Melanozetes meridianus Sellnick, 1928

Seniczak, Stanisław, Seniczak, Anna & Mistrzak, Marcin, 2024, Morphological ontogeny of Melanozetes meridianus (Acari, Oribatida, Ceratozetidae), Zootaxa 5485 (1), pp. 155-177 : 160-173

Melanozetes meridianus Sellnick, 1928

( Figs 1–18 )

Diagnosis. Adult dark brown, of medium size (527–605), and with characters of Melanozetes given by Seniczak et al. (2023). Translamella present, incomplete or absent, lamellar cusp long. Notogastral setae (14 pairs, including c 2 and c 3) long and barbed, porose areas (4 pairs) rounded, Aa slightly larger than other porose areas. Trochanters III and IV and all femora flattened, femora I and II with large ventral carina.

Juveniles light brown with brown prodorsum, gastronotum, sclerites and legs, nymphs with well-developed lamella. Humeral organ and humeral macrosclerite present, seta c 1 on humeral macrosclerite, setae c 2 and c 3 on unsclerotized integument. Gastronotal shield of larva with seven pairs of setae (d -, l -series and h 1), that of nymphs with 10 pairs of setae (d -, l -, h -series and p 1), setae p 2 and p 3 inserted on large posteroventral macrosclerite. Legs of juveniles stocky, femora flattened, femora I and II with large ventral carina, solenidion ω 2 on tarsus I longer than solenidion ω 1.

Morphology of adults. Adults oval in dorsal and ventral aspect ( Figs 1a , 2 , 4a, 4c ), similar to those redescribed by Behan-Pelletier (1986), but see Remarks. Mean measurements (with standard deviation and range): length of females 585.6±16.2 (559–605, n= 24), width 385.2±11.3 (371–397); length of males 550.6±12.7 (527–579, n= 26), width 358,9±8.5 (345–377). Notogastral setae (14 pairs, including c 2 and c 3) long, barbed, with longer barbs in distal half ( Figs 1a, 1c , 2 , 3a , 4 , 5a, 5b, 5d , 6a ), and four pairs of porose areas, Aa slightly larger than other porose areas. Most ventral setae short and smooth, discidium ( Dis ) triangular, rounded, custodium (cus) short and pointed, genital setae short and smooth ( Figs 2 , 3a , 4c , 6c ). Adanal setae slightly longer than anal and other ventral setae, ad 1 and ad 2 with short barbs, ad 3 smooth ( Figs 2 , 6d ). Chelicera chelate-dentate, seta cha longer than chb, both barbed ( Figs 3b , 5a, 5c , 6b ). Most palp setae smooth ( Figs 3c , 5a, 5c , 6b ). Trochanters III and IV and all femora flattened, femora I and II with large ventral carina, most leg setae with short barbs ( Figs 4 , 5a, 5c, 5d , 6a–c , 7 ). Some leg setae thickened (d on all femora, l’ on femora I and II, l” on genua I and II, v” on tibiae I and II). Solenidion ω 2 on tarsus I is located anteriorly to solenidion ω 1 and is longer than ω 1. Formulae of leg setae [trochanter to tarsus (+ solenidia)]: I—5-3(1)-4(2)-18(2); II—1-5-3(1)-4(1)-15(2); III—2-3-1(1)-3(1)-15; IV—1-2-2-3(1)-12. Tarsi heterotridactylous.

Remarks. The adults of M. meridianus investigated herein are smaller than those investigated by Sellnick (1928) and Willmann (1931) —body size 600 x 405, Shaldybina (1975) —body size 600 x 400, Weigmann (2006) — body length 525–635. Our adults are also smaller than those studied by Behan-Pelletier (1986) from Canada —mean measurements (and range): length of females 644 (622–661, n = 9) and width 424 (402–447), length of males 607 (596–622, n= 7) and width 389 (376–415). The body size of our adults of M. meridianus is larger than that from Japan investigated by Fujikawa (1972) —mean length 495 (471–500) and width 312 (300–329), and Aoki (1973) —body length 538. In the adults studied herein the translamella can be present, incomplete, or absent, which well explains the presence of the translamella in the adult drawn by Shaldybina (1975) and Fujikawa (1972), and its absence in the adult drawn by Willmann (1931), Behan-Pelletier (1986), and Weigmann (2006). The adult drawn by Behan-Pelletier (1986) has shorter lamella and lamellar cusp and thicker head of bothridial seta than in other papers, and seta d on femur I is thin, whereas in our individuals it is thickened. In all papers, the notogastral setae of M. meridianus are long, and they are distributed as in our individuals, and porose area Aa is slightly larger than other porose areas on the notogaster. In almost all papers the adults of M. meridianus have six pairs of genital setae, but those studied by Fujikawa (1972) have 4–6 pairs. The morphology and body size of adults from Śnieżne Kotły (Karkonosze National Park, Poland) investigated herein were similar to those from the steep west slope of Skrajny Granat (Tatry Mts, Poland).

Redescription of juveniles. Larva oval in dorsal and ventral view ( Figs 8 , 9a , 11a, 11c ), body light brown with brown prodorsum, gastronotum, sclerites and legs. Prodorsum subtriangular, rostrum rounded, seta ro and le of medium size, seta in longer, all finely barbed, seta ex short and smooth ( Figs 8 , 10a , 11a, 11b , 12a , Table 1 ). Mutual distance between setal pair le nearly two times longer than that between setae ro, and mutual distance between pair in over three times longer than that between pair ro. Setal pair le inserted approximately midway between setal pairs in and ro. Opening of bothridium rounded, bothridial seta clavate, with barbed head. Ridge present between opening of bothridium in direction of seta in, and lateral depression present above basal part of leg I. Prodorsum finely porose with small pits.

Gastronotum of larva with 12 pairs of setae, including h 3 inserted lateral to anterior part of anal valves ( Figs 8 , 9a , 10a , 11a–c , 12d ); most short and smooth, except for slightly thicker c 3 and longer and barbed dp. Humeral organ located anteriorly to seta c 3, humeral sclerite elongated and porose, with seta c 1, other setae of c -series on unsclerotized integument. Gastronotal shield porose, with small pits and with seven pairs of setae (d -, l -series and h 1), other setae on unsclerotized integument ( Figs 8 , 10a ). Cupule ia located posteriorly to seta c 3, cupule ih lateral to anterior end of anal opening, other cupules not observed between pits. Opisthonotal gland opening gla posterolateral to seta lm. Paraproctal valves (segment PS) glabrous. Legs stocky, femora I and II flattened, with large ventral carina and small pits, most leg setae finely barbed ( Figs 11a–c , 12 , 13 ).

Shape and colour of protonymph and other nymphs as in larva, but lamella present between opening of bothridium and insertion of seta in. Gastronotum with 15 pairs of setae due to appearance of p -series in protonymph, retained by subsequent nymphs ( Figs 9b , 10b , 14 , 15 ), most gastronotal setae short smooth except for slightly thicker c 3 and longer, finely barbed dp. Humeral organ as in larva, humeral sclerite with seta c 1, other setae of c - series on unsclerotized integument. Gastronotal shield porose, with small pits and 10 pairs of setae (d -, l -, h -series and p 1), setae p 2 and p 3 on large posteroventral sclerite ( Figs 9b , 10b , 14 , 15 , 16 ), small pits present on lateral parts of posteroventral sclerite. In protonymph one pair of genital setae present on genital valves, and two pairs are added both in deutonymph and tritonymph ( Figs 9b , 14 , 16c ). In deutonymph, one pair of aggenital setae and three pairs of adanal setae appearing, and remain in tritonymph, all short and smooth. In tritonymph small porose areas present, Aa anteromedial to seta la, A1 anteromedial to seta lm, A2 anterior to seta h 3, and A3 anterior to seta h 1 ( Figs 10b , 15 ). Cupule ia posterior to seta c 3, cupule im posterior to seta lm, cupule ip anterior to seta p 1, cupule iad lateral to anterior part of anal valves, cupules ih and ips pushed laterally to cupule iad ( Figs 10b , 14b ). Opisthonotal gland opening gla anterolateral to seta lp. Anal valves of protonymph and deutonymph glabrous, and those of tritonymph with two pairs of short and smooth setae. Legs stocky, femora I and II flattened, with large ventral carina and small pits ( Figs 16–18 ). Most leg setae finely barbed, and some leg setae thickened (d on all femora and genu IV, v on tibia I and II, v’ on tibia III and IV). Solenidion ω 2 on tarsus I located anteriorly to solenidion ω 1 and longer than ω 1.

Summary of ontogenetic transformations. In all instars the prodorsal seta in is the longest, and ex is the shortest, and le is longer than ro. In all juveniles, the bothridium is rounded, whereas in the adult it is larger and gains lateral and medial scales. In all instars, the bothridial seta is clavate, with barbed head, but in the adult it is relatively shorter and has slimmer head than in the juveniles. The larva has 12 pairs of gastronotal setae, including h 3, while the nymphs have 15 pairs. The notogaster of adult loses seta c 1, such that 14 pairs of notogastral setae remain. The formula of gastronotal setae in M. meridianus is 12-15-15-15-14 (from larva to adult), and formulae of epimeral, genital, aggenital setae and segments PS−AN is as in M. interruptus ( Seniczak et al. 2023) . The ontogeny of leg setae and solenidia is as in M. interruptus except for setae l” and v’ on tarsus I of adult, which are absent in M. meridianus , but are present in M. interruptus .

Distribution, ecology and biology. Melanozetes meridianus has a Holartic distribution and is less frequent in the southern part (Subías 2024). This species is considered by Schatz (1983) tyrphobiont, hygrophilous and panphytophagous. The European M. meridianus inhabits montane forests and wetlands, including raised bogs, alpine and subalpine dwarf shrubs and pastures, i.e., from xeric to rather moist habitats ( Tarman 1970, Horak & Woas, 2010, Schatz 2020). In Sphagnum mosses, a remarkable peak of glycogen granules was observed in this species in autumn and early winter, and low in spring (Smrž & Materna 2000). The North American M. meridianus inhabits bogs, peatlands, tussock heath, wet mosses in flood plains, and dry tundra of North American Arctic and subarctic ( Haarlov 1967; Behan-Pelletier 1986, 1997; Gjelstrup & Solhøy 1994; Presthus-Heggen 2010; Behan-Pelletier & Lindo 2019; Barendregt et al. 2021). The Japanese M. meridianus is more abundant in mountain soil than in moss, litter, humus, and bark of standing trees in a beech forest, a Glehn’s spruce-grass bamboo forest, and a Glehn’s spruce-reed forest ( Fujikawa 1972). In Poland, this species was recorded only from the southern mountain areas ( Seniczak 1989a, Olszanowski et al. 1996, Gabryś et al. 2008), including Tatry Mts ( Rafalski 1966). The latter author found this species at the elevation of 1000 m a. s. l.

In the litter of dwarf mountain pine in Śnieżne Kotły ( Karkonosze National Park , Poland), the density of this species was 693 individuals per 500 cm 3, whereas in moss and grass on the steep west slope of Skrajny Granat ( Tatry Mts , Poland) it was 304 individuals per 500 cm 3. In population of M. meridianus from Śnieżne Kotły, the juveniles dominated (80% of all individuals), and the stage structure of this species was the following: 134 larvae, 118 protonymphs, 218 deutonymphs, 83 tritonymph and 140 adults. The sex ratio (females to males) was 1:1.2, and no female was gravid, indicating seasonal reproduction of this species. In population of M. meridianus from the slope of Skrajny Granat also the juveniles dominated (65% of all individuals), and the stage structure of this species was: 70 larvae, 59 protonymphs, 38 deutonymphs, 32 tritonymph and 105 adults. The sex ratio of M. meridianus was 1:0.8, and 64% of females were gravid, usually carrying two large eggs (215 x 132 each), comprising 37% of total body length of females.

Comparison of Melanozetes meridianus with congeners and remarks

Seniczak et al. (2023) compared chosen morphological characters of adults of Melanozetes species, from which the largest is M. avachai (see Seniczak et al. 2016b), and smallest is M. montanus ( Fujikawa, 2003) . The body size of European M. meridianus is similar to that of M. stagnatilis and M. exobothridialis Bayartogtokh et Aoki, 1998 , but general morphology is also similar to that of M. mollicomus , except for slightly larger and stockier body, and absence of deep, rounded individual depressions at notogastral and adanal setae, which in M. mollicomus are present. Moreover, some leg setae of European M. meridianus are thickened (d on all femora, l’ on femora I and II, l” on genua I and II, v” on tibiae I and II), whereas in M. mollicomus they are thin. The Melanozetes species compared by Seniczak et al. (2023) differ also from one another by the presence of translamella, shape of lamellar cusp and notogastral setae, and the size of porose area Aa.

The adult of M. meridianus from Canada drawn by Behan-Pelletier (1986) differs from those studied herein by having shorter lamella and lamellar cusp, thicker head of bothridial seta and thin seta d on femur I (only leg I was drawn), which in our individuals are thickened, which arises the question if Canadian specimens belong to M. meridianus . The diagnosis of adult M. meridianus given by Sellnick (1928) was imprecise, and this author also noted this species in Iceland and Greenland ( Sellnick 1960), which is close to Canada. Similarly, the Japanese adults of M. meridianus are clearly smaller than those from Europe and Canada, and have 4−6 pairs of genital setae, instead of 6 pairs that are present in European and Canadian adults. Therefore, more detailed investigations are required on European, Canadian and Japanese adults of M. meridianus , including the juvenile stages and molecular investigations to find out if these populations belong to M. meridianus or represent unknown cryptic species.

Seniczak et al. (2023) also compared chosen morphological characters of adults, tritonymphs and larvae of M. avachai , M. azoricus , M. interruptus , M. paramollicomus and M. stagnatilis . Based on the morphology of these stages, M. meridianus is most similar to M. azoricus , M. paramollicomus , and M. stagnatilis , differing from them in four morphological characters, and is most different from M. avachai , differing from it in 13 morphological characters. Generally, M. avachai differs clearly from other species of Melanozetes and is more similar to some species of Fuscozetes Sellnick, 1928 ( Seniczak 1989b, 1993a; Seniczak et al. 2016a). For example, the larva of M. avachai lacks the gastronotal shield as that of F. fuscipes (C. L. Koch, 1844) and F. tatricus Seniczak, 1993 , and seta c 1 is inserted on microslerite, as in F. tatricus ( Seniczak 1993a) . In the larvae of Melanozetes species this seta is inserted on humeral sclerite ( Seniczak 1989a, 1993b). In M. avachai setae c 2 and c 3 are inserted on microslerites, as in F. tatricus , whereas in other Melanozetes species they are inserted on unsclerized integument. In the nymphs and adult of M. avachai femora I and II are oval in cross section, as in Fuscozetes species, whereas in other species of Melanozetes they are flattened, with a large ventral carina.

The adult and tritonymph of M. meridianus have relatively large number of thickened leg setae comparing to most species of Melanozetes ( Table 2 ). The highest number of these setae has M. paramollicomus (30 setae), smaller number has M. meridianus (25 setae), whereas M. avachai and M. azoricus have 13 and nine setae, respectively. In M. interruptus and M. stagnatilis thickened leg setae are absent. The thickened leg setae are common in Trichoribatinae sensu Shaldybina (1975), especially l ʺ on genua and tibiae of adults ( Shaldybina 1971, 1975; Behan-Pelletier 1986; Bayartogtokh & Ermilov 2016; Seniczak et al. 2018, 2019).

Figures

FIGURE 1. Melanozetes meridianus, female. (a) Dorsal aspect, legs I and II partially drawn, scale bar 50 μm; (b) presence of translamella in another adult; (c) shape of seta la (enlarged).

FIGURE 2. Melanozetes meridianus, female, ventral aspect, legs partially drawn, scale bar 50 μm.

FIGURE 3. Melanozetes meridianus, female. (a) Lateral aspect, legs partially drawn, scale bar 50 μm; mouthparts, right side, antiaxial aspect, scale bars 20 μm, (b) chelicera, (c) palp.

FIGURE 4. Melanozetes meridianus, adult, SEM micrographs. (a) Dorsal view, (b) lateral view, (c) ventral view, (d) anterior part of body, dorsal view.

FIGURE 5. Melanozetes meridianus, adult, SEM micrographs, lateral view. (a) Anterior part of body, (b) bothridial seta, (c) mouthparts, (d) middle part of body.

FIGURE 6. Melanozetes meridianus, adult, SEM micrographs. (a) Posterior part of body, lateral view; (b) mouthparts, dorsal view; ventral view, (c) genital plates, (d) anal plates.

FIGURE 7. Melanozetes meridianus, leg segments of adult (part of femur to tarsus), right side, scale bar 20 μm. (a) Leg I, tarsus (pl’ not illustrated); (b) leg II; (c) leg III; (d) leg IV.

FIGURE 8. Melanozetes meridianus, larva, dorsal aspect, legs I and II partially drawn, scale bar 20 μm.

FIGURE 9. Melanozetes meridianus, posterior part of hysterosoma, legs III and IV partially drawn, scale bar 20 μm. (a) Larva, (b) protonymph.

FIGURE 10. Melanozetes meridianus, lateral aspect, legs partially drawn, scale bars 50 μm. (a) Larva, (b) tritonymph.

FIGURE 11. Melanozetes meridianus, larva, SEM micrographs.(a) Dorsal view, (b)lateral view, (c) ventral view, (d) mouthparts, ventral view.

FIGURE 12. Melanozetes meridianus, SEM micrographs larva. Dorsal view, (a) anterior part of body, (b) bothridial seta; lateral view, (c) anterior part of body, (d) posterior part of body.

FIGURE 13. Melanozetes meridianus, leg segments of larva (part of femur to tarsus), right side, scale bar 20 μm. (a) Leg I, tarsus (pl’ not illustrated); (b) leg II, (c) leg III.

FIGURE 14. Melanozetes meridianus, posterior part of hysterosoma, legs IV partially drawn, scale bar 50 μm. (a) Deutonymph, (b) tritonymph.

FIGURE 15. Melanozetes meridianus, tritonymph, dorsal aspect, legs I and II partially drawn, scale bar 50 μm.

FIGURE 16. Melanozetes meridianus, tritonymph, SEM micrographs. (a) Dorsal view, (b) lateral view (c) ventral view, (d) anterior part of body, dorsal view.

FIGURE 17. Melanozetes meridianus, SEM micrographs, tritonymph. (a) Bothridial seta, dorsal view, (b) mouthparts frontal, ventral view; (c) leg II, lateral view, (d) leg I, dorsal view.

FIGURE 18. Melanozetes meridianus, leg segments of tritonymph (part of femur to tarsus), right side, scale bars 20 μm. (a) Leg I, tarsus (pl’ not illustrated); (b) leg II; (c) leg III; (d) leg IV.

Tables

TABLE 1. Measurements of some morphological characters of juvenile stages of Melanozetes meridianus (mean measurements of 10 specimens in μm).

Morphological characters		Larva	Protonymph	Deutonymph	Tritonymph	Adult
Body length		208	390	423	533		590
Body width		158	205	267	371		389
Length of prodorsum		96	115	128	138		191
Length of:	seta ro	22	30	37	42		48
	seta le	25	34	46	52		58
	seta in	46	58	78	88		104
	seta bs	44	50	53	56		45
	seta c 1	16	14	18	30		lost
	seta c 2	24	17	29	54		55
	seta c 3	15	19	26	35		66
	seta da	16	19	23	45		74
	seta dp	22	35	43	56		78
	seta la	10	13	21	40		64
	seta lp	20	25	35	45		88
	seta h 1	21	25	33	48		72
	seta h 3	12	17	22	29		58
	seta h 3	8	20	32	35		51
	seta p 1	not developed	14	16	19		53
genital opening		not developed	33	41	61		75
anal opening		71	80	110	138		107