Ceratozetes parvulus Sellnick 1922

Ceratozetes parvulus Sellnick, 1922

(Figs. 2–19)

Diagnosis

Adult rather small (length 270–310), light-brown, with 10 pairs notogastral setae of medium size, small, rounded porose areas and other characters of Ceratozetes (Behan-Pelletier 1985). Bothridial seta short, clavate with barbed, and distally rounded head. Tutorium with wide, short cusp, and without dorsal spines. Lamellar cusp short and relatively broad. Palpal femur lacking seta inf. Tarsi monodactylous. Anteroventral projection on genua I and II and femur II absent, legs lacking setae l’ on femur III, v’ on genua I and II, it on all tarsi, pl, l” and v’ on tarsus I, and pv on tarsus IV.

Juveniles egg-shaped in dorsal view, unpigmented, and with short and smooth prodorsal and gastronotal setae. Opening of bothridium small, rounded, bothridial seta setiform, of medium size. Cuticle with tubercles, humeral organ weakly formed. Most leg setae relatively short and thick, lacking setae l’ on femur III of deutonymph and tritonymph, pl on tarsus I, it on all tarsi, and pv on tarsus IV of nymphs.

Morphology of adult

Adult rather small, with notogastral setae of medium size (Figs. 2, 3, 4a, 6, 7a, 7b, 7d, 8a–c) similar to that described by Sellnick (1922) and redescribed by Behan-Pelletier (1985) but see Remarks. Body length (and range) of our females 289.4 (277–290) and body width (and range) 174.7 (155–178), males absent. Subcapitular setae m and h slightly longer than a, all smooth (Fig. 3). Most palp setae short and all smooth (Figs. 4a, 4c, 7c, 9a, 9b), seta inf on femur absent, formula of setae (and solenidion): 0-1-1-3-9(1). Chelicera chelate, seta cha longer and thicker than chb, cha finely barbed, chb smooth (Figs. 4b, 7c, 9a, 9b). Epimeral setae short and smooth (Fig. 3). All femora and trochanters III and IV flattened, with ventral carina, largest on femur IV, porose areas on paraxial side (Figs. 5, 6–9). Anteroventral projection on genua I and II and femur II absent, legs lacking setea l’ on femur III, v’ on genua I and II, it on all tarsi, pl, l” and v’ on tarsus I and pv on tarsus IV. Formulae of leg setae (and solenidia, from trochanter to tarsus): I—1-5-2(1)-4(2)-14(2); II—1-5-2(1)-4(1)-13(2); III—2-2-1(1)-3(1)-13; IV—1-2-2-3(1)-10. Legs monodactylous.

Remarks. In our individuals, the range of body length and width is slightly smaller than in those described by Sellnick (1922 —length 270–300, width 170–175), redescribed by Behan-Pelletier (1985 —length 268–308, width 168–196), and investigated by Weigmann (2006 —length 270–310), but other morphological characters are similar as in these papers. The exception is smaller porose area A 3 in our individuals, which is placed more distant laterally from seta h 1 than in the adult drawn by Behan-Pelletier (1985).

Description of juvenile stages

Larva egg-shaped in dorsal view (Fig. 10), unpigmented. Prodorsum subtriangular in dorsal view, all prodorsal setae short and smooth (Table 1). Mutual distance between setal pair le slightly longer than between setal pair ro, but between setal pair in nearly four times longer than between setal pair ro; pair le inserted approximately midway between ro and in (Fig. 13a). Opening of bothridium small, rounded, bothridial seta setiform and of medium size (Figs. 11a, 11b, 11c).

Gastronotum of larva (Figs. 10, 12a, 13a) with 12 pairs of setae, including h 3 inserted laterally to medial part of anal valves; all short and smooth, except for minute h 3 (Table 1), h 2 about 1.5 times longer than h 1. Cuticle with tubercles and transverse folds. Cupules ia, im, ip and opisthonotal gland opening not observed among tubercles of cuticle, cupule ih lateral to anterior end of anal opening (Fig. 12a). Humeral organ weakly formed anterior to seta c 3 (Fig. 13a). Paraproctal valves (segment PS) glabrous, cuticle of lateral part of anal region with tubercles. Most leg segments relatively thick, with short and thick setae, except for longer distal setae on tarsi; setae pl lacking on tarsus I (Fig. 14).

Prodorsum of nymphs, prodorsal setae and bothridium as in larva, but mutual distance between setal pair in only three times longer than between setal pair ro, and part of prodorsum between setal pair in porose. Gastronotum of protonymph with 15 pairs of setae because setae of p -series appearing and present in subsequent nymphs (Figs. 12b, 13b, 15a, 15b); all these setae short (Table 1) and smooth. Cuticle with tubercles and transverse and inclined folds. In protonymph, one pair of genital setae appearing on genital valves (Fig. 12b), and two pairs added in deutonymph and tritonymph each (Figs. 15a, 15b); all short and smooth. In deutonymph, one pair of aggenital setae appearing, and present in tritonymph. Anal valves of protonymph and deutonymph glabrous, in tritonymph with two pairs of anal setae, all short and smooth. Cupules ia, im, ip and opisthonotal gland opening not observed between tubercles of cuticle, ips lateral to anterior part of anal valves of protonymph and iad lateral to anterior part of anal valves of deutonymph and tritonymph. Humeral organ weakly formed, anterior to seta c 3 (Fig. 13b). Cuticle of tritonymph with tubercles and transverse and inclined folds (Figs. 16, 17, 18a, 18b). All femora oval in cross section, most leg segments relatively thick, with short and thick setae, except for longer distal setae on tarsi (Figs. 18c, 18d, 19); some setae absent (l’ on femur III of deutonymph and tritonymph, pl on tarsus I, it on all tarsi, and pv on tarsus IV of all nymphs).

Summary of ontogenetic transformations

In all juveniles, the prodorsal setae ro, le, in and ex are short and smooth, whereas in the adult ro and le are of medium size, in is long, and ex remains short. In all instars, the opening of bothridium is small, but in the adult it is clearly larger and gains a large, inner scale. In all juveniles, the bothridial seta is setiform and smooth, whereas in the adult it is clavate, with short barbs. The larva has 12 pairs of gastronotal setae, the nymphs have 15 pairs (p -series are added in protonymph), whereas the notogaster of adults loses two pairs of setae of c -series (c 1 and c 3), and all setae of d -series, such that 10 pairs of setae remain. The formula of gastronotal setae is 12-15-15-15-10, whereas formulae of epimeral, genital and aggenital setae, and formula of segments PS−AN in C. parvulus are as in C. helenae (Seniczak et al. 2016b). The ontogeny of leg setae and solenidia is given in Table 2.

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Note: structures are indicated where they are first added and are present through the rest of ontogeny; pairs of setae in parentheses, dash indicates no additions; 1 rarely appears in the deutonymph and tritonymph.

Distribution, ecology and biology

Ceratozetes parvulus has a Holarctic distribution (Weigmann 2006). It has been found in many countries in Europe—in the Czech Republic (Starý 1988, 2006), Germany (Weigmann et al. 2015, Lehmitz et al. 2020), England (Monson 1997), Finland (Karppinen 1962, Markkula 1986, Huhta et al. 2010), Poland (Willmann 1939, Żbikowska- Zdun et al. 2006, Seniczak et al. 2016a), Russia (Leonov 2020), Sweden (Tarras-Wahlberg 1961), Switzerland (Borcard 1995, 1997), in Asia—Russian Siberia (Mordkovich et al. 2003, Kozlov 2014), in North America— numerous localities in Canada (Behan-Pelletier & Eamer 2009) and USA —in New York (Belanger 1976 after Mumladze et al. 2013) and Alaska (Behan-Pelletier 1985, Blackford et al. 2014). However, it is rather not abundant. In Norway, only one specimen has been reported from the area near Oslo (Thor 1937), despite several studies in peatlands have been carried out later (Solhøy 1976, 1979; Seniczak et al. 2010, 2019; Markkula 2014). Regarding reproduction mode, C. parvulus is considered parthenogenetic since no males have been found yet (Behan-Pelletier & Eamer 2009).

Ceratozetes parvulus was first found in Sphagnum and other mosses in a bog (Sellnick 1922), and since then it has been most often recorded from peatlands (Behan-Pelletier & Eamer 2009, Weigmann et al. 2015, Seniczak et al. 2016a, Lehmitz et al. 2020, Leonov 2020). Among Ceratozetidae, C. parvulus is a constant component of the peatland oribatid fauna (Behan-Pelletier & Bisset 1994) and was found in 39% of a total 46 studied Holarctic bogs (Mumladze et al. 2013). However, it has also been reported from other moist arctic, subarctic and boreal habitats in different types of forests, meadows and tundra (Behan-Pelletier & Eamer 2009, Huhta et al. 2010, Kozlov 2014, Leonov 2020). It has also been found in water bodies, but its presence there was probably accidental (Behan- Pelletier & Bisset 1994). Krivolutsky and Lebedeva (2004) found C. parvulus in feathers of common reed bunting (Emberiza schoeniclus L.).

In this study, 1733 specimens of C. parvulus were collected in total. This species was particularly abundant in Finse (Fig. 20a), where it achieved the density of 17,800 ind./m 2, while in Trøndelag (Hitra and Høstedmyra) its average density was 2,100 ind./m 2. In the samples collected in Finse at the beginning of August, C. parvulus was represented by all developmental stages and the adults dominated (53% of population), whereas the larvae, protonymphs, deutonymphs and tritonymphs constituted 10%, 6%, 4%, and 27% of population, respectively (Fig. 20b). All studied adults were females and 13% of them were gravid, carrying one large egg (144 x 78), constituting about 50% of the body length of females.

In the samples collected at the end of June in Trøndelag, the adults also dominated (making on average 74% of populations in both peatlands), but the participation of juveniles was lower than in Finse and tritonymphs were absent there. The larvae, protonymphs and deutonymphs made on average 18%, 5% and 3% of populations, respectively.

In Trøndelag, C. parvulus was found in all microhabitats sampled, but preferred hummocks, where it occurred most abundantly and with the highest constancy, while in hollows it was the least abundant and occurred only in 20% of the samples (Fig. 21a). In hummocks, the juveniles were more abundant than in the other microhabitats. This microhabitat may be the most favorable for the development of this species (Fig. 21b). Ceratozetes parvulus is a terrestrial component of the oribatid fauna (Behan-Pelletier & Bisset 1994) that supports its preference to drier peatland microhabitats. According to Popp (1962), hummocks are important for overwintering of C. parvulus. Ceratozetes parvulus was also recorded from Sphagnum lawns (Monson 1997), while in a virgin bog in Finland it was restricted to hollows (Markkula 1986). According to our study hollows seem to be the least favorable for this species (it was not abundant and found only in few samples).

Hummocks and hollows are very distinct peatland microhabitats, which differ from one other in moisture and soil temperature. For example, in one peatland studied in China the temperature was significantly higher in hummocks than in hollows, differing by 4.3°C in May and July, while in September this difference was insignificant (Wang et al. 2021). It can be assumed that in Trøndelag C. parvulus started its reproduction in the spring, because in the sampling season no tritonymphs were found. Temperature is an important factor affecting the development of oribatid mites, so higher participation of juvenile stages in hummocks can be explained by the higher temperature of hummocks.