Sellnickia caudata (Michael, 1809)

Sellnickia caudata (Michael, 1809) View in CoL ( Notaspis )

Grandjean (1958) provided a very detailed and comprehensive re-description of the adults of this species. Herein, we only repeat and complement the most important features

and provide the first SEM images of this species as well as the first description of juveniles.

Adult

Diagnosis. Dark brown, large sized (656–734 µm) oribatid mites. Sexually dimorphic rostral lobe. Sensillum short globular. Prodorsal setae long, robust and ciliate. Lamellar ridge present. Dorsosejugal scissure incomplete. Octotaxic system present, large porose areas. Ten pairs of almost completely reduced notogastral setae. Large pygidial protuberance present. Legs heterotridactylous, with tarsal, tibial and femoral porose areas and remarkably pectinate unguinal tarsal setae.

Characters of the adult

Measurements. Females (N = 4), length: 656–734 µm (mean 684 µm), width: 425–456 µm (mean 442 µm); males (N = 2), length: 656–719 µm (mean 688 µm), width: 419–431 µm (mean 425 µm).

Integument. Colour dark brown. Cuticle smooth, except for gastronotic area showing faint foveate pattern ( Figure 7a,e View Figure 7 ). Cerotegument absent, except for thin finely granular layer covering lateral area between bothridium and leg acetabula ( Figure 7b,c View Figure 7 ).

Prodorsum ( Figures 7c View Figure 7 and 8a,c View Figure 8 ). Rounded in dorsal view; striking sexual dimorphism present, females with median rectangular

rostral lobe, not seen in dorsal view, males also with median rectangular rostral lobe but lateral parts of this structure are strongly notched resulting in a frontal orientation of the lobe, which is then very well visible in dorsal view ( Figure 8c View Figure 8 ). Rostral seta (ro) long (ca. 80 µm), setiform and slightly ciliate, lamellar (le) and interlamellar seta (in) very long (150–165 µm), robust setiform and strongly ciliate. Exobothridial seta (ex) shorter (ca. 75 µm) but also robust and strongly ciliate. Prominent lamellar ridge running from bothridium to the base of lamellar seta, nearly straight in dorsal view, curved in lateral view ( Figure 7c View Figure 7 ). Tutorium (tu) developed as cuticular enfolding with sharp edge reaching from anterior of bothridium to insertion of rostral seta ( Figure 7c View Figure 7 ). Sensillum short with slender stalk and large globular head covered by minute barbs.

Notogastral region. Circular in dorsal view, with conspicuous large caudal protuberance (ca. 50 µm) at the posterior end ( Figures 7a View Figure 7 and 8 View Figure 8 ). Protuberance with median porose area at its terminal part; no difference in this structure between male and female. Dorsosejugal scissure medially interrupted. Two pairs of small inconspicuous ridges in humeral area. Ten pairs of minute notogastral setae (c 2, la, lm, lp, h 1–3, p 1–3), seta p 1 located on caudal protuberance ( Figures 7d View Figure 7 and 8a,c View Figure 8 ), seta p 2–3 only visible in lateral view. Octotaxic system consisting of large elliptical or round porose areas. Porose area Aa largest near vestige la, A1, A2 and A3 slightly smaller, all located on posterior half of notogaster ( Figures 7e,f View Figure 7 and 8a,c View Figure 8 ). Orifice of opisthonotal gland gla circular and laterad of seta lp.

Lateral aspect. Pedotectum I PtI large shield-like, pedotectum II PtII smaller, but still well-developed ( Figure 7c View Figure 7 ). Discidium absent.

Podosoma and venter. Epimeral setation 3-1-3-3, all setae smooth and spiniform (19–30 µm). Six pairs of genital setae, genital orifice slightly larger in females. One pair of aggenital setae (ag). Two pairs of anal setae (an 1–2) and three pairs of adanal setae (ad 1–3), ad 1 and ad 2 located posterior of anal valves. Lyrifissure iad small in anterior paranal position.

Gnathosoma. Chelicera chelate with strong interlocking teeth on digits ( Figure 9a View Figure 9 ). Seta cha and chb long spiniform and unilaterally barbed. Trägårdh’s organ Tg a blunt thin triangular lamellar blade in paraxial position. Palp setal formula 0-2-1-3-8 (+ solenidion ω), tarsal eupathidium acm and solenidium ω associated forming a “corne double” ( Figure 9b View Figure 9 ). Mentum diarthric, seta h spiniform smooth (ca. 30 µm). Rutellum with three distal teeth, the most lateral largest and blunt, two inner teeth slightly smaller but acute, all almost fully covered by thin lamella in ventral view ( Figure 9c View Figure 9 ). Setae a and m spiniform and unilaterally barbed (15–20 µm).

Legs ( Figure 10). Heterotridactylous, median claw strongest, lateral claws slightly weaker and with distal ventral indentation. All claws borne on an elongated slender pretarsal stalk, whereas stalk of tarsus I is slightly shorter than that of other legs. Cerotegument finely granular. Large porose areas on paraxial aspect of all femora and elongated porose areas on ventral side of each tarsus and tibia. A small dorsal porose area on tarsus I and II surrounding the base of solenidia. Tibia and tarsi elongated whereas in posterior legs they are relatively longer. Solenidia φ 1–2 of tibia I borne on small apophysis. Setae (u) on tarsus I remarkably pectinate ventrally, the same applies to setae (p) and (u) on all other tarsi ( Figure 7g,h View Figure 7 ). Leg setation and solenidia: I (1-5-2-4-18) (1-2-2), II (1-5-2-4-15) (1-1-1), III (2-3-1-3-15) (1-1-0); IV (1-2-2-3-12) (0-1-0).

Remarks

Grandjean (1958) provided the most detailed description for Sellnickia caudata and although his specimens originated from Queensland, Australia, the specimens from New Zealand investigated herein exactly match their morphology. We could not find any deviating characters. The second known species of this genus, Sellnickia heveae ( Oudemans 1927) , was synonymized with S. caudata by Subías (2022) based on their almost identical appearance. However, Oudemans (1927) noted that his species shows a thick, lanceolate and unilaterally ciliate palptibial seta unlike that of S. caudata which is typically setiform. This difference was confirmed by Grandjean (1958) and thus the supposed synonymy should be discarded ( Behan-Pelletier 2015). Sellnickia heveae also possesses the characteristic pygidial protuberance and Oudemans (1927) hypothesized that it consists of a sticky material that allows the mites to glue themselves to the leaves when strong winds blow. Grandjean (1958) clearly contradicts that, stating that the pygidial tubercle rather secretes a typical cerotegument layer instead of a sticky substance. Observations in the field are clearly needed to answer if the protuberance is involved in attachment or in some kind of pheromonal communication, or in something else.

The sexual dimorphism of rostral lobes found in Sellnickia caudata is unusual and the function is yet unknown. Norton and Alberti (1997) stated that the male lobe has got the right size and shape to fit over the pygidial tubercle of the female, and thus males could possibly “nuzzle” this protuberance. In that case, the question remains why males show the exact same pygidial tubercle with a probable secretory function. Sexually dimorphic rostral structures are known to occur in several other oribatid taxa, as for example in the genus Symbioribates where rostral setae are modified or in Nasozetes where there is a spatulate protuberance in the males (e.g. Behan-Pelletier 2015). These dimorphic structures in oripodoid and other mites may all be somehow involved in some kind of mating behaviour (e.g. Bayartogtokh et al. 2017, 2022).

The higher-level taxonomy of Sellnickia species shows some inconsistencies and needs clarification. The family Sellnickiidae , consisting of S. caudata and S. hevea , is accepted by Norton and Behan-Pelletier (2009) and Schatz et al. (2011), but Subías (2022) includes these two species in the family Oribatulidae . Grandjean (1958) was the first to discuss the family placement of Sellnickia and he argued that the genus shares many characters with Oribatulidae , as for example the elongated pretarsi, the notched lateral claws, or the tarsal porose area which is also present in Phauloppia and Lucoppia . At the same time, he admitted that there are certain characters in Sellnickia that are not present in Oribatulidae , e.g. the lack of centrodorsal setae, a sclerotized spermatopositor and a specifically shaped preanal organ, which could justify the erection of a separate family. However, Grandjean (1958) refrained from doing the latter and stated that a final decision should only be made when the juvenile morphology of Sellnickia is known in detail. Balogh and Balogh (1984) established the subfamily Sellnickiinae in Oribatulidae and listed the reduction of notogastral setae, the absence of the dorsosejugal suture, 5 to 6 pairs of genital setae and the adanal position of lyrifissure iad as diagnostic characters for this group. They also included the monotypic genus Grandjeania into this subfamily. Later, Norton and Behan-Pelletier (2009) were the first to mention the family name “ Sellnickiidae ” and they listed femur II with 5 setae, the caudal protuberance bearing porose area and seta p 1, a complete circumpedal carina posterior to acetabulum IV and the possession of one solenidion on tarsus II as diagnostic traits for the family. The legs of Grandjeania are not known in detail but its notogaster exhibits two caudal protuberances that do not bear seta p 1 and the circumpedal carina is absent, at least according to the figures ( Balogh 1963, p. 42); as a consequence, it was excluded from Sellnickiidae . Schatz et al. (2011) outlined the higher-lever classification of Oribatida and listed the Sellnickiidae as monogeneric family with two species, S. caudata and S. heveae and they mentioned Balogh and Balogh 1984 as family authors. Shortly after, Behan-Pelletier (2015) published a review on sexual dimorphism in oribatid mites which included details of Sellnickia caudata , but it was listed as a member of the family Oribatulidae in all associated tables. The recent world catalogue of oribatid mites ( Subías 2022) conforms to that and includes Sellnickia in Oribatulidae , and accordingly the placement of this group of mites remains unclear. Based on the present data on adult and juvenile morphology (which follows in the next section), we think that Sellnickia should be placed in the separate family Sellnickiidae and not in Oribatulidae , although they are closely related to the latter group. The diagnostic familial characters given by Norton and Behan-Pelletier (2009) should be accepted whereas the presence of tarsal pulvilli in the immatures should be added to the diagnosis. The Sellnickiidae should only include the two Sellnickia species for now, but when more morphological details on Grandjeania and their juvenile morphology are reported in the future, the family might be adjusted in the sense of Balogh and Balogh (1984).

Ecology

Members of the genus Sellnickia have been exclusively found on leaves of trees and thus they are apparently arboreal. Specimens of S. heveae were collected from Hevea sp. tree leaves ( Oudemans 1927), while individuals of S. caudata were collected from leaves of lemon trees ( Grandjean 1958), from green leaves of an undetermined plant ( Hammer 1966) and from twigs and leaves of green button mangrove C. erectus (present study). The occurrence on different species of plants indicates that the mites are not adapted to a specific type of plant. In the present study, we found high numbers of S. caudata specimens on nearly every investigated leaf, and they were observed to feed on the plant tissue and to form moulting aggregations on the underside of the leaves. Whether such strong infestations can cause harm to the plant is yet unknown, at least such cases have not been reported yet.

Distribution

Sellnickia caudata was originally described from Auckland, New Zealand ( Michael 1908), but details about the exact collection site were not provided in the respective publication. This species was also collected in the Rotoehu State Forest in the Bay of Plenty ( Hammer 1966) and now it was found on Waiheke Island in the Hauraki Gulf, New Zealand. Consequently, S. caudata may show a common distribution on the landmasses of north-eastern North Island. Outside New Zealand, there is a single report from Maryborough in Queensland ( Grandjean 1958) pointing to a wider distribution in Australia as well. The second species, S. heveae was reported from Medan in Sumatra ( Oudemans 1927). Presently, all members of the genus are only known from the Australasian region.

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Description of juveniles

Common features of juvenile stages. Apheredermous. Bideficient; all opisthonotal setae associated with small porose excentrosclerites Figure 11a–c View Figure 11 . Colour of prodorsum light brown, gastronotic region pale yellow. Integument soft, slightly plicate, except for stronger sclerotized prodorsum. Posterior edge of the prodorsum with a pair of lighter spots. All prodorsal setae long and barbed ( Figure 11a View Figure 11 ). Bothridium small, cup-like. Sensillum short globose, surface of distal globe rough, covered with minute spines. A pair of small light spots adjacent to dorsosejugal scissure. Gastronotum oval in dorsal view, slightly convex in lateral view. Legs monodactylous with large lobe-like subunguinal pulvillus ( Figure 11d View Figure 11 ). Porose areas present on femora, tibiae, and tarsi (difficult to observe). Seta d absent from genua and tibiae although respective solenidion is present.

Larva. (N = 1): length 262 µm.

Prodorsum ( Figure 12a). Interlamellar (in) and lamellar seta (le) long, robust, and ciliate (ca. 50 µm), the former slightly thicker. Exobothridial seta (ex) long, spiniform, weakly ciliate (ca. 30 µm). Rostral seta (ro) long, thin, weakly ciliate (ca. 34 µm).

Gastronotic region ( Figure 12a). Ten pairs of notogastral setae, c 1-2, da, dm, dp, la, lm, lp, h 1–2. Setae c 1–2, da, dm, la and lm long (length 15–25 µm), robust and ciliate; other setae short (8–11 µm), spiniform and smooth. At the base of each seta there is a small porose sclerite, slightly invaginated, difficult to observe. Orifice of opisthonotal gland (gla) circular, ring-like; gland small elliptical sac.

Podosoma and venter ( Figure 12b). Epimeral setation 2-1-2, all setae short (approx. 8 µm) and spiniform. Claparède’s organ (clp) globular. Aggenital, genital, adanal and anal setae not developed. Cupule of lyrifissure ih anterior of anal opening.

Legs. Setation and solenidia: leg I (0-2-2-2-16) (1-1-1), leg II (0-2-2-2-13) (1-1-1), leg III (0-2-1-1-13) (1-1-0).

Protonymph. (N = 1): length 286 µm.

Prodorsum. Interlamellar (in) and lamellar seta (le) long, robust, and ciliate (52–56 µm), the former slightly thicker. Exobothridial seta (ex) long, spiniform, weakly ciliate (ca. 40 µm). Rostral seta (ro) long, thin, weakly ciliate (ca. 47 µm).

Gastronotic region. Fourteen pairs of notogastral setae; c 1-2, da, dm, dp, la, lm, lp, h 1–3, p 1–3. All short (ca. 5 µm), spiniform, smooth with small porose sclerites (excentrosclerite) at base. More than 40 additional small, slightly invaginated porose sclerites, distributed over gastronotic region.

Podosoma and venter. Epimeral setation 3-1-2-1, all setae spiniform and approx. same length (8 µm). Setae 1c and 4b added in this stage. One pair of short (ca. 5 µm) genital setae. Adanal and anal setae not developed. Cupule of lyrifissure ip anterior of anal opening.

Legs. Porose areas paraxially on femora I-III and ventrodistally on tarsi and tibiae I-III; difficult to observe. Setation and solenidia: leg I (0-2-2-3-16) (1-1-2), leg II (0-2-2-2-13) (1-1-1), leg III (0-2-1-1-13) (1-1-0), leg IV (0-0-0-0-7) (0-0-0).

Deutonymph. (N = 3): length 331–388 µm (mean 365 µm).

Prodorsum ( Figure 13a View Figure 13 ). Interlamellar (in) seta very long, robust and ciliate (ca. 90 µm), lamellar seta (le) shorter and thinner (ca. 63 µm). Exobothridial seta (ex) long, spiniform, weakly ciliate (ca. 40 µm). Rostral seta (ro) long, thin, weakly ciliate (ca. 53 µm).

Gastronotic region ( Figure 13a View Figure 13 ). Fourteen pairs of notogastral setae, same positions, shapes and lengths in protonymph. Microsclerites and porose areas same as in protonymph.

Podosoma and venter ( Figure 13b View Figure 13 ). Epimeral setation 3-1-2-2, seta 4a added in this stage (ca. 9 µm). Two pairs of short (ca. 6 µm) genital setae g 1–2. Three pairs of adanal setae (ca. 11 µm) flanking anal orifice. Cupule of lyrifissure iad anterior of anal opening.

Legs ( Figure 14). Porous areas same as in protonymph, but in this stage also on leg IV. Setation and solenidia: leg I (0-3-2-3-16) (1-2-2), leg II (0-3-2-2-13) (1-1-1), leg III (1-2-1-1-13) (1-1-0), leg IV (0-2-1-1-12) (0-1-0).

Tritonymph. (N = 1): length 494 µm.

Prodorsum ( Figure 13c View Figure 13 ). Interlamellar (in) seta very long, robust and ciliate (139 µm), lamellar seta (le) shorter and thinner (120 µm). Exobothridial seta (ex) long, spiniform, weakly ciliate (62 µm). Rostral seta (ro) long, thin, weakly ciliate (65 µm).

Gastronotic region ( Figure 13c View Figure 13 ). Fourteen pairs of notogastral setae, no difference to deutonymph (microsclerites included). Additional microsclerites same, positions and shapes as in deutonymph.

Podosoma and venter. Epimeral setation 3-1-2-3, seta 4c added in this stage, close to trochanter IV, length of all epimeral seta ca. 9 µm. Four pairs of genital setae g 1–4 (6 µm). Three pairs of adanal setae, same positions, shapes, and sizes as in deutonymph. Two pairs of short anal setae (6–8 µm), fully developed in this stage.

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Legs. Porose areas same as in deutonymph. Setation and solenidia: leg I (0-3-2-3-18) (1-2-2), leg II (0-3-2-3-15) (1-1-1), leg III (1-2-1-1-15) (1-1-0), leg IV (1-2-2-1-12) (0-1-0).

Conspicuous changes during ontogeny. The larva shows small porose excentrosclerites associated only with the notogastral setae. From the protonymph, numerous additional small, slightly invaginated porose sclerites appear on the gastronotum and these are not associated with other structures. The anterior notogastral setae c 1–2, da, dm, la and lm are remarkably long, robust and ciliate in the larva, but in the protonymph they become small spiniform and smooth, and they keep this appearance throughout the subsequent ontogenetic development.

Remarks

Oudemans (1927) also found a tritonymph but gave very limited information on it in his work about S. heveae . He mainly mentions the ambulacrum which consists of a pretarsus equipped with a single claw and a conspicuous pulvillus (“carunkel”). Immatures of S. caudata show the exact same feature on their legs. Tarsal pulvilli, only present in the juvenile stages, can also be found in Ametroproctus , Cymbaeremaeus ( Behan-Pelletier 1987, 1988), Dendroeremaeus ( Behan-Pelletier et al. 2005) and Megeremaeus ( Behan-Pelletier 1990) . Apart from the latter, all taxa are exclusively arboreal species; therefore, this morphological feature is clearly an adaption to this lifestyle. Most of these groups, including Sellnickia , are not related to each other, consequently juvenile adhesive pulvilli have evolved independently in these lineages (e.g. Pfingstl 2023).

Herein, we report the presence of opisthonotal excentrosclerites for the first time in the juveniles of Sellnickia , a feature that is characteristic for immatures of Oripodoidea (e.g. Norton and Behan-Pelletier 2009).

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