpC, SpD, SpE, SpF—suprapleural sclerites; (SpC)—indistinct part of suprapleural sclerite SpC; PA, PP—anterior, posterior pleural sclerites; ro, le, il, bo, exa—rostral, lamellar, interlamellar, bothridial, anterior exobothridial seta; exp—posterior exobothridial porus; c1-3 , d1-3 , e1 , e2 , f1 , f2 , h1- 3, p1-3 — notogastral setae; ia, im, ih, ip, ips—notogastral lyrifissures; mf—muscle fibres; 1a-c, 2a, 3a-c, 4a-d— epimeral setae; g1 , g7 —genital setae; ag—aggenital seta; ad1-3 — adanal setae; an1-2 —anal setae; pe—peranal seta; SC—subcapitulum; ru—rutellum; h, m, ma, a—subcapitular setae; or—adoral seta; MD—movable digit of chelicera; cha—cheliceral seta; ω, d, l, v, cm, acm, u, su—solenidion and setae on palp; σ, σ1, σ2, φ, ω, ω1 , ω2, ω3 —solenidia on genu, tibia, tarsus; dσ, dσ1 , dσ2 , dφ—coupled seta d on solenidion; ε—famulus on tarsus of leg I; ft, pl, tc, it, p, u, a, s, pv, d, l, v—setae on legs I–IV.
Type species: Arcochthonius roynortoni n. sp.
Diagnosis — Adults unique among known Brachychthoniidae (Thor, 1934) by following combination of character states: prodorsum and notogaster with sclerotized plates and ridges, rostral apex and lateral edge of prodorsum with teeth, anterior edges of medial and posterior notogastral sclerites with transverse thickenings, spanned by arched clamp-like ridges, notogastral seta d2 situated close to d1 , far from lateral notogaster margin, seta c3 inserting on distinct embossed plate on anterior notogastral sclerite NA, suprapleural sclerite SpE distinct, lateral to anterior transverse scissure, other suprapleural sclerites absent, suprapleural incision distinct, anterior and posterior pleural sclerites separated, epimeres IV with 4 pairs of setae, genital sclerites with 7 pairs of setae, aggenital sclerites present, adanal sclerites posteriorly fused, adanal seta ad2 hypertrophic, femur IV with 2 setae, genua III and IV each with 2 setae, tarsal setal formulas I–IV 18–14–13–11.
Description of adult — Enarthronote oribatid mite, placed in the family Brachychthoniidae. Body in lateral view arched, prodorsal and notogastral sclerites thickened and sculptured with plates and ridges. Prodorsum strongly elevated, anterior edge bent ventrad. Prodorsal surface with transverse and longitudinal ridges. Rostral apex with three teeth. Lateral edge of prodorsum serrated sawtooth-like, with 3–4 larger teeth posterior to serration. Five pairs of prodorsal setae (rostral, lamellar, interlamellar, anterior exobothridial, bothridial seta). Exobothridial hump protruding, posterior exobothridial seta represented by small pore. Bothridial seta small, with numerous small bristles. Notogaster elevated, divided in three separated sclerites by two broad transverse scissures. Anterior edges of notogastral sclerites NM and NP with distinct transverse thickenings, spanned by arched clamp-like ridges. All sclerites with larger and smaller plate-like ridges. Marginal suture on notogastral sclerite NA absent. 16 pairs of notogastral setae, c1 , c2 , c3 , d1 , d2 , d3 , e1 , e2 , f1 , and h1 inserting on tubercles on distinct plates, f2 , h2 , h3 , p1–p3 inserting on small tubercles on NP. Pygidium strongly arched caudally. Pygidial and pleural sclerites ventrally surrounded by disjugal suture. Pygidial sclerite with deep suprapleural incision, separating posterior pleural sclerite laterally about halfway from pygidium. Suprapleural sclerite SpE distinct, SpC partially fused with notogastral sclerite NA, visible part indistinctly protruding caudad. Sclerites SpD and SpF absent. Anterior and posterior pleural sclerites separated by distinct furrow. Lyrifissures ia – ips in pleural region. Subcapitulum anarthric, with 4 pairs of setae and 1 pair adoral setae. Rutellum atelobasic. Chelicera of chelate type, with one seta. Palp setal formula (trochanter to tarsus, solenidion in parentheses): 0–2–1–3–10(1), (u) fused and forked eupathidium on palptarsus, solenidion on palptarsus originating near base of seta cm. Epimeres I and II fused medially, separated from III and IV by sejugal furrow. Epimeres III and IV separated medially. Epimeral setal formula 3–1–3–4. Genital sclerites with 7 pairs of setae, aggenital sclerites undivided with 1 pair of setae, peranal sclerite narrow with 1 seta each, anal sclerites with 2 pairs of setae, adanal sclerites fused posteriorly, with 3 pairs of setae, seta ad2 hypertrophic. All legs monodactylous, claws smooth. Setal formulas of legs (trochanter to tarsus, famulus included, solenidia in parentheses): leg I 0–3–3(2)–5(1)– 18(3), leg II 0–4–3(1)–4(1)–14(1), leg III 2–3–2(1)–3(1)–13, leg IV 1–2–2(1)–4(1)–11. All setae setiform, smooth.
Etymology — The genus name has two meanings: The prefix ''Arco-'' derives from the arched (latin arcus) clamp-like ridges on the anterior edges of notogaster sclerites. It is also a homage to the municipality of Arco/Trentino (Northern Italy), my second home, where I prepared part of this work. It is compounded with ''-chthonius'', a common generic root for Pseudoscorpions and enarthronote genera (from ancient Greek χθόνιος, meaning ''underworld'' or ''subterranean'').
As for the genus.
Measurements — Total body size (n=11): L 267 (range: 260–280), W 154 (150–165), length of notogaster 202 (196–211, ~75% of total body length), lateral height (genital sclerite – base of seta d1 in lateral view) 131 (125–140).
Integument — Prodorsum sculptured with longitudinal and transversal ridges, notogaster with numerous smaller and larger plates-like sculpturing, ridges and lines. Ventral part of posterior notogastral sclerite (PP) with small tubercles and mosaic-like plates. Body colour pale yellowish to light brown, some specimens whitish.
Prodorsum (Figs 1, 3, 4D, E) — Rostrum extended anteriad and ventrad, apex with 3–5 larger teeth, laterally serrated. Lateral edge of prodorsum with thickened ridge, with some teeth at level of lamellar setae. Between rostral setae two protrusions bulging anteriad, with small, not protruding scale medially (Fig. 4D). Anterior prodorsal region with 4 transverse ridges with small knobs, additional transverse and longitudinal ridges between interlamellar setae, posterior 3–4 pairs of muscle sigillae visible. Prodorsal setae smooth, stout, attenuating, inserting on small tubercles, of medium length. Rostral seta (ro) directed anteriomediad (18–22), lamellar seta (le) directed posteromediad (25–30), interlamellar seta (il) directed anteromediad (30), thus le and il forming an ''X'', exobothridial seta (exa) on well-developed lateral hump anterolateral of bothridium, strongly bent posteriad (25). Posterior exobothridial pore (exp) lateral to bothridium. Bothridium cup-shaped, directed posterolaterad, bothridial seta (bs, 40–50) spoon-shaped, with short stalk and long and narrow head, unilaterally with numerous small bristles.
Notogaster (Figs 1–3, 4F) — Shape subquadrangular in dorsal view, lateral edges almost parallel. Anterior edges of medial and posterior notogaster sclerites with distinct and broad (12–15) transverse thickenings, spanned by arched clamp-like ridges, formed by raised sculpturing. Number of ridges variable (NM 11/12, NP 14/15). On both sclerites one single ridge medially, proximal two ridges posteriorly connected, others partially with posterior appendices, also formed by raised sculpturing. Two medial bands of embossed sculpturing traversing notogaster longitudinally, at level of setal row c1–d1 on anterior notogastral sclerite (NA), at level of e1 on medial (NM), and at level of row f1–h1 on posterior sclerite (NP). Additional plate-like sculpturing at base of most setae, posterolateral of c1 and along posterior edges of sclerites NA and NM. Regular medial field groups on all notogastral sclerites. Some maculae on sclerite NA, mainly around insertion plate of seta d2 . Notogastral setae smooth, stout, attenuating, setae of rows c, d, e, and seta f1 and h1 on distinct sculptured plates, seta p1 and p2 on common plate, f2 , h2 , h3 , p3 not on sculptured plates. All setae inserting on tubercles and of medium length (c1 40, c2 35–40, c3 27–32, d1 42–45, d2 , 45–50, d3 40, e1 45–50, e2 30–35, f1 , f2 , h1–h3 , p1 , p2 25–30, p3 15–18). Seta d2 close to d1 , seta p2 posterior to p1 . Setae p2 and p3 visible in lateral and ventral view.
Pleural region (Figs 2, 3) — Anterior and posterior pleural sclerites (PA, PP) separated, in ventral region closer and almost touching, but never overlapping. Sclerite PA triangular, large (75–80 x 50), anterior apex reaching close to region of lyrifissure ia. Insertion of fine muscle fibres (?, mf) visible on anterior pleural sclerite. Anteroventral part of sclerite PP with polygonal tesselate plate-shaped sculpturing. Suprapleural sclerite SpE rhombic (50 x 20), lateral to notogastral sclerites NA and NM, with thin suture at level of scissure between NA and NM. Lyrifissures as small cupules, in usual position (cf. Moritz 1976a).
Gnathosoma (Figs 2, 4A–C) — Size of subcapitulum 45–55 x 40–45. Four pairs of short attenuating setae (h, m, a 7–10, ma 10–12), 1 pair of spiniform adoral setae (or, 8–10). Rutellum (ru) small (15 x 4–5), with distal teeth directed anteromediad. Palp (45–50) setal formula as for the genus, setae (u) fused, bifid. Lyrifissure on palptarsus positioned proximodorsally. Chelicera (55–60) with one setiform and smooth seta on dorsum of fixed digit (cha, 13), directed anteriad. Movable digit (MD) small. Cheliceral teeth very weakly developed.
Epimeral region (Fig 2) — Epimeral setal formula as for genus, setae setiform (~8–10), setae 1a, 2a on round plates.
Anogenital region (Figs 2, 3) — Genital sclerites large (L 60–65, W ~25), 7 pairs of genital setae in two longitudinal rows, 4 medially, 3 laterally (all ~10–12), aggenital sclerites distinct, large, 1 pair of aggenital setae (~8), peranal sclerites narrow, tapering posteriad, 1 pair of peranal setae (~8), anal sclerites small, long, tapering posteriad, 2 pairs of anal setae (~8–10) on small ridge, adanal sclerites fused, forming a common U-shaped sclerite (W 10–12), width posteriorly slightly diminished by broadened ridge around anal sclerite, adanal setae on small raised ridge, ad1 , ad3 setiform (12), ad2 hypertrophic, lance-shaped, with small velum (~20–25).
Legs (Figs 5 A–D) — Length of legs (incl. claws) moderate (leg I–III 39–42%, leg IV 52% of body length). Leg IV considerably longer than other legs (trochanter IV 30–32). Leg I 110–125, leg II 100–115, leg III 105–120, leg IV 135–150. Setal formula of legs as for the genus. Solenidion φ on tibia I long (68–77), on tibiae II shorter (37–45), other solenidia (σ, ω) short (~15–25), σ1 on genu I very short (7–9). Famulus ε on tarsus I of medium length (~10). Coupled setae and solenidia on genua and tibiae I–IV. Proximodorsal lyrifissures present on all tarsi. Homology of leg setae and solenidia as indicated in Table 1.
Male — unknown (all Brachychthoniidae species are supposed to be parthenogenetic, Norton and Palmer 1991, Cianciolo and Norton 2006).
Immatures — unknown.
Eggs — One specimen with one egg (127 x 77), surface smooth.
Variation — Two specimens with 7/8 genital setae (see remark #6)
Austria, Central Alps, Ötztal Alps, Obergurgl (municipality Sölden), summit of Festkogel mountain (13. June 1999, 46°51.17'N, 11°3.07'E, 3030 m a.s.l.), soil samples, TH 359 BP2b, moss cushion (1 adult specimen), TH 359 BPc, grass tuft (10 adult specimens), all leg. Konrad Thaler.
The holotype (adult female) and two paratypes (females) from the type locality, preserved in ethanol, are deposited in the collection of the Senckenberg Museum, Görlitz, Germany (SMNG). Additional material is deposited in the collection of the author, which will finally also be placed in the Senckenberg Museum, Görlitz.
This species is named in honour of my friend and colleague Dr. Roy A. Norton, who has extensively contributed to our knowledge of oribatid mites. He supported and motivated my work in many ways.
Arcochthonius roynortoni n. sp. shares several morphological character states with known Synchthonius species: body surface strongly sclerotized and arched in lateral view, rostral apex anteriorly with large teeth, lateral edge of rostrum serrated and with large teeth, position of notogastral seta d2 close to d1 , prosomatic suprapleurite absent, suprapleural sclerite SpE present, sclerite SpC partially fused with NA, other suprapleural sclerites absent, suprapleural incision deep, aggenital sclerite present, adanal seta ad2 hypertrophic, setal formula on legs.
Arcochthonius differs from the Synchthonius species in the following: pleural sclerites separated in A. roynortoni n. sp. (vs. fused in Synchthonius, see remark #5), seven pairs of genital setae in A. roynortoni n. sp. (vs. six pairs in Synchthonius, see remark #6), large exobothridial hump in A. roynortoni n. sp. (vs. small in Synchthonius), notogastral seta c3 on distinct embossed sculpturing in A. roynortoni n. sp. (vs. directly on integument of notogastral sclerite NA in Synchthonius, see remark #4),
The genus Synchthonius Hammen, 1952 includes two described species, S. crenulatus (Jacot, 1938) and S. elegans Forsslund, 1956. Moritz (1976b: p. 243) proposed Synchthonius boschmai Hammen, 1952 as a synonym of S. crenulatus (Jacot, 1938). Subías (2017, unpublished electronic update of his 2004 checklist) placed Liochthonius lentus Chinone, 1974 to Synchthonius without any comment, a suggestion I reject. This species has some faint irregular spots, but lacks the character states of known Synchthonius species. The figure of the ventral side of this species (Chinone 1974, fig. 71) depicts different numbers of setae (6/7) on the genital sclerites (see also remark #6).
Many characters of Arcochthonius are shared with other Brachychthoniidae genera (Table 2). In addition to Synchthonius, species of Poecilochthonius Balogh, 1943 and Verachthonius Moritz, 1976 show most similarities with Arcochthonius n. gen.: Poecilochthonius: prodorsal and notogastral surface ornamented, lateral rostral teeth present, prosomatic suprapleurite absent, suprapleural sclerite SpE present, pleural sclerites separated, aggenital sclerite present; Verachthonius: prosomatic suprapleurite absent, d2 close to d1 , suprapleural sclerite SpE present, pleural sclerites separated, p2 posterior to p1 , adanal sclerites fused caudally.
The new species differs from the Poecilochthonius and Verachthonius species in the following: ornamentation of prodorsum and notogaster (vs. smooth in Verachthonius, different ornamentation pattern in Poecilochthonius), rostral apex with three larger teeth (vs. serrations without teeth in Poecilochthonius, smooth and without teeth in Verachthonius), lateral rostral teeth present (vs. absent in Verachthonius), seta d2 close to d1 (vs. d2 near notogastral margin in Poecilochthonius), suprapleural sclerite SpC fused with NA (vs. distinct with seta c3 in Poecilochthonius, fused with NA with distinct lateral lobe, seta c3 on unsclerotized integument in Verachthonius), seta p2 posterior to p1 (vs. p2 laterocaudal to p1 in Poecilochthonius), suprapleural incision deep and distinct (vs. short in Poecilochthonius and Verachthonius), aggenital sclerite present (vs. absent in Verachthonius), adanal sclerites fused caudally (vs. separated in Poecilochthonius), adanal seta ad2 hypertrophic (vs. ad2 not hypertrophic, setiform in Verachthonius, ad2 and ad3 hypertrophic in Poecilochthonius).
The morphological differences of Arcochthonius roynortoni n. sp. to species of these and other Brachychthoniidae genera justify the erection of a separate genus.
In the course of previous studies in the Obergurgl area, Central Alps, 12 Brachychthoniidae species were recorded (Fischer & Schatz 2013, Schatz 1979, 2004). Some species of this family were found in considerable abundances in raw soils (Schatz 2004). Remarkably, on the summit of the Festkogel mountain a further six Brachychthoniidae species beside Arcochthonius roynortoni n. sp. were encountered (Schatz in prep.). As mentioned above, this locality is only sparsely vegetated on the border between alpine grassland and subnival cushion plant formations with scarce pioneer vegetation. Numerous records of Brachychthoniidae species are reported from raw soil and lichens (e.g. Schweizer 1956, Moritz 1976a, b, Leonov 2020).
Arcochthonius roynortoni n. sp. seems to be a rare species. It was detected only in two soil samples in 1999 on the summit of the Festkogel mountain despite numerous additional sampling in the same spot. Also, this species was not found in the course of intensive collecting at lower elevations on the slope of the Festkogel (Schatz 1979). Possibly it survived as preglacial relict on a nunatak or it evolved in this altitude in isolation (cf. Schatz 2020).
Little is known on the feeding preferences of Brachychthoniidae species. Schuster (1956) demonstrated a relationship between relative size of chelicerae and feeding type. Based on the morphology of the chelicera, Brachychthoniidae species can be assumed to be ''microphytophagous'', consuming microorganisms as fungi, bacteria and algae (Lebrun 1971). Microphytophagous oribatid species are very abundant in alpine grassland and subnival cushion plants (Schatz 1979). These observations also apply to the locality of Arcochthonius roynortoni n. sp. This species has a small movable digit with weakly developed teeth and can also be classsified as microphytophagous in the sense of the above authors.
Sincerest thanks to the late Konrad Thaler, the renowned Arachnologist, who collected this material, and to Barbara Thaler-Knoflach for valuable information about the location. The institutes of Ecology and Zoology, University of Innsbruck gave logistic support. Thanks are also due to Valerie Behan-Pelletier, Irene Schatz and two anonymous reviewers for their valuable suggestions and comments on the manuscript.