Morphological Ontogeny, Ecology, and Biogeography of Fuscozetes fuscipes (Acari, Oribatida, Ceratozetidae)

Simple Summary The systematic status of Fuscozetes is not clear in the literature. Therefore, the morphological ontogeny of F. fuscipes, the type species of this genus, was investigated and compared with its congeners in this study, and a new diagnosis of Fuscozetes is given. The juveniles of F. fuscipes are light brown, with a brown prodorsum, sclerites, epimeres, and legs. In all juveniles, a humeral organ and a humeral macrosclerite are present. The gastronotum of the larva has 12 pairs of setae (h3 is present), while the nymphs have 15 pairs. In the larva, the gastronotal shield is weakly developed, most of the gastronotal setae are short and inserted on the microsclerites, and several other macrosclerites and many microsclerites are present on the hysterosoma. In the nymphs, the gastronotal shield is well developed, with 10 pairs of setae (d-, l-, and h-series, and p1), and setae p2 and p3 are located on a large posteroventral macrosclerite. In all the instars, femora I and II are oval in cross-section, without a large ventral carina. Mitochondrial COI sequence data revealed a deep split between the Nearctic and Palearctic populations of F. fuscipes, and a less, but significant, divergence within each continent. These strong geographical barriers were contrasted with multiple cases of shared haplotypes over long distances in the Palearctic, indicating high migration rates in modern times. Abstract The systematic status of Fuscozetes Sellnick, 1928, is not clear in the literature. Therefore, the morphological ontogeny of F. fuscipes (C.L. Koch, 1844), the type species of this genus, was investigated and compared with its congeners in this study, and a new diagnosis of Fuscozetes is given. The juveniles of F. fuscipes are light brown, with a brown prodorsum, sclerites, epimeres, and legs. In all juveniles, a humeral organ and a humeral macrosclerite are present. The gastronotum of the larva has 12 pairs of setae (h3 is present), whereas the nymphs have 15 pairs. In the larva, the gastronotal shield is weakly developed, and most gastronotal setae are short except for a slightly longer h2. Most of the gastronotal setae are inserted on the microsclerites except for h3, and several other macrosclerites and many microsclerites are present on the hysterosoma. In the nymphs, the gastronotal shield is well developed, with 10 pairs of setae (d-, l-, and h-series, and p1), and setae p2 and p3 are located on a large posteroventral macrosclerite. In all the instars, femora I and II are oval in cross-section, without a large ventral carina. Mitochondrial COI sequence data revealed a deep split between the Nearctic and Palearctic populations of F. fuscipes, and a less, but significant, divergence within each continent. These strong geographical barriers were contrasted with multiple cases of shared haplotypes over long distances in the Palearctic, indicating high migration rates in modern times.

The systematics of Fuscozetes are not clear in the literature, and its diagnosis has varied over time.Sellnick [3] paid particular attention to the skeletal characters of adults, whereas Shaldybina [2] added the number of notogastral setae, which can provide information on the phylogeny of moss mites [4][5][6].Based on the morphology of the juvenile stages and adults of the Fuscozetes species, Shaldybina [2,7,8] limited the number of setae on the notogaster of adults to 10 or 11 pairs.Behan-Pelletier [9,10] expanded the diagnosis of Fuscozetes to 10-14 pairs, including c 3 , but this diagnosis was problematic because it also included Melanozetes Hull, 1916, which has 14 pairs of notogastral setae.Based on the morphology of the juvenile stages and adults of the Fuscozetes species, Seniczak et al. [11] restricted the diagnosis of Fuscozetes to 10-13 pairs of notogastral setae, including seta c 2 , and some or all the setae of the d-series.Next, Seniczak et al. [12] and Seniczak and Seniczak [13,14] added the length and position of solenidion ω 2 on tarsus I, which clearly separated Fuscozetes from Melanozetes, both in the nymphs and the adults.In Fuscozetes, solenidion ω 2 is shorter than ω 1 and is placed posterolaterally to ω 1 , whereas in Melanozetes, solenidion ω 2 is as long as or longer than ω 1 and is placed anteriorly to ω 1 .
Shaldybina [2] included Fuscozetes in the subfamily Sphearozetinae, which clearly differs from Ceratozetinae and Trichoribatinae in both the adult and juvenile stages.Weigmann [15] omitted this proposal and included Fuscozetes and related genera in Ceratozetidae, indicating that the systematics of this family need further investigation.
The juvenile instars of the Fuscozetes species are relatively well studied.According to the catalogue by Norton and Ermilov [16], Seniczak et al. [12], and Seniczak and Seniczak [13,14], the morphological ontogeny of seven species of Fuscozetes is known: F. coulsoni A. and S. Seniczak, 2020; F. fuscipes; F. kamchatkicus Seniczak et al., 2016; F. pseudosetosus Shaldybina, 1969; F. setiger (Trägårdh, 1910); F. setosus (C.L. Koch, 1839); and F. tatricus Seniczak, 1993.The morphological ontogeny of F. fuscipes has already been investigated by Seniczak [17], but this study was general and omitted the lateral aspect of the larvae, tritonymphs, and adults, as well as the leg setation, which is important for the morphological comparison of species within the genus.Here, we investigated specimens of F. fuscipes from Norway, which differed slightly from those studied by Seniczak [17] from Poland, illustrating some regional variability in this species.We also illustrated the morphological structures of F. fuscipes with SEM figures to clarify the miniscule characters of this species.In addition, we compared the molecular data (COI) of F. fuscipes from different locations, based on our own and public data.

Morphological and Biological Studies
The specimens of F. fuscipes used in this study were collected on 15 June 2018 by A. Seniczak from patches of Sphagnum mosses on the shore of lake Skomakerdiket (Bergen, Vestland, Norway, 60 • 23 ′ 39.7 ′′ N, 5 • 21 ′ 04.7 ′′ E, 320 m a. s. l.).The samples were extracted in Berlese funnels in the laboratory of the Department of Natural History, University Museum of Bergen (Norway), over ten days.The juveniles of F. fuscipes were identified using the specific characters given by Seniczak [17].In 30 adults selected at random, the sex ratio and the number of gravid females were determined, as well as the body length and width.We also measured the morphological characters of all the instars of F. fuscipes, namely the total body length (in the lateral aspect, from the tip of the rostrum to the posterior edge of the notogaster), the width (in the dorsal aspect, at the widest part of the notogaster), and the length of the anal and genital openings and the setae, perpendicularly to their length.All measurements are given in µm.All light microscopy was performed using a Nicon Eclipse Ni.
For scanning electron microscopy (SEM), the mites were air-dried, coated with Au/Pd in a Polaron SC502 sputter-coater, and placed on Al stubs with double-sided sticky carbon tape.The observations and micrographs were made with a Quanta™ 450 FEG scanning electron microscope.

DNA Barcoding
The specimens of F. fuscipes used for molecular studies were collected in Southern, Central, and Northern Norway.We also used public sequences from the BOLD database, which originated from Canada, Finland, and Germany.The specimens of F. fuscipes were DNA-barcoded at the Canadian Centre for DNA Barcoding (CCDB) in Guelph, Canada.Before sending the samples to the CCDB, each specimen was photographed, and these photos are vouchers available at the Barcode of Life Data System (BOLD, http://boldsystems.org,accessed on 20 November 2023).The specimens were placed in a well containing 50 mL of 90% ethanol in a 96-well microplate and submitted to the CCDB.The mites were sequenced for the barcode region of the COI gene according to standard protocols at the CCDB (www.ccdb.ca,accessed on 20 October 2020), using either the LepF1/LepR1 [23] or the LCO1490/m HCO2198 [24] primer pairs.The DNA extracts were placed in archival storage at −80 • C, mainly at the CCDB, and some (sequencing code starting with UMNFO) at the University Museum of Bergen (ZMUB).The COI sequence chromatograms were checked for double peaks and potential NUMTs, and were blasted in GenBank to detect and exclude possible contaminants.The sequences are available in GenBank (Table 1).The sequences were aligned by eye in MEGA11: Molecular Evolutionary Genetics Analysis, version 11 [29].A Bayesian inference (BI) analysis was conducted in MrBayes 3.2 using a GTR + G + I model of nucleotide substitutions [30].Posterior probabilities were generated from 10 million generations of sampling from two independent runs using one cold (temp = 0.3) and three heated chains, excluding the first 25% of generations as burn-in.The chain convergence was assessed using a standard deviation of the split frequencies approaching 0.01 and a potential scale reduction factor (PSRF) of 1.0.The consensus tree topology was visualized in FigTree 1.4.2(available at http://tree.bio.ed.ac.uk/software/ figtree, accessed on 20 November 2023) and edited in Adobe Illustrator.

Diagnosis of Fuscozetes Sellnick, 1928
Based on the morphological characters given by Seniczak et al. [11,12] and Seniczak and Seniczak [13,14] and F. fuscipes studied herein, the diagnosis of Fuscozetes is as follows: the adults are of a medium size (423-897), brown to dark brown, and with characters of Ceratozetidae [2].A translamella is present or absent, the lamellar cusp is rounded or with teeth, and the bothridial seta is clavate or fusiform.The notogastral setae are short to long (10-13 pairs, including c 2 and all or some setae of the d-series), and the porose areas (4 pairs) are of a similar size or with a larger Aa.Femora I and II are oval in cross-section, and solenidion ω 2 on tarsus I is shorter than ω 1 and is located posterolaterally to solenidion ω 1 .

Diagnosis
The adults are brown, of a medium size (629-897), and with the characters of Fuscozetes given above.The translamella is present, the lamellar cusp is long with teeth, and the bothridial seta is fusiform.The notogastral setae are long (10 pairs, including c 2 ), and porose area Aa is rounded and slightly larger than the other porose areas.
In the juveniles, the bothridial seta is fusiform and a humeral organ and humeral macrosclerite are present.In the larva, the gastronotal shield is absent; most of the gastronotal setae are short and inserted on the microsclerites; and other sclerites and microsclerites are also present.In the nymphs, the gastronotal shield is present, the setae of the c-series are located on the microsclerites, and setae p 2 and p 3 are placed on a large, posteroventral sclerite.
Animals 2024, 14, x FOR PEER REVIEW 7 of 27 630-765), but the length and distribution of the notogastral setae were similar in all studies.According to Shaldybina [8] and Weigmann [15], the porose area Aa was found to be larger than in the adults investigated herein and as reported by Bayartogtokh [47], which may reflect regional variation in this species.Shaldybina [8] drew dark areas around the notogastral setae, which are only observed in young, light-brown adults.

Redescription of Juveniles
The larva is egg-shaped in its dorsal and ventral view (Figures 9c, 10a, 11a and 13b), and its body is light brown with a darker prodorsum, sclerites, epimeres, and legs.The prodorsum is subtriangular, the rostrum is rounded, setae ro and in are of a medium size and finely barbed, and the other setae are short and smooth (Figures 9c,d, 10a, 12a, 13 and Table 2).The mutual distance between setal pair le is almost two times longer than that between setae ro, and the mutual distance between pair in is about four times longer than that between pair ro.Setal pair le is placed approximately midway between the setal pairs in and ro.The opening of the bothridium is rounded and the bothridial seta is fusiform, with a barbed head.A ridge is present between the opening of the bothridium and the insertion of seta in.The prodorsum is finely porose.
The gastronotum of the larva has 12 pairs of setae, including h 3 inserted laterally to the anterior part of the anal valves (Figures 9d, 11a, 12a and 13b).The gastronotal shield is poorly developed and most of the gastronotal setae are short and smooth (Figures 9c,d, 10a, 11a, 12a, 13a-c and 14a,c), except for a slightly longer h 2 .Most of the setae are located on basal microsclerites except for h 3 .The humeral organ is rounded and located anteriorly to seta c 3 .The humeral sclerite is oval and porose, three or four other macrosclerites are present laterally to setae da and dm, and many microsclerites are present in the central and posterior parts of the gastronotum.Three large macrosclerites are present on the lateral side of the gastronotum, including one around the gla opening, along with 3-4 small sclerites (Figures 10a, 11a and 12a).A large sclerite is present posteriorly to leg III.Cupule ia is located posteriorly to seta c 3 , cupule im is located posteriorly to seta lm, im is located between setae h 1 and h 2 , and ih is lateral to the anterior end of the anal opening (Figures 10a, 11a and 12a).The gland opening gla is located laterally to seta lp.The paraproctal valves (segment PS) are glabrous.The chelicera is chelate-dentate (Figure 14b,d).All the femora are oval in cross-section, a large ventral carina is absent, and most leg setae are finely barbed (Figures 9c,d, 13, 14b-d and 15).
The shape and color of the protonymph and other nymphs are the same as in the larva, but the head of the bothridial seta is slimmer, and the prodorsum and gastronotal shield have a reticulate cuticle (Figures 12b,17,18,b).The gastronotum has 15 pairs of setae because the setae of the p-series appear in the protonymph and are retained in subsequent nymphs; all of these setae are short and smooth except for the medium-sized c-series setae (Figures 11b,12b,16,17,18,20a-c and Table 2).The humeral organ is located in the same location as in the larva, the humeral sclerite has seta c 1 , and the other setae of the c-series are located on the basal microsclerites.The gastronotal shield is well developed, with 10 pairs of setae (d-, l-, and h-series, and p 1 ), and setae p 2 and p 3 are placed on a large posteroventral sclerite.Small sclerites are present laterally to the setae of the l-series, and a large macrosclerite is present posteriorly to leg IV (Figures 11b, 12b and 16).In the protonymph, one pair of genital setae is present on the genital valves, and two pairs are added in both the deutonymph and the tritonymph (Figures 11b, 16, 18d and 19b).The genital valves are placed on a large macrosclerite.In the deutonymph, one pair of aggenital setae and three pairs of adanal setae appear, and they remain in the other nymphs; all are short and smooth.In the tritonymph, cupules ia and im are placed in the same manner as in the larva, cupule ip is between setae p 1 and h 2 , cupule iad is lateral to the anterior part of the anal valves, and cupules ih and ips are pushed laterally to cupule iad (Figures 11b, 12b and 16).The gland opening gla is placed anterolaterally to seta h 3 .The chelicerae are chelate-dentate (Figures 18d and 19a).The anal valves of the protonymph and deutonymph are glabrous, and those of the tritonymph have two pairs of short and smooth setae.All the femora are oval in cross-section, a large ventral carina is absent, and most of the leg setae are finely barbed (Figures 18,19,20d and 21).In one deutonymph, two setae v' were present on trochanter III.

Summary of Ontogenetic Transformations
In the larva, the prodorsal setae ro and in are of a medium size, and the setae le and ex are short, whereas in the nymphs and adult, seta in is clearly longer than ro, and seta le is of a medium size.In all the juveniles, the bothridium is rounded, whereas in the adult, it is larger and gains scales.In all the instars, the bothridial seta is fusiform with a barbed head, but in the nymphs and adult, the head is slimmer than in the larva.The larva has 12 pairs of gastronotal setae (h 3 is present), while the nymphs have 15 pairs.The notogaster of the adult loses setae c 1 , c 3 , and those of the d-series, such that 10 pairs of setae remain on the notogaster.The formula of gastronotal setae in F. fuscipes is 12-15-15-15-10 (from larva to adult).The formulae of the epimeral setae are: 3-1-2 (larva), 3-1-2-1 (protonymph), 3-1-2-2 (deutonymph), and 3-1-3-3 (tritonymph and adult); the formula of the genital setae is 1-3-5-6 (protonymph to adult); the formula of the aggenital setae is 1-1-1 (deutonymph to adult); and the formula of segments PS-AN is 03333-0333-022.The ontogeny of the leg setae and solenidia of F. fuscipes is given in Table 3.

Leg
Trochanter Femur Genu Tibia Tarsus , (tc), (p), (u), (a), s, (pv), Note: structures are indicated where they are first added and are present through the rest of the ontogeny; pairs of setae are in parentheses, and a dash indicates no additions. 1Added in some deutonymphs; if not, this seta is added in the tritonymph.

Mitochondrial Genetic Variation
The Bayesian analysis of the COI sequences revealed two deeply diverged clades of F. fuscipes corresponding to exclusively Nearctic and Palearctic populations, respectively (Figure 22), and separated by a 15.5-18.4% uncorrected p-distance.Each regional clade contained 11 haplotypes, with a maximum of 7.2% divergence in the Nearctic clade and 5.8% in the Palearctic clade.Several Palearctic haplotypes were shared between distant locations, including Finnmark and Vestland in Norway, and one haplotype was shared Animals 2024, 14, 538 20 of 27 between Finnmark and SW Finnland.Near-identical haplotypes were found in Agder and Nordland (Norway), and in SW Finland and Germany.
5.8% in the Palearctic clade.Several Palearctic haplotypes were shared between distant locations, including Finnmark and Vestland in Norway, and one haplotype was shared between Finnmark and SW Finnland.Near-identical haplotypes were found in Agder and Nordland (Norway), and in SW Finland and Germany.
Figure 22.Bayesian tree topology based on COI nucleotide sequences (658 bp).Specimen numbers correspond to those in the BOLD database (http://boldsystems.org/, accessed on 20 November 2023).Information about barcoded specimens is presented in Table 1.

Ecology and Biology
We collected F. fuscipes in Sphagnum mosses on the shore of lake Skomakerdiket (Bergen, Norway), where this species achieved a density of 102 individuals per 500 cm 3 .In this

Ecology and Biology
We collected F. fuscipes in Sphagnum mosses on the shore of lake Skomakerdiket (Bergen, Norway), where this species achieved a density of 102 individuals per 500 cm 3 .In this population, the juveniles made up 52% of all individuals, with the following stage structure: 8 larvae, 33 protonymphs, 4 deutonymphs, 7 tritonymphs, and 50 adults.The female-to-male sex ratio was 1:0.3, and 7% of the females were gravid and carried one or two large eggs (290 × 175), comprising 34% of the length of the females.

Morphology and Development
The juvenile stages of F. fuscipes from Norway are generally similar to those from Russia [48] and Poland [17], except for the shape of some setae and sclerites and the number of microsclerites in the larvae.In the larvae from Norway, the prodorsal seta in is of a medium size and barbed, as in the larvae from Poland [17], whereas in the larvae from Russia [48], this seta is short and smooth.In all regions, the larvae of F. fuscipes lack a gastronotal shield, most of the gastronotal setae are located on the microsclerites and other sclerites, and microsclerites are also present on the gastronotum.However, the shape of the macrosclerites differs in these larvae, and the larvae from Norway and Russia have more microsclerites than those from Poland.In the tritonymphs from Norway and Poland, the gastronotal setae are slightly longer than in the specimens from Russia.All these differences probably illustrate regional variation in the species.
Seniczak et al. [14] compared a selection of morphological characters in several Fuscozetes species.In light of this comparison and this investigation, the largest is F. fuscipes, the smallest is F. setiger, and the body length of most species overlaps.These species also differ from one another in the shape of their bothridial seta, translamella, lamellar cusp, and porose area Aa, and in the number and shape of their notogastral setae.Most species have 10 pairs of notogastral setae (c 2 is present); two species (F.novus Shaldybina, 1969, and F. tatricus) have 11 pairs (c 2 and dp are present), and F. setosus has 10-13 pairs of setae (c 2 and some or all the setae of the d-series are present).
Seniczak et al. [14] also compared 23 morphological characters of the larvae and tritonymphs of F. coulsoni, F. fuscipes, F. kamchatkicus, F. setiger, F. setosus, and F. tatricus.The juveniles of F. fuscipes are the most similar to those of F. setosus, differing from them in six morphological characters, and the most dissimilar from those of F. setiger, differing from them in 21 morphological characters.This is not too surprising, in view of the distant phylogenetic relationship to F. setiger indicated by our COI data analysis (Figure 22).
The morphological ontogeny of F. fuscipes is generally similar to that of F. coulsoni, F. kamchatkicus, F. pseudosetosus, F. setiger, F. setosus, and F. tatricus [8,[12][13][14]17,[49][50][51], except for the shape of the larval gastronotum.In F. fuscipes, F. pseudosetosus, and F. tatricus, the gastronotal shield is absent, but macrosclerites and microsclerites can be present, whereas in the other species, the gastronotal shield and a humeral macrosclerite are present, and sometimes other macrosclerites and microsclerites.Among these species, the ontogeny of the leg setae and solenidia were investigated in detail in F. coulsoni, F. kamchatkicus, and F. setiger [12][13][14].The ontogeny of the leg setae and solenidia of F. fuscipes studied herein is most similar to that of F. setiger, differing from it in two morphological characters, and most dissimilar to that of F. kamchatkicus, differing from it in five morphological characters (Table 4).Some leg characters can be diagnostic.
The morphology of the adults and juveniles of Fuscozetes is generally similar to those of Melanozetes [9][10][11][52][53][54][55][56][57][58][59][60], except for the diagnostic characters for these genera.The adults of Fuscozetes have 10-13 pairs of notogastral setae, including c 2 and some or all the setae of the d-series, whereas those of Melanozetes have 14 pairs of setae, including c 2 and c 3 [11,[56][57][58][59].The separation of the juveniles of Fuscozetes from those of Melanozetes is more difficult than the adults.The juveniles of Fuscozetes have a generally smaller area of sclerites on the gastronotum that those of Melanozetes, except for the larvae of some species.For example, the larva of Melanozetes avachai Seniczak et al. 2016 [58] has a weakly developed gastronotal shield, most of its gastronotal setae are located on microsclerites, femora I and II are oval in cross-section, and a large ventral carina is absent, as with that of F. fuscipes studied herein.The separation of the nymphs and adults of Fuscozetes from those of Melanozetes is easier than for the larva, mainly using the length and location of solenidion ω 2 on tarsus I; in Fuscozetes, this solenidion is shorter than ω 1 and is placed posterolaterally to ω 1 , whereas in Melanozetes, solenidion ω 2 is as long as or longer than ω 1 and is placed anteriorly to ω 1 .
The three-dimensional SEM figures of F. fuscipes correspond well with the line drawings of this species, which are, to some degree, subjective and depend on the technique of preparation and the author.For example, in the larva of F. fuscipes, some macrosclerites and microsclerites are observed in different aspects as depressions, which are rarely observed in SEM figures.In the tritonymph, the reticulation of the gastronotum and a humeral sclerite are well observed.

Ecology and Distribution
Fuscozetes fuscipes has a Holarctic distribution [1].In Norway, it has been found in moist mosses in the north (Finnmark), west (Vestland), and south [60][61][62][63].Fuscozetes fuscipes is a hygrophilous [64,65] or meso-hygrophilous species [66,67].It inhabits wet tundra [10], Sphagnum mosses, and wet habitats [65,[68][69][70] close to pools and lakes [71][72][73][74], as well as wet-to-humid forest soils and meadows [45].In an oligotrophic bog in Norway, it was found only in the lower Sphagnum layer, 10-17 cm deep [63,75].Solhøy Wunderle and Solhøy [76] consider F. fuscipes a true arctic and high-mountain species, and Schatz [77] and Murvanidze et al. [45] confirmed the dominance of this species in subalpine zones, while Solhøy found it in an alpine zone in Norway [75].This species is sensitive to some pesticides [78], but it is cold-tolerant and able to survive winter temperatures of −28 • C [79].Wallwork [80] investigated various biological aspects of F. fuscipes in the laboratory.Adults and nymphs were fed on the macerated leaf tissue of hemlock (Conium maculatum L.) and on the moist decaying leaves and petioles of yellow birch (Betula alleghaniensis Britt.), and the nymphs also fed on dead mites and springtails.Madge [65] observed no clear response by adults and juveniles to a higher air humidity in similar experiments, whereas in dry air, these wet-adapted mites quickly died.Nevertheless, this species survives much longer in a lower humidity than another hygrophilous species, H. rufulus C.L. Koch, 1835, probably due to its thicker waxy cuticle on the body [66,81].
Shaldybina [82] cultivated F. fuscipes under laboratory conditions at 18-20 • C; fed it on lichens, mosses, and raw potatoes; and estimated a development time of 86 days for this species.The mean time of the development of successive instars (+ immobile period between stages) was in days: egg, 12; larva, 11.5 + 4; protonymph, 12 + 4; deutonymph, 14.5 + 4; and tritonymph, 17.5 + 6.5.Among the 18 species cultivated by this author, the time of development varied between 40 and 180 days.Under natural conditions in cooler climates, the time of the development of F. fuscipes probably lasts longer than 86 days, which limits its population growth.In our investigation, only 7% of the females of this species were gravid, each carried one or two large eggs, and in June, the number of juveniles was approximately similar to that of the adults.In earlier studies carried out in a similar area at two lake shores, also in June, the juveniles were more abundant than the adults and made up 71% and 81% of each local population [83].
The several shared or near-identical haplotypes between distant sites sampled in this study indicate high migration rates of this species in Europe, at least in modern times.A similar pattern of haplotype identity was found for Platynothrus peltifer (C.L. Koch, 1839) in Western Norway, Belgium, and Germany [102]; P. punctatus (L.Koch, 1879) in Svalbard, Western Norway, and Southern Spain [103]; and Nanhermannia coronata Berlese, 1913, in Northern, Central, and Southern Norway, Ireland, and Finland [104].We may hypothesize that their migration in a latitudinal direction is largely influenced by migrating birds.In contrast, the longitudinal separation of subclades in Canada, and between Canada and Scandinavia, in two different clades of F. fuscipes indicates much less migration in this direction.The morphology of F. fuscipes from Canada has not been studied in detail, including the juvenile stages, so it is not possible to pinpoint any morphological differences between the Nearctic and Palearctic populations.To enable a firm test of species boundaries and taxon validity, more genetic data and morphological studies are needed, because a single and rapidly evolving mitochondrial marker is not sufficiently informative to conclude on such issues.
Furthermore, we note that other oribatid genera have mixed patterns of genetic differentiation across the Atlantic, e.g., Platynothrus troendelagicus Seniczak and Seniczak, 2022, which has identical COI haplotypes across Europe (Norway and Ireland) and Canada [105], whereas populations of P. peltifer have diverged significantly between Europe, USA, and Japan.It is, therefore, possible that more intensive sampling of F. fuscipes will support a similar cryptic-species scenario.In this context, it will be useful to study the ecological traits that may affect long-distance colonization in this species group.

1.
Fuscozetes species are a well-formed morphological group of mites that differ clearly from closely related Melanozetes species, both as nymphs and adults.The adults of Fuscozetes have fewer notogastral setae (10-13 pairs, including c 2 and some or all the setae of the d-series) than those of Melanozetes (14 pairs, including c 2 and c 3 ), and the adults and nymphs have a shorter solenidion ω 2 on tarsus I than ω 1 , and it is placed posterolaterally to ω 1 .In Melanozetes, solenidion ω 2 is as long as or longer than ω 1 and is placed anteriorly to ω 1 .

2.
Fuscozetes fuscipes is a hygrophilous species and prefers wet tundra, Sphagnum mosses, and wet habitats close to pools and lakes.

3.
Mitochondrial genetic data revealed deeply diverged populations across the Holarctic, a high local and regional genetic diversity, and several examples of haplotypes shared between distant Scandinavian localities, indicating latitudinal long-distance migration.

Figure 7 .
Figure 7. SEM micrographs of adult Fuscozetes fuscipes.Ventral view of (a) medial part of body and (b) posterior part of body; (c) anterior part of body, lateral view; and (d) part of leg I, dorsal view.

Figure 8 .
Figure 8. SEM micrographs of Fuscozetes fuscipes.Part of leg I of adult, (a) dorsal view and (b) lateral view; larva, (c) dorsal view and (d) lateral view.

Figure 7 . 27 Figure 7 .
Figure 7. SEM micrographs of adult Fuscozetes fuscipes.Ventral view of (a) medial part of body and (b) posterior part of body; (c) anterior part of body, lateral view; and (d) part of leg I, dorsal view.

Figure 8 .
Figure 8. SEM micrographs of Fuscozetes fuscipes.Part of leg I of adult, (a) dorsal view and (b) lateral view; larva, (c) dorsal view and (d) lateral view.

Figure 8 .
Figure 8. SEM micrographs of Fuscozetes fuscipes.Part of leg I of adult, (a) dorsal view and (b) lateral view; larva, (c) dorsal view and (d) lateral view.

Figure 13 .
Figure 13.SEM micrographs of Fuscozetes fuscipes larva.(a) Dorsolateral view; (b) ventral view; and dorsal view of (c) anterior and medial part of body and (d) anterior part of body.

Figure 14 .
Figure 14.SEM micrographs of Fuscozetes fuscipes larva.(a) Posterior part of body, dorsolateral view; ventral view of (b) anterior part of body, (c) medial part of body, and (d) gnathosoma.

Figure 13 . 27 Figure 13 .
Figure 13.SEM micrographs of Fuscozetes fuscipes larva.(a) Dorsolateral view; (b) ventral view; and dorsal view of (c) anterior and medial part of body and (d) anterior part of body.

Figure 14 .
Figure 14.SEM micrographs of Fuscozetes fuscipes larva.(a) Posterior part of body, dorsolateral view; ventral view of (b) anterior part of body, (c) medial part of body, and (d) gnathosoma.

Figure 14 .
Figure 14.SEM micrographs of Fuscozetes fuscipes larva.(a) Posterior part of body, dorsolateral view; ventral view of (b) anterior part of body, (c) medial part of body, and (d) gnathosoma.

Figure 19 .
Figure 19.SEM micrographs of Fuscozetes fuscipes tritonymph.Ventral view of (a) anterior part of body and (b) posterior part of body; dorsal view of (c) anterior and medial part of body and (d) bothridial seta.

Figure 19 . 27 Figure 19 .
Figure 19.SEM micrographs of Fuscozetes fuscipes tritonymph.Ventral view of (a) anterior part of body and (b) posterior part of body; dorsal view of (c) anterior and medial part of body and (d) bothridial seta.

Table 1 .
Information about sequenced specimens of Fuscozetes fuscipes and other oribatid species used in this study.Na-not available; these sequences are public in BOLD, but without some data.

Table 2 .
Measurements of some morphological characters of juvenile stages of Fuscozetes fuscipes (mean measurements of 4-10 specimens in µm).

Table 2 .
Measurements of some morphological characters of juvenile stages of Fuscozetes fuscipes (mean measurements of 4-10 specimens in µm).
Bayesian tree topology based on COI nucleotide sequences (658 bp).Specimen numbers correspond to those in the BOLD database (http://boldsystems.org/, accessed on 20 November 2023).Information about barcoded specimens is presented in Table1.