Siphonaria planucosta sp. nov.

(Figs 68D–E, L–N, 69C–D)

Material examined. Type material. Holotype, from Dolokoan Beach, 8°31.424’S, 125°37.091’E, N of Dili, Timor-Leste; coll. B.W. Jenkins, TL01-1, 14 July 2019 (AM C.584826 [M446, SK229 (RS + SPM)], Fig. 68D) . Paratypes: same data as holotype (AM C.585443 10+p, C.585442 p [M451, SK165], Fig. 68E) .

Other, non-type material. Timor-Leste: Dolokoan Beach N of Dili, 8°31.424’S, 125°37.091’E TL01-1, (AM C.585925 p [SK231]) .

External morphology. Foot sole dark grey, foot edge cream; foot wall, cephalic folds and pneumostomal lobe evenly grey; black pigment shading at join of foot wall and mantle, concentrated over cephalic folds; mantle thin, narrower than foot wall, edge lobed with black bands aligning with shell rib interstices and interstice width; pneumostome under mantle between right adductor muscles.

Shell (Figs 68D–E, L; Table S9). Small sized (max sl mean = 8.3 mm, SD = 0.47 mm, n = 2), ovate; low, fragile, thin; apex offset strongly posterior and to left, apical sides strongly convex, protoconch direction weakly homostrophic to central (n = 1, Fig. 68L),below apex, curls to posterior, shell whorl dextral; growth striae indistinct, shell edge uneven, faintly scalloped; rib count (mean = 33, SD = 4, n = 2), primary ribs flat, pale white, fairly straight bent in places, broaden to shell lip, 2–3 secondary ribs between primary ribs, interstices dark brown; paired primary ribs form siphonal ridge, usually not prominent, may appear raised, protoconch area darker; Interior shell margin white, dark chocolate brown rays extend from shell lip to spatula appearing paired, aligning under rib interstices; spatula and indistinct siphonal groove evenly dark chocolate brown; ADM scar indistinct, CMS convex; thickening of shell lip occurs, infills and reduces lip scalloping. Shell resembles other small dark normalis group.

Reproductive system (Fig. 69C; n = 2). Positioned within entire right side of coelom, against foot wall on foot muscle, under the respiratory cavity and intestine occupying large proportion of animal body volume; epiphallic parts positioned over BM. GA medium, with singular GP through foot wall; AO very small, narrow, bluntly pointed, joined to lower ED and upper GA; ED very short, broad, centrally bent, joins to side of GA; GA, AO, ED all white muscular fibrous tissue; EG very large, soft whitish tissue, slightly folded, joins ED; single very broad flagellum (F1), similar length and width to ED, appears as an extension of ED, EG and F1 joined in parallel; BD and CD connect side-by-side into GA between ED join and GP, both ducts short, slightly bent, smooth, thickened, whitish, featureless, pass closely together between outside RAM and inner foot wall (BD over CD) into soft white folded tissues of MG; MG / AG complex medium in size; CD connecting to ducts, BC embedded in folds close to embedded blackish SV; BD without distal loop and MA, with loop immediately prior to BC; BC relatively large, spherical, thin whitish translucent test, (4 SPM in holotype AM C.584826 TL01- 1 [M446, SK229]); HD short, narrow, coiled, links ducts in soft white folded tissues of AG to yellowish granulated HG; outer edge of MG lobbed; AG larger than HG, sides match curvature of inner foot wall.

Spermatophore (Fig. 69D). Broad head with short flagellum (length = 1.71 ± 0.08 mm, n = 2); head section cylindrical, bulbous, rounded tip; test thin, smooth, featureless, translucent encasing a white opaque central core; short looped tapering section merges head to filamentous flagellum; head slightly shorter, wider than translucent flagellum (head length = 1.16 ± 0.17 mm, flagellum length = 0.55 ± 0.088 mm, ~ 68% of SPM length, head width = 119 ± 8 μm, flagellum width = 11 ± 0 μm, n = 2); 4 SPMs tightly coiled in BC (AM C.584826 holotype [M446, SK229]).

Comparative remarks. In our mitochondrial phylogeny (Figs 1, 4), S. planucosta sp. nov. ( normalis group, unit 79) is the sister species of S. radiata (unit 12). Siphonaria planucosta sp. nov. differs from S. radiata by COI distances of ≥ 18.4% (Table S8).

We found S. planucosta sp. nov. in sympatry with five congeners in TL. For comparisons with S. forticosta sp. nov., S. alba, S. javanica, S. viridis, and S. campestra sp. nov. refer to comparative remarks under these species.

Distribution and habitat. Recorded from Dolokoan Beach, N of Dili, Timor-Leste (Fig. 70). In this study, found at sheltered positions (mainly in crevices) on exposed shore boulders, at mid littoral levels (Fig. 68L).

Etymology. From ‘planus’ (Latin = level, even), for the level, even primary ribs on the shell of this species; adjective.