The morphological ontogeny of Limnozetes solhoyorumsp. nov. from Norway is described and illustrated. This species is most similar to L. guyiBehan-Pelletier, 1989 as adult, but differs from it mainly by the sculpture of notogaster, shorter prodorsal seta in and notogastral setae, especially p1, and the distribution of posterior notogastral setae. The tritonymph of L. solhoyorum differs from that of L. guyi by shorter gastronotal setae c3, dm, dp, lm and lp. In the deutonymph, tritonymph and adult of both species, setae d and l' from genu IV are absent.
The mites of Limnozetes Hull, 1916 live abundantly, including the juveniles, in semiaquatic and aquatic habitats, bogs, fens and at the edges of lotic habitats (Behan-Pelletier 1989; Seniczak 2011; Seniczak et al. 2016, 2019a, b; Lehmitz et al. 2019). Behan-Pelletier (1989) gave the diagnosis of the genus Limnozetes, from which the main diagnostic characters are well-developed pteromorph, presence of dorsal expansion of bothridium, tutorium and genal tooth, weakly developed bothridial seta, lack of lenticulus and one or two setae a from tarsi, and presence of very long and curved seta d on femora I–III. By contrast, the juveniles of Limnozetes have long bothridial seta, but the gastronotum can be rounded, with plicate cuticle and short, thin setae or elongated, with slightly folded cuticle and blade-like marginal setae. Based mainly on the morphology of juveniles of nine species of Limnozetes from North America and Europe, Seniczak and Seniczak (2009a) divided Limnozetes species in ‘rugosus group', with stocky juveniles, plicate cuticle, and short and thin gastronotal setae, and ‘ciliatus group’, with boat-shaped juveniles, slightly folded cuticle and blade-like marginal setae on the gastronotum.
The morphological ontogeny of Limnozetes species is insufficiently known. According to the catalogue of juvenile oribatid mites by Norton and Ermilov (2014), the full morphological ontogeny of four species of this genus is known: L. ciliatus (Schrank 1803), L. foveolatus Willmann, 1939 (= L. palmerae Behan-Pelletier, 1989), L. lustrum Behan-Pelletier, 1989 and L. rugosus (Sellnick, 1925). The morphological ontogeny of L. amnicus Behan-Pelletier, 1989, L. borealis Behan-Pelletier, 1989, L. feuerborni Willmann, 1932b, L. guyi Behan-Pelletier, 1989, L. latilamellatus Behan-Pelletier, 1989 and L. onondaga Behan-Pelletier, 1989 is only partially known.
While working on the oribatid mite fauna from a poor bog carpet near lake Langvotnevatnet (Kvam, Vestland, Norway), we found in a bog pool in submerged moss Warnstorfia fluitans (Hedw.) a large population of undescribed Limnozetes species, with all juvenile stages. These juveniles have plicate cuticle on the gastronotum, indicating their membership in the ‘rugosus group'.
The aim of this paper is to describe and illustrate the morphological ontogeny of this species, as Limnozetes solhoyorum sp. nov., and to compare its morphology with congeners.
Materials and methods
The juveniles and adults of L. solhoyorum sp. nov. were collected on 25th June 2008 by the senior author from a bog pool, from submerged moss Warnstorfia fluitans, in a poor bog carpet near lake Langvotnevatnet (Kvam, Vestland, Norway: 60.370718°N, 6.024418°E, 340 m a. s. l.). For a better understanding of the morphology of the ‘rugosus group’, we also investigated the ontogeny of leg setae and solenidia of L. rugosus and the shape of chelicerae and palp of this species, which were omitted by Seniczak and Seniczak (2010). We also present the scanning electron microscopy (SEM) micrographs of both species because they illustrate better than the line drawings the surface of the body – an important diagnostic character of Limnozetes species. The latter species was collected from mire in Finse (Vestland, Norway, 60.585722°N, 7.516167°E, 1300 m a. s. l.).
The illustrations of instars are limited to the body regions that show substantial differences between instars, including the dorsal and lateral aspects of the larva, tritonymph and adult, ventral regions of all instars and some leg segments of the adult and tritonymph. The chelicera and palp of the adult are also illustrated. We measured the total length (from tip of rostrum to posterior edge of notogaster) and width (widest part of notogaster without pteromorphs), and length of setae and some parts of the body of mites in µm. Illustrations were prepared from specimens mounted temporarily on slides in lactic acid (Grandjean 1949a). In the text and figures, we used the following abbreviations: rostral (ro), lamellar (le), interlamellar (in) and exobothridial (ex) setae, lamella (La), bothridium (bo), bothridial seta (bs), notogastral or gastronotal setae (c-, d-, l-, h-, p-series), lyrifissures or cupules (ia, im, ip, ih, ips, iad), opisthonotal gland opening (gla), subcapitular setae (a, m, h), cheliceral setae (cha, chb) and Trägårdh organ (Tg), palp setae (sup, inf, l, d, cm, acm, lt, vt, ul, su) and solenidion ω, genal tooth (gt), pedotectum 1 (Pd1), tutorium (Tut), discidium (Dis), epimeral setae (1a–c, 2a, 3a–b, 4a–b), adanal and anal setae (ad-, an-series), aggenital seta (ag), leg solenidia (σ, φ, ω), famulus (ε) and setae (bv, ev, d, l, ft, tc, it, p, u, a, s, pv, v). The terminology used follows that of Grandjean (1949b, 1951, 1953) and Norton and Behan-Pelletier (2009). The species nomenclature follows Subías (2004, 2019).
For scanning electron microscopy (SEM), mites were fixed in 90% ethanol and placed on Al-stubs with a double-sticky carbon tape and coated with Au/Pd in a Polaron SC502 Sputter coater. Observations and micrographs were made with a ZEISS Supra 55VP scanning electron microscope.
Adults of medium size (344–364), with characters of Limnozetes. Lamella narrow, with well developed cusp, translamella absent. Seta in reaching half length of lamella, sensillus short, setiform. Notogaster convex, with elongated and rounded foveae, short and thin setae, length to width of notogaster and length to width of pteromorph similar (1.2:1). Famulus on tarsus I present, but setae d and l' from genu IV absent.
Juveniles plicate, with characters of Limnozetes. Prodorsal setae le and in and bothridial seta long and thin, whereas gastronotal setae short and thin, except slightly longer lp, h1 and h2 in larva, and p1 and h-series in nymphs. In deutonymph and tritonymph, setae d and l' from genu IV absent.
Adult. Measurements: mean body length (and range): females 353.9 (344–364, n= 50) and width 240.1 (234–244).
Prodorsum. Rostrum rounded, lamella long, without translamella, lamellar cusp of medium size (14), rounded, with lamelar seta (Figs. 1a, 2, 3a, 4a, 4b, 4c, 5a, Table 1). Setae le and in longer (46–48) than ro (33–35), and in thinner than le and ro. Seta ro inserted on lateral part of rostrum, whereas seta in inserted close to inner border of lamella and anterior border of notogaster. Bothridium rounded, with well developed dorsal expansion (Figs. 1a, 5a), bothridial seta short, setiform. Medial and posterior parts of prodorsum punctate.
Notogaster. Convex, ratio length to width of notogaster similar to ratio length to width of pteromorph (1.2:1, Figs. 4a, 5c). Ten pairs of notogastral setae present, including c2, all short (Table 1) and thin. Notogaster with elongated and rounded foveae, anterior part and pteromorphs with more elongated foveae than central part (Figs. 1–3a, 4, 5a). Lyrifissures ia and im posterolateral to seta c2 and posterior to seta la, respectively, ip posterolateral to seta h1, ips and ih anterior and anteromedial to seta p3, respectively, and iad anteromedial to seta ad3 (Figs. 1a, 2, 3a).
Gnathosoma. Subcapitular seta h as long as m (23), seta a slightly shorter (16), all smooth (Figs. 2, 5b). Chelicera (length 87–89, width 38) with setae of similar length (12), cha thicker than chb, both barbed (Fig. 3b). Palp (length 55–57) with short and smooth setae (Figs. 3c, 5b, 6b, 6d), palpal eupathidium acm fused with solenidion ω at some distance from insertion of solenidion, eupathidia ul1, ul2 and su short (Figs. 3c, 6b, 6d). Formula of palp setae (trochanter to tarsus + solenidion ω): 0-2-1-3-9(1).
Ventral aspect. All epimeral setae short and smooth (Figs. 2, 4d), formula of epimeral setae 3-1-2-2. Six pairs of genital setae, all short and smooth, inserted on inner part of genital plates. One pair of aggenital setae as short as genital setae. Anadal setae short, but slightly longer than anal setae. Ventral and anal plate with elongated and rounded foveae (Figs. 4d, 5c).
Legs. Seta d on femora I–III very long, curved and barbed (Fig. 7). Seta l'' on genua I and II relatively thick, famulus ε on tarsus I short, solenidia ω1 and ω2 on tarsi I and II fused at some distance from insertions (Figs. 6a, 7a, 7b, 7e), setae d and l' absent from genu IV, distal setae on tarsi short, thick and barbed (Fig. 6c, 7). Formulae of leg setae [trochanter to tarsus (+ solenidia)]: I—1-4-3(1)-4(2)-15(2); II—1-4-3(1)-4(1)-14(2); III—2-3-1(1)-3(1)-13; IV—1-2-0-3(1)-10. Tarsi heterotridactylous.
Larva oval in dorsal aspect, light brown, cuticle plicate (Fig. 8). Prodorsum subtriangular, all prodorsal setae thin and smooth; ro of medium size, le and in long and ex short. Mutual distance between setal pair le about two times longer than between setal pair ro, and mutual distance between setal pair in about four times longer than between pair ro. Pair le inserted closer to pair ro than to pair in. Opening of bothridium small, oval, bothridial seta setiform and longer than seta in. Area between bothridia with transverse folds, lateral parts with longitudinal folds.
Gastronotum of larva with 12 pairs of setae, including h3 inserted laterally to medial part of anal valves (Figs. 9a, 10a). Setae of c-series and h3 minute, other gastronotal setae short, but posterior setae, especially seta lp, longer than other setae (Fig. 8, 10a, Table 1), all smooth. Cupules not observed in plicate cuticle. Opisthonotal gland opening anterolateral to lp (Fig. 10a). Paraproctal valves (segment PS) glabrous. Most leg setae short, setae l' and l'' on all genua and tibiae thicker than v' on tibiae (Fig. 11).
Prodorsum of protonymph porose, prodorsal setae and bothridial seta as in larva. Gastronotum of protonymph with 15 pairs of setae due to appearance of setae of p-series (Fig. 9b), retained in subsequent stages (Figs. 12a, 12b); all short and smooth. In protonymph, one pair of genital setae present on genital valves, and two pairs added in deutonymph and tritonymph each, all short and smooth. In deutonymph, one pair of aggenital setae and three pairs of adanal setae added, and retained in tritonymph (Figs. 12a, 12b), all short and smooth. In protonymph and deutonymph, anal valves glabrous, in tritonymph two pairs of short and smooth anal setae present. Cupules not observed in plicate cuticle, opisthonotal gland opening anterolateral to seta lp (Fig. 10b). Some anterior and medial setae hardly visible in plicate cuticle, posterior marginal setae on gastronotum longer than other setae (Figs. 10b, 12a, 12b, 13, 14, 15a). Palp of tritonymph with short and smooth setae, palpal eupathidium acm fused with solenidion ω at some distance from insertion, eupathidia ul1, ul2 and su short (Figs. 15c, 15d). Leg segments stocky, most leg setae short, setae l' and l'' on genua I and II and tibiae I and II thicker than seta v' on tibiae I and II (Fig. 16a, 16b). Famulus ε on tarsus I short, solenidia ω1 and ω2 on tarsi I and II fused together at some distance from insertions (Figs. 16a, 16b, 16e, 17a, 17b). In tritonymph, seta l'' on femora I–III present or absent, and setae d and l' from genu IV absent (Fig. 16).
Summary of ontogenetic transformations
In all juveniles of L. solhoyorum, setae le and in are long, ro is of medium size and ex is short, whereas in the adult in, le and ro are long, and ex is short. The bothridium is small and rounded in all juveniles, but in the adult it gets larger, and develops a dorsal expansion. In all juveniles, the bothridial seta is long, setiform, whereas in adults it is short, setiform or absent. In all instars, the gastronotal setae are short. The larva has 12 pairs of gastronotal setae, including h3, the nymphs have 15 pairs (p-series is added). The notogaster of adult loses setae c1, c3 and of d-series, such that 10 pairs of setae remain, all are short and smooth. The formula of gastronotal setae of L. solhoyorum is 12-15-15-15-10 (from larva to adult), formulae of epimeral setae are 2-1-2 (larva), 3-1-2-1 (protonymph) and 3-1-2-2 (deutonymph, tritonymph and adult). Formula of genital setae is 1-3-5-6 (protonymph to adult) and formula of aggenital setae is 1-1-1 (deutonymph to adult). Fomula of segments PS–AN is 03333-0333-022. The ontogeny of leg setae and solenidia of L. solhoyorum is given in Table 2.
Distribution, ecology and biology
We found a large population of L. solhoyorum sp. nov. in a small pool in submerged moss Warnstorfia fluitans. In this pool, 5,994 individuals of L. solhoyorum were collected in 2 dm3 of moss, corresponding to density 150,000 individuals per m2. This species constituted 82% of all Oribatida, being accompanied by Hydrozetes octosetosus Willmann, 1932a (17% of Oribatida) and single individuals of Hydrozetes lacustris (Michael 1882), Limnozetes foveolatus Willmann, 1939, Tyrphonothrus maior (Berlese 1910), Oppiella nova (Oudemans 1902) and Diapterobates humeralis (Hermann 1804). Juveniles of L. solhoyorum constituted 24% of all individuals of this species and the stage structure was: 119 larvae, 539 protonymphs, 732 deutonymphs, 50 tritonymphs and 4,554 adults. In the same bog, one adult of L. solhoyorum was also found in Sphagnum pulchrum (Lindb. ex Braithw.) Warnst and two adult individuals were found in S. riparium Ångstr. (mentioned as Limnozetes. sp. 1 in Seniczak et al. 2019b). Only females were noted, and 70% of them were gravid, carrying one or two large eggs. The eggs were relatively large (157 × 94), comprising 44% of total body length of females.
The holotype (female) and five paratypes (females) with above collection data are deposited in University Museum of Bergen, University of Bergen, Bergen, Norway.
This species is named in honour of two oribatologists working in Norway: Dr. Ingrid Wunderle-Solhøy and Prof. Torstein Solhøy, University of Bergen.
Comparison of the morphology of Limnozetes solhoyorum with congeners
Limnozetes Hull, 1916, with the type species L. ciliatus, comprises medium sized mites as adults (266–390 µm), with Limnozetes characters. Subías (2019) listed 14 species in Limnozetes, including Limnozetes palmerae Behan-Pelletier, 1989, but the latter species we consider a junior synonym of L. foveolatus Willmann, 1939, as suggested by Siepel et al. (2009) and supported by Lehmitz et al. (2019). Among these species, the largest is L. similis Pérez-Íñigo & Baggio, 1989 and the smallest is L. foveolatus. The body length of most species overlaps (Table 3). These 14 species also differ one from another by the presence of translamella, shape of bothridial seta, shape of notogaster and pteromorph, length of some setae and presence of setae d and l' on genu IV.
Based on the ontogeny of leg setae (Table 2), setae d and l' are also absent from genu IV in deutonymphs and tritonymphs of the ‘rugosus group’ (L. borealis, L. guyi, L. latilamellatus, L. rugosus, L. solhoyorum sp. nov.), with plicate cuticle (Behan-Pelletier 1989; Seniczak & Seniczak 2010), whereas these nymphs of the'ciliatus group', with boat-shaped juveniles, have these setae. Among the adults of the ‘rugosus group’, with plicate juveniles, L. borealis and L. rugosus have an incomplete translamella, whereas other species lack the translamella. However, in L. latilamellatus the lamella is wide, without lamellar cusp, which is unique in Limnozetes, whereas in L. solhoyorum and L. guyi the lamella is narrow, with lamellar cusp. From this comparison it is evident that L. solhoyorum is most similar to L. guyi, but differs from it mainly by shorter prodorsal seta in and notogastral setae, especially p1, and by the distribution of posterior setae on the notogaster. In L. solhoyorum, seta h1 is inserted more medially than in L. guyi (Behan-Pelletier 1989). The tritonymph of L. solhoyorum differs from that of L. guyi mainly by shorter gastronotal seta c3, dm, dp, lm and lp. The body size of the adult of L. solhoyorum overlaps with that of L. rugosus, and the shape of body, pteromorph, leg setae, solenidia and famulus ε is also similar in both species (Figs. 4, 5a, 5c, 6a, 7, 18, 19, 20). The latter species has incomplete translamella, which in the former species is absent, and differs from it also by the length:width ratio of the notogaster and pteromorph (Table 3).
The tritonymph of L. solhoyorum differs from that of L. rugosus in few morphological characters (Table 4). The former species has higher number of longer setae on the gastronotum than the latter species both in the larva (lp, h1, h2, and lm, respectively) and nymphs (p1, h-series, and lp, h1, h2, respectively), and differs also from it by the shape of setae in and h3 in the larva. Moreover, the shape of leg setae and solenidia of the adult and tritonymph of L. rugosus (Figs. 20, 21) is similar as in L. solhoyorum presented above, but the ontogeny of setae differs in these species. In L. rugosus, seta l" is more often added on femora I and II than in L. solhoyorum, l" can appear on tibia II in the deutonymph or tritonymph (versus in tritonymph in L. solhoyorum), and l' appears in the deutonymph (versus in the tritonymph in adult of L. solhoyorum). The shape of body, palp and leg setae, solenidia and famulus ε of tritonymph of L. rugosus (Figs. 21, 22) is similar as in the tritonymph of L. solhoyorum (Figs. 14–17).
Measurements of some morphological characters of juvenile stages and adult of Limnozetes solhoyorum sp. nov. (mean values of 10 individuals in µm); Nd: not developed.
Ontogeny of leg setae (Roman letters) and solenidia (Greek letters) in Limnozetes solhoyorum sp. nov.
Selected morphological characters of Limnozetes species.
The juveniles of L. solhoyorum sp. nov. are stocky, with plicate cuticle and short setae on the gastronotum, as other species of the ‘rugosus group' of Limnozetes proposed by Seniczak and Seniczak (2009a). All species also lack setae d and l' on genu IV in the deutonymph and tritonymph (Behan-Pelletier 1989; Seniczak & Seniczak 2010). By contrast, the juveniles of the'ciliatus group’ are boat-shaped, with slightly folded cuticle and blade-like marginal setae on the gastronotum, and they have setae d and l' on genu IV in the deutonymph and tritonymph.
In the adults of Limnozetes, only the presence of setae d and l' on genu IV supports the division of Limnozetes in ‘rugosus group' and'ciliatus group', whereas using the other morpholological characters is risky. For example, in most species of the ‘rugosus group', the notogaster of adults is more convex than in most species of the'ciliatus group', but it is also convex in L. lustrum from the'ciliatus group' (see Seniczak & Seniczak 2010). In the ‘rugosus group’, the range of length to width ratio of notogaster is smaller (1.1:1–1.3:1) than in the ‘ciliatus group' (1.2:1–1.3:1), and this ratio of pteromorph is larger (0.9:1–1.9:1) than in the ‘ciliatus group’ (0.6:1–0.9:1), but in several species of both groups each ratio overlaps.
Grandjean (1951) compared several morphological characters of Limnozetes and Hydrozetes Berlese, 1902 and stated that these genera are closely related, and morphological characters are more derived in Limnozetes than in Hydrozetes. This relationship is also well-observed in a similar way of loss of setae of c-series in these genera, which reflects their phylogeny and was supported by molecular studies (Krause et al. 2016). In both genera, the loss of these setae starts with seta c1 and continues to c3, according to the hypothesis of Shaldybina (1972). However, in some species of Hydrozetes, all setae of c-series are present (Seniczak et al. 2007, 2009), whereas in some individuals of L. rugosus only c2 and c3 are present (Seniczak & Seniczak 2010), indicating more derived character than in some species of Hydrozetes.
Selected morphological characters of juveniles of Limnozetes solhoyorum sp. nov. and L. rugosus.
The ontogeny of leg setae of Limnozetes also differs from that of Hydrozetes. For example, some species of Limnozetes lost the famulus ε on tarsus I and many leg setae (setae pl on tarsus I, a'' on tarsi I and II, setae a' and a'' on other tarsi, l' and d on genu IV in deutonymph to adult), which are present in Hydrozetes (Seniczak et al. 2017). However, in Limnozetes species, tarsal setae u and p are short, thick and barbed, similarly as in Hydrozetes species (Seniczak & Seniczak 2008, 2009b; Seniczak et al. 2007, 2009, 2017), and these setae reflect their ecological importance. They cooperate with claws and help the mites to stick to water plants, which can be easily observed when manipulating the mites with the needle.
Our study shows the importance of SEM micrographs for better understanding of the morphology of Limnozetes species. For example, the hand made figures show similar sculpture of the adult of L. solhoyorum to that of L. rugosus and L. borealis (Behan-Pelletier 1989; Seniczak & Seniczak 2010), whereas the SEM micrographs clearly differentiate these species. The famulus ε on tarsus I is small and can be easily overlooked in light microscopy, but is well observed in SEM micrographs. The latter micrographs also show the shape of leg and palp setae, fusion of solenidia ω1 and ω2 on tarsi I and II in a certain distance from their insertions. However, some palp setae observed in SEM micrographs look thicker than those in the hand made figures, which probably results from coating the mites for the SEM study.
We are grateful to Dr. V.M Behan-Pelletier (Invertebrate Biodiversity Program, Research Branch, Agriculture and Agri-Food Canada, Ottawa, Canada) for her suggestion that this Limnozetes species is new for science. We are grateful to Prof. Kjell Ivar Flatberg (Norwegian University of Science and Technology, Trondheim, Norway) for identification of moss Warnstorfia fluitans. This study was supported by the grant from The Norwegian Taxonomy Initiative - Knr 35-16 Norwegian Forest Oribatida (NFO) - highly diverse, but poorly known. This study was also done under the program of the Polish Minister of Science and Higher Education "Regional Initiative of Excellence" in 2019–2022 (Grant No. 008/RID/2018/19).