Fabaeformiscandona monticulus Peng & Zhai & Smith & Wang & Guo & Zhu 2021, n. sp.

Fabaeformiscandona monticulus n. sp.

(Figs 23–26)

Type locality. A pond in Kangding, Garze, Sichuan, China (30.07745ºN, 101.79605ºE, altitude 4207 m) (OS09-1, Table 1).

Type material. Holotype: one dissected male (WOC14) from type locality (Table 1). Allotype: one dissected female (WOC15) from type locality. Paratype: one dissected male (WOC17) from type locality (Table 1).

Derivation of name. From the Latin monticulus, the diminutive form of mountain, and referring to the large and triangular a lobe of the Hp, which is reminiscent of a mountain peak. The name is a noun in apposition.

Dimensions. Male (LV, n = 2) length 0.82–0.84 mm, H/L ratio 0.55. Female (LV, n = 1) length 0.79 mm, H/L ratio 0.54.

Description of male. Carapace (Fig. 23) sub-reniform in lateral view. Dorsal margin nearly evenly arched, with highest point at about 1/3 of length from posterior end. Both anterior and posterior ends broadly rounded, maximum curvature of anterior margin in antero-ventral area. Ventral margin significantly concave. Valve surface sparsely covered with fine setae. In dorsal view, carapace moderately compressed, with anterior end more pointed than posterior end. In interior view, antero-ventral area of calcified inner lamella unusually wide in LV of both sexes, with inner margin straight or even slightly curved inwards.

A1 (Fig. 24A, B) with eight segments, first two fused forming large base, carrying two dorsal setae and two unequally long ventro-apical setae. Third segment short and rectangular, with one dorso-apical seta. Fourth segment short, with one dorso-apical seta. Fifth segment sub-quadrate, with one dorso-apical seta extending beyond terminal segment with about half of length, and one short ventro-apical seta. Sixth segment sub-quadrate, with two long dorso-apical setae and one short ventro-apical seta. Seventh segment elongate, with two long dorso-apical setae and two short ventro-apical setae. Eighth segment slender, with three unequally long setae and dorso-apically situated aesthetasc y a.

A2 (Fig. 24C, I, J) first segment (coxa) with one long posterior seta and two unequal ventral setae, shorter one of which robust and plumose. Second segment (basis) robust, with one slender ventro-apical seta extending beyond terminal of next segment. Exopod plate small, carrying three progressively long setae, longest of which extending to about end of first endopodal segment. Endopod with penultimate segment sub-divided. Third segment (first endopodal segment) robust, with slender aesthetasc Y situated behind mid-length, extending to about ventro-apical end of this segment, and with two sub-equal ventro-apical setae extending slightly beyond terminal segment. Fourth segment sub-rectangular, ventrally carrying small aesthetasc y1, distally carrying three t-setae, two of which transformed into male bristles, and one dorso-apical seta. Fifth segment quadrate, ventro-apically with small aesthetasc y2. z1 and z2 transformed into robust claws, with z2 longer and thicker than z1. z3 small, claw-like. G2 longest claw, two times length of G1. G3 slender. Terminal segment small. Gm extending to about tip of G2. Gm less than half length of GM. Aesthetasc part of y3 slender.

Md (Fig. 24D, E) coxa elongate, interiorly bearing ca. eight slender teeth. One stout seta present on antero-subapical part of coxa. Palp four-segmented. First segment robust, with four ventral setae, including smooth unnamed seta, plumose seta S1, short plumose seta S2, and thin, smooth seta α. Second segment short, with two dorso-apical setae and ventro-apically, group of four smooth setae, shorter accompanying seta and tiny seta β. Third segment with three slender dorso-subapical setae, two stiff intero-apical setae (including seta γ) and one long and one tiny ventro-apical setae. Fourth segment slightly elongate, with two more robust setae/claws and two slender setae.

First segment of Mx palp (Fig. 24F) with four dorso-apical setae. Second segment elongate, with two more robust claws/setae and two smaller setae. Tooth bristles on distal gnathobasic endite smooth. Two robust setae present on proximal side of proximal endite.

L5 (Fig. 24G, H) gnathobasic endite with ca. 14 apical setae, two of which bifurcate distally (one observed in left L5 of holotype). Right L5 palp progressively slenderer distally, bearing two sub-apical setae. Right L5 palp with blunt dorsal bump on median part of trunk, finger slender and elongate, one of two sub-apical setae claw-like.

L6 (Fig. 25A) with five segments. First segment elongate and less sclerotized, carrying intermediately long seta d1. Second segment long, with seta e reaching distal part of third segment. Third segment and fourth segment subequally long, each with one distal seta. Fifth segment elongate and trapezoidal, with seta h1 longer than h3. Claw h2 much longer than total length of three terminal segments, slightly serrated at sub-apical part.

L7 (Fig. 25B) generally well sclerotized, with four segments, penultimate segment undivided. First segment with setae d1 and dp. Second segment without seta. Third segment distally with curved seta g. Fourth segment elongate and rectangular, with setae h1, h2 and h3 progressively longer. Seta h3 only faintly curved at sub-apical part.

Ur (Fig. 25C, D) ramus stout and robust, basal part swollen. Sa slim. Ga sub-equally long to Gp but slightly more robust, both serrated. Sp situated at ca. 1/4 length of ramus from distal end, longer than Sa. Ur attachment medially with two branches and proximally with tri-ramous branch.

Hp (Fig. 25E) with large, triangular outer lobe a, small, triangular median lobe h, and interiorly, pointed, setalike structure. M-process only slightly enlarged distally.

ZO (Fig. 26) with 5+2 rosettes each bearing numerous spines.

Additional description of female. Carapace (Fig. 23B & G) dorsal margin nearly straight and sloping anteriorly. Posterior end narrowly rounded (not broadly rounded as in male), with maximum curvature at postero-ventral area, postero-dorsal margin gently curving up to hinge. Anterior margin similar to that of male. Calcified inner lamella wide, especially in antero-ventral area; here inner edge of calcified inner lamella almost straight. Posterior part of both valves with polygonal pattern (Fig. 23G).

Distal morphology of A2 (Fig. 24I). t-setae subequally long but with t2 slightly longer, extending beyond terminal segment with half of length. z-setae extending slightly beyond terminal segment. G1 and G3 longest claws. G2 about half length of G1, slender. GM extending to or slightly beyond G1. Gm about 3/4 length of GM.

GL (Fig. 25C) very small and narrowly rounded.

Remarks. The smooth γ seta on the Md and the relatively laterally compressed carapace indicates that this species belongs to the genus Fabaeformiscandona. Of the 55 species in this genus, most have carapaces noticeably different in shape to those of Fabaeformiscandona monticulus n. sp., but three do show a resemblance in the lateral view of the female: Fabaeformiscandona balatonica (Daday, 1894), Fabaeformiscandona levanderi (Hirschmann, 1912) and Fabaeformiscandona danielopoli Yin & Martens, 1997. The wide calcified inner lamella in the anteroventral area of the female valves of Fabaeformiscandona monticulus n. sp. is a feature not seen to such an extent in the other species; it is also wide here in F. levanderi but not as wide as in Fabaeformiscandona monticulus n. sp. and it does not have an almost straight edge in F. levanderi. Males of all three species are sufficiently different in carapace and Hp morphologies to clearly distinguish them from Fabaeformiscandona monticulus n. sp. In particular the lobe a of the Hp of Fabaeformiscandona monticulus n. sp. is very large and triangular, much larger than the a lobes of the other species, and the posterior margin of the male more evenly rounded.

The arrangement of 4+1+β setae on the inner edge of the second Md palp segment indicates that Fabaeformiscandona monticulus n. sp. belongs to the acuminata -group of Fabaeformiscandona, the same as F. levanderi and F. danielopoli, but different to F. balatonica (balatonica -group).

The polygonal pattern on the posterior part of the female carapace is a feature shared by some other Candonidae, such as Fabaeformiscandona myllaina Smith & Kamiya, 2007, Neglecandona angulata (G.W. Müller, 1900), Candona muelleri jakutica Pietrzeniuk, 1977, and Candona xizangensis Huang, 1982 (in Huang et al. 1982).

Although in a different genus, Candona xizangensis, also reported from the Tibetan Plateau, shows some similarity to Fabaeformiscandona monticulus n. sp. in overall carapace shape and the presence of a polygonal pattern in the female carapace (see figs in Akita et al. 2016). The female of C. xizangensis is more tightly curved posteriorly and the postero-dorsal margin longer than those in Fabaeformiscandona monticulus n. sp. The male carapaces of both species are similar, but the male of C. xizangensis is slightly more rounded posteriorly, and has a small convex expansion in the mouth region (absent in Fabaeformiscandona monticulus n. sp.). The appendages of C. xizangensis remain unknown, hindering further comparisons and confirmation of its generic status.

Fabaeformiscandona monticulus n. sp. was collected once during this study, from a pond with sparse aquatic plants near the shore at an altitude of 4207 m (Table 1).