Aquanothrus montanus Engelbrecht, 1975

Norton, Roy A. & Franklin, Elizabeth, 2018, Paraquanothrus n. gen. from freshwater rock pools in the USA, with new diagnoses of Aquanothrus, Aquanothrinae, and Ameronothridae (Acari, Oribatida), Acarologia 58 (3), pp. 557-627 : 593-598

Aquanothrus montanus Engelbrecht, 1975

Aquanothrus montanus was described from samples of three populations in the Free State,

South Africa. Two were from sediment in temporary water bodies (‘natural aquaria’) on aeolian sandstone at the top of mountains. The type locality is ‘mountain Vegkop’ (Vechtkop),

which according to Dr. Lizel Hugo-Coetzee (based on collection records of the Museum) has coordinates of 28°47.36′S, 27°13.87′E (there are several mountains or hills with that name in South Africa). The other mountainous site is Thaba Patchoa (29°19.9′S, 27°05.4′E; Fig.

18H-I). The geological substrates are similar in origin and age to that of the type locality of

Paraquanothrus grahami in Utah. The third sample came from a very different site: the bottom of the H. F. Verwoerd Dam (now Gariep Dam; 30°37.5′S, 25°30.4′E), 30 m below the surface .

According to Dr. Louise Coetzee, who was involved with the collection, all the latter specimens came from a ‘ball of debris’ and she speculated that they might have been washed down the watershed to the dam in heavy rain, rather than representing in-situ an population.

Like Paraquanothrus species , A. montanus moves clumsily, can dig into the substrate,

and survives extended dry periods. The mites feed on sediment but also on aquatic plants.

There have been no further morphological studies, but mites identified as this species were part of rock pool faunas investigated in several community ecology studies. Vanschoenwinkel et al. (2007, 2008 a, b, 2009) studied metacommunity structure and dynamics in sandstone rock pools on Korannaberg Mountain, also in the Free State. They found the mite to be dispersed by wind and by overflowing water, and these probably are also the dispersal mechanisms of

Paraquanothrus species. Mites attributed to A. montanus (but see below) were also part of the rock pool fauna on granite outcrops in arid to semiarid southeastern Botswana, studied by

Jocqué et al. (2006, 2007).

Engelbrecht (1975) viewed his South African material as representing a single species that varied greatly in body surface patterns, notogastral setation and the form of certain leg setae.

However, he did not discuss these variations in the context of the three populations, nor did he identify the source for illustrated specimens or indicate correlation among the different versions of traits. In essence, he concluded that the general sympatry of the variants precluded the existence of several species or subspecies. But the total extent of reported morphological variation exceeds that of any other single known species of oribatid mite. For example, the tarsal setation in the limited number of adults from Thaba Patchoa available to us is strikingly (and consistently) different from that of the other two sites (see below), and those of the Botswana sample are different still. We think there are at least three endemic species of Aquanothrus in southern Africa, represented by the Vegkop (and Verwoerd Dam), Thaba Patchoa, and Botswana populations, and hereon we treat the latter two as representing undescribed species. Considering the highly insular nature of the habitat and the isolation-potential of mountainous environments, fine-grained speciation should not be surprising.

pool. Scale bars: 50 μm (A, G); 20 μm (B, C); 5 μm (D-F). Abbreviations (other than setae): AN – anal plate; dph – dorsophragma; RU – rutellum; pph – pleurophragma; prp – preanal plate; str – strut of preanal organ; vw —vertical wall of preanal organ (muscles dissolved).

The morphological studies of Engelbrecht (1975) are extensive and detailed but include certain errors and omissions that hamper comparisons with other ameronothroid mites. Below, we: correct these errors based on studies of paratypes; provide new data on leg setal ontogeny; and propose a new and expanded generic diagnosis for Aquanothrus , comparable to that for Paraquanothrus , that encompasses all populations studied by us.

Corrections and comments on leg setation of adult

Engelbrecht (1975) presented illustrations (his Figs 18 -21) of all four legs from one variant, which corresponds with the paratypes from the type locality at Vegkop Mountain studied by us, as well as the sample from Verwoerd Dam. He also illustrated (his Figs 22-25) the four tarsi from another variant, which consistently matches the paratypes from Thaba Patchoa studied by us. The most striking differences between variants relate to the form of tectal and fastigial setae but solenidial placement and the famulus also differ. On comparing these figures with dissected paratype specimens, we noted the problems described below; further explanations are given in reference to the complete setal ontogeny.

Engelbrecht (1975), Fig. 18 (leg I) — On the genu, the long structure drawn as a solenidion (labeled σ and provided with the traditional transverse cross-hatching) is actually seta d (it is birefringent in polarized light); solenidion σ is the short one (it is isotropic). The presence of d seems typical of the population, but not invariable (see Table 3 and below). Several problems relate to the tarsus. Seta ft″ was drawn (just distal to?pl) but not labeled; of the two setae labeled pv, the proximal one is pv″, the distal one is the subunguinal, s. The seta he labeled?pl

is actually pv′ and is incorrectly drawn on the abaxial (= ‘antiaxial’) side (cf. the correct Fig. 19). This position of fundamental seta pv′, high on the adaxial face of tarsus I in all instars (and also tarsus II; see below), is unusual.

Fig. 19 (leg II) — On the genu, observations in polarized light showed that the long dorsal structure is seta d; solenidion σ is the short one. On the tarsus: setae labeled it′ and it″ are respectively tc′ and tc″; the fastigial pair (ft) are drawn (in the vicinity of solenidia) but not labeled; of the two setae labeled pv, the proximal one is pv″, the distal one is seta s. As for the previous figure, the seta labeled?pl is an unusually placed pv′. It is not a primilateral seta, since these never occur on tarsus II in Brachypylina ( Grandjean 1959). Also, the seta is clearly not a proximal accessory seta in the l′ row since it is present from the larva on both tarsi I and

II.

Fig. 20 (leg III) — On the tarsus, the seta labeled pv″ is the subunguinal, s.

Fig. 21 (leg IV) — On the genu, the dorsal hatched structure is seta d (as in nearly all Brachypylina there is no solenidion on this segment; see below). On the tibia the dorsal hatched structure is seta d; solenidion φ is at its base, minute and not drawn. On the tarsus the single fastigial is ft″, not ft′; the setae labeled?Ad, pv′ and pv″ are respectively pv″, pv′ and s; therefore, pair (pv) has a ′ disjunction. In the Vegkop population, no tarsal seta is added in the adult on any leg (see below).

Fig. 22 — The pair labeled it′ and it″ are respectively tc′ and tc″. The seta labeled ft′ is solenidion ω1, tightly coupled to the large ft″. The structure labeled ω2 is ft′, and the solenidion labeled ω1 is actually ω2. The small, non-eupathidial proral seta p′ is present, but hidden from view, and the unlabeled ventral seta is s.

Fig. 23 — The pair labeled it′ and it″ are respectively tc′ and tc″. The small proral seta p″

is present, but hidden from view, and the unlabeled ventral seta s. isSolenidia ω1 and ω2 are correctly labeled; they are closely coupled, but in separate alveoli.

Fig. 24 — The pair labeled it′ and it″ are respectively tc′ and tc″. The small proral seta p″

is present, but hidden from view, and the unlabeled ventral seta s. isIn none of our specimens was seta a′ truncated as shown in the figure; all had a′ attenuate, similar to that of tarsus IV

(Fig. 25).

Fig. 25 — The pair labeled it′ and it″ are respectively tc′ and tc″. Setae p″, a″ and u″ are present, but hidden from view, and the unlabeled ventral seta s. is

Ontogeny of leg setation

Table 3 outlines the development of leg setation for the population of Aquanothrus montanus at the type locality, Vegkop Mountain. Highlights of differences with Paraquanothrus species include: (1) the addition of seta v′ (tritonymphal) to trochanters I, II; (2) the retention by the adult of seta d on genua I-II and tibia IV; (3) the addition of v′ (deutonymphal) to genua I,

II; (4) the absence of the famulus from tarsus I in all instars (but see below); (5) the normal structure of proral setae (not eupathidial) on tarsus I of all instars; (6) the close coupling of tarsus I solenidion ω1 to ω2 beginning with the protonymph (coupled with ft″ in the larva, as in Paraquanothrus ); (7) the presence of fundamental seta pv′ on tarsi I-III; (8) the presence of seta pv″ on tarsus IV, giving the normal oribatid mite complement of seven protonymphal setae.

Noted variations within and differences among the three studied populations include the following. Genual seta d: of four examined paratype adults from Vegkop, one lacked seta d

1

All data relate to the population at the type locality at Vegkop Mountain , Free State, South Africa. Structures are placed where they are first added and are assumed present through the rest of ontogeny, unless noted in brackets. Setae in parentheses represent pseudosymmetrical pairs ; dash indicates no additions; asterisk (*) indicates solenidion is coupled to seta d, in same alveolus. Underlined notation indicates seta with variable ontogeny (see text for details).

bilaterally from genua I and II; on these segments the Thaba Patchoa species retains seta d,

but the Botswana species consistently loses d in the adult. Seta d of tibia III is highly variable in the Vegkop population, being absent from the leg illustrated by Engelbrecht (1975, his Fig. 20) and three of seven legs III of Vegkop paratypes studied by us (retained in the other four) ; the single dissected specimen from Thaba Patchoa lacked d bilaterally, as did the four examined Botswana specimens. In the Vegtop population seta l′ varies on three segments: it was absent from two of the four tritonymphal trochanters III examined, and also from about half those of adults ; on femur II it was absent from one of four deutonymphal legs and one of four tritonymphal legs examined, but was present in all studied adults; on tibia III it was absent from two of four deutonymphal legs III examined, but present consistently in later instars.

Other comments and corrections

Respiratory organs of legs — Engelbrecht (1975) neither mentioned nor illustrated the respiratory organs of legs in A. montanus : in juveniles, the trochantero-femoral system is in the typical form of porose areas, but in adults of all three species of Aquanothrus that we studied, they are invaginated as tracheae. All four femora have a conspicuous stigma and dark tracheal vestibule (similar to those of legs I and II in Paraquanothrus grahami ; cf. our Fig. 6G ), from which two tracheae extend: the proximal one is short and remains within the segment, but the long distal trachea reaches various distances into other leg segments, according to species. Unlike P. grahami , the stigma on each femur is high on the adaxial face, and similarly placed on all legs. Trochanters III and IV have a smaller vestibule, also high on the adaxial face, and a single short trachea.

Famulus — Mites from the type population of A. montanus at Vegtop Mountain (and those from Verwoerd Dam) lack a famulus. Such a loss is highly unusual among oribatid mites, though one of the instances is in the podacarid genus Halozetes (reviewed by Fuangarworn and Norton 2013). In the Aquanothrus species from Botswana, the famulus is represented only by an alveolar vestige between solenidia ω1 and ω2. By contrast, in the Thaba Patchoa species there is a minute, spiniform famulus that is difficult to see, inconspicuously located immediately distal to the cluster containing the massive seta ft″, the tightly coupled solenidion ω1, and the adjacent (but with separate insertion) ω2; it is not shown in Engelbrecht’s (1975,

his Fig. 22) illustration, which we think represents this species.

Proral setae of tarsus I — In the vast majority of oribatid mites, setal pair p () on tarsus I are eupathidia (hollow, pale, smooth uniporous chemosensilla that are homologues of normal setae), but in all examined specimens from the Vegkop and Verwoerd Dam samples of A. montanus they are normal, pigmented, and indistinguishable from those of tarsi II-IV. Since the subunguinal seta of tarsus I is also normal, there are no eupathidia on legs A of. montanus from Vegkop, nor in the Thaba Patchoa species. These are the first such examples known to us, with the previous minimum being a single eupathidium, p′, in the enarthronote family Psammochthonidae ( Fuangarworn and Norton 2013) . The Botswana species of Aquanothrus has both proral setae of tarsus I formed as eupathidia in all instars.

Gnathosoma — Engelbrecht (1975) did not indicate the orientation of the illustrated chelicera (his Fig. 26) but it is abaxial. Seta chb is low on the abaxial face, with Trägårdh’s organ

(unlabeled) on the opposite side. Seta cha is high on the adaxial face, not the abaxial face, and the movable digit is well inserted into the cheliceral body in the usual manner, so that its base should have been drawn with a broken line. As in Paraquanothrus species , there is a long,

looping portion of trachea, terminating in each chelicera. The palp does not vary noticeably among our specimens from Vegkop, Thaba Patchoa and Botswana, but all differ from that illustrated in Engelbrecht’s Fig. 27 in several ways (cf. our Fig. 18E, F ): (1) tibial seta l′ is present, high on the adaxial side, so the segment has the usual three setae; (2) seta cm is the largest, most proximal seta of the tarsus and dorsally positioned, not a small spine as in his figure; (3) solenidion ω is baculiform, inserted on the abaxial face of the large basal tubercle. It seems clear that Engelbrecht confused seta cm with the smaller ω, but in none of our specimens is any seta as minute and spiniform as that labeled ‘cm’ in his figure. Engelbrecht did not mention the presence of eupathidia, but the same four are present in all studied specimens: acm , (ul) and su. The somewhat separate tubercle of acm is difficult to see in abaxial view, but it is present in all specimens (our Fig. 18F ).

Prodorsal setae of larva — Engelbrecht (1975) described and illustrated (his Figs 9 , 30) A.

montanus as lacking prodorsal setae le and ro. In our specimens from Vegtop, pair ro are present and relatively conspicuous. Pair le are also present, in a position similar to that illustrated for nymphs (his Figs 31, 33, 35), but are small, fine and difficult to see.

Gastronotic sclerites of larva — Engelbrecht (1975) described and illustrated (his Fig. 29)

the presence of ‘slightly chitinized sclerites’ on the larval gastronotum (‘notogaster’); two are illustrated, but three are indicated in the text, with the third perhaps being the prodorsum. We studied several larvae from Vegkop Mountain, the type locality, and found no such discrete sclerites. All juveniles have patterns of plication that result in a vague ridge on each side,

running longitudinally between setal rows d and l, and in the larva these outline the regions of the supposed sclerites. Perhaps some residual retention of cerotegument pigmentation in this region, when specimens were fresher, gave an impression of a cuticular difference, but there is no sclerite or other formation resembling the porose sclerites of juvenile Podacaridae ( Ermilov et al. 2012) , or even the leathery patches of Paraquanothrus species.

Claparède’s organ — Engelbrecht (1975) did not illustrate (in Fig. 30) or mention this larval organ but it is present. As in Paraquanothrus species it is represented by a porose dome sunken within a cavity. There is no scale-like protective seta (1c), and the seta he labeled as

1c lies just medial to the organ, so is instead probably 1b; 1c appears in the protonymph as in Paraquanothrus .

Paraproctal atrichosy — Engelbrecht (1975) did not recognize that the paraproctal segment is glabrous (atrichous) in the larva, protonymph and deutonymph A of. montanus , as it is in Paraquanothrus species and a majority of other Brachypylina ( Grandjean 1949a). Setae of the respective segment are added in the subsequent instar. In his illustrations of the larva (his Figs 29, 30) the setae labeled h2, ps2 and ps3 are respectively h1, h2 and h3. The small, faint structure labeled ps1? in Fig. 30 probably is cupule ip; there is no seta at that location.

Line of dehiscence in juveniles — Engelbrecht (1975) did not illustrate it, but the larva and nymphs of A. montanus and other species have an inconspicuous, permanent line δ. It is U-shaped, incomplete anteriorly, and passes well dorsal to gland opening gla; it effaces at a level just anterior to the periglandular leathery patch. Posteriorly, the line has a normal path, passing above cupule ip in the larva, but below ip in nymphs.

Figures

Figure 6 Paraquanothrus grahamin. sp., transmitted-light micrographs of adult: A – legs I-IV (trochanters absent from I, II); B – partly extended ovipositor; C – palp, abaxial view; D – partial tarsus I; E – same, partial enlargement; F – partial tarsus IV; G – partial femur I, adaxial view; H – base of femur IV; I – distal part of tectal seta of tarsus I; J – female, strongly cleared to show eggs; K – egg from freshly killed specimen; L – portion of preserved egg, showing microgranular surface; M – portion of chelicera, adaxial view; N – same location, internal focus showing part of looping trachea and terminal bulb. Scale bars: 100 μm (A, J); 50 μm (K); 20 μm (B, L); 10 μm (C, F-H, M, N); 5 μm (D, E, I). Abbreviations (other than setae):dt – claw-depressor tendon (vague);e – famulus; gt – guide tube;rt – ‘root’ of famulus; st – stigma;Tg

Figure 9 Paraquanothrus spoonerin. sp., adult (legs omitted): A – dorsal view and enlargement of flattened rostrum; B – ventral view; C – lateral view; D – bothridial region, anterolateral view. Scale bars: 100μm (A-C), 20 μm (D). Abbreviations (other than setae and lyrifissures): asc– ascleritic band of notogaster;bng – border of notogaster;bt – base of tectum;bvp –border of ventral plate;cms – circumgastric muscle sigilla;Le – lenticulus; of – oblique fold. For coxisternal setae, see Remark 15.

Figure 18 A-F, Aquanothrusadults (from Thaba Patchoa unless noted otherwise; see text): A – prodorsum (notogaster removed); B – anterior part of anal region, external view; C – same, internal view; D – apobasic notogastral setap1 from undescribed Botswana species (arrow at deeply invaginated insertion, asterisk in alveolar canal); E – right palp and rutellum; F – dorsal base of palp tarsus, adaxial view; G – encrusting epibiotic diatoms and debris on adults (clockwise from upper left: postanal margin (Verwoerd Dam), prodorsum margin (same), posterior notogaster (Botswana species); H – locality at top of Thaba Patchoa Mountain, South Africa (29°19.86′S, 27°05.39′E); I – same, inhabited rock