Xestoleberis concava Chand & Kamiya, 2016, n. sp.

Xestoleberis concava n. sp.

Type series. Holotype: male UMUT RA32550 (Figs. 4 F–F’, H–L, 12A–B, D1–D4). Paratypes: females: UMUT RA32552 (Fig. 4 A–B’, M), UMUT RA32553 (Fig. 2 C–D), UMUT RA32554 (Fig. 12 E–F), UMUT RA32555 (Fig. 12 C). All type material was collected from the type locality. The holotype and paratypes are deposited at the University Museum, University of Tokyo, Japan. Valves on paleontological paper/cavity slides and soft parts mounted on glass slides. Additional paratypes preserved in 70% ethanol: four males and three females UMUT RA32551.

Type locality. A flat and fine sand beach coast in front of Naselesele Village at the north east tip of Taveuni Island (P6, Fig. 1, Table 1); habitat: a green calcareous alga (Halimeda opuntia).

Etymology. Concava means arched or curved in Latin; the proximal structure of the hemipenes of this species is curved to form a concave, cupped structure.

Diagnosis. Carapace with widely rounded posterior and anterior ends; wider posterior than anterior. Anterior and ventral edges of valves lined with numerous simple marginal pores. Coloration in living specimens translucent brown. BO short segment with numerous fine terminating setae. Ejaculatory ducts arranged in S-shapes with inward exits. Furca reduced to two short setae.

Description. Carapace ovate, strongly inflated and vertically compressed (Figs. 2 A–B, 12A–F). Maximum valve length range: 524 µm–546 µm, maximum valve height range: 324 µm–335 µm (Table 2). Maximum height at mid-length. Dorsal margin convex, ventral margin slightly sinuous. Both anterior and posterior vestibula narrow; posterior narrower than anterior. Merodont hinge. Normal sieve pores scattered over carapace; those lining anterior and antero-ventral edges of carapace more recessed and some with smooth circumferences. Scar pattern: topmost scar of posterior row of four adductor scars U-shaped, single anterior scar V-shaped.

An1 with six podomeres; first two big, wide and rectangular, third to sixth small and quadrate (Fig. 4 A–A’). Second podomere with one medial seta, third podomere with one dorsal apical seta, fourth and fifth podomeres with one long and one fine dorsal apical setae. Terminating setae of sixth podomere: one stout with pointed tip, one slender and round tipped, one whip-like, and two fine. An2 with one ventral apical seta on first endopodite podomere (Fig. 4 B–B’). Two dorsal and ventral medial setae (longer ventral medial setae hirsute) and stout hirsute ventral apical seta on second endopodite podomere. Very fine serrations on one of two terminating claws. Md coxa with six pointed and two lobate teeth and one fine seta (Fig. 4 C–E), palp with four podomeres: two medial setae at junction of first and second podomeres, and second and third podomeres, second podomere with one dorsal and two ventral apical setae, third podomere with five long dorsal and one stout ventral apical setae, fourth podomere with three stout terminating setae (two long and one short). Exopodite with at least two long setuled setae. Mx with two segmented palp; first segment with three distal dorsal-apical setae and second podomere with one fine ventral medial seta and three terminating setae (two thick and one fine) (Fig. 4 F–G). Branchial plate with 16 setuled setae. BO is symmetrical; short segment with numerous fine terminating setae (Fig. 4 H).

Basal setal formula for L5 1+1:2:1 and L6 and L7 1+1:1:1 (Fig. 4 I–K). Knee setae of all legs hirsute. Terminating claws L5 stout, short and curved, L6 smooth, stout, long and curved and L7 smooth, stout, and straight. Hp with asymmetrical, wide, rounded to sub-rounded distal processes; one has very rounded, U-shaped end, other with sub-rounded, almost quadrate end (Fig. 4 L). Proximal ends of capsules acutely angled and proximal support structure shaped like inverted T that forms cup-like structure. Furca two short fine setae on small rounded base.

Distribution. Xestoleberis concava n. sp. specimens were collected from two locations, Naselesele, Taveuni Island (type locality) and the fine sand gently sloping beach coast of Viani Village in Natewa Bay, Vanua Levu Island (P5, Fig. 1, Table 1). In addition to the holotype habitat (Halimeda opuntia), prominent counts of the specimens were also obtained from a short red alga (Pterocladiella sp.) and a green filamentous alga (Ulva intestinalis).

Remarks. Xestoleberis concava n. sp. is similar to Xestoleberis posterotruncata Titterton & Whatley, 2005 (Fig. 4, No. 27, Pl. 4, Figs. 1–5, Titterton & Whatley 2005) reported from Solomon Islands and Xestoleberis cauticola Hartmann-Schröder, 1978 (Abb. 370–384, Tafel XII Figs. 12–13, Hartmann-Schröder 1978) reported from northwestern Australia. Xestoleberis posterotruncata has both simple and branching pore canals in the anterior marginal zone, while X. concava n. sp. only has simple anterior marginal pores. The L7 of X. cauticola has a very thin and straight terminating claw tapering to a sharp point and with a mass of short, fine setae at the base of the claw. The L7 claw of Xestoleberis concava n. sp. tapers to a point with a slight curve. Xestoleberis concava n. sp. and X. cauticola ejaculatory duct S-shaped arrangements varies, Hp distal processes of X. cauticola are triangular with narrow, rounded tips, while those of X. concava n. sp. are sub-triangular with widely rounded tips. Xestoleberis cauticola has sub-rounded proximal capsule ends in contrast with the acutely angled proximal capsule ends of X. concava n. sp.