Siphonaria tongatapuensis, B. W. Jenkins & Köhler, 2024

Jenkins, Bruce & Köhler, Frank, 2024, Hidden in plain sight: Systematic review of Indo-West Pacific Siphonariidae uncovers extensive cryptic diversity based on comparative morphology and mitochondrial phylogenetics (Mollusca, Gastropoda), Megataxa 13 (1), pp. 1-217 : 155

Siphonaria tongatapuensis sp. nov.

( Figs 57F–H, O–P, S, 59G–H)

Material examined. Type material. Holotype, from Halafuoleva Beach , 21°12.021’S, 175°14.680’W, S coast of Tongatapu, Tonga; coll. B.W. Jenkins, TO03-1-2, 23 May 2019 ( AM C.585279 [M420, SK107], Fig. 57F) . Paratypes, same data as holotype ( AM C.585540 20+p; AM C.585281 p [SK224], Fig. 57G; AM C.585911 p [SK387], Fig. 57H) .

Other, non-type material. Tonga: Halafuoleva Beach, S coast Tongatapu, 21°08.358’S, 175°02.443’W, TO03-1- 2 ( AM C.586002 3p, C.585282 p [SK222]) .

External morphology ( Fig. 57P). Foot sole dark grey; foot edge and cephalic folds cream; foot wall, mantle and pneumostomal lobe evenly grey, darker to foot edge, some irregular blotches of black pigmentation on foot wall; mantle thin, translucent, narrower than foot wall, edge thickened, lobed, with weak black banding aligning with rib interstices, pneumostome wide.

Shell ( Figs 57F–H, O, P; Table S9). Small sized (max sl mean = 11.1 mm, SD = 1.5 mm, n = 2), circular ovate; height medium; apex offset central, apical sides convex, protoconch direction central to homostrophic (n=1, Fig. 57O); shell whorl dextral; initial growth profile very tall, profile flattens with maturity, exterior pale blue/ grey uneven, protoconch area darker, shell thick, growth striae prominent; rib count (mean = 34.5, SD = 7.5, n = 2), primary ribs pale grey, fairly straight, ridges rounded, increasingly raised to and extend beyond uneven shell lip to unevenly scallop the lip; two prominent primary ribs extending strongly beyond shell edge form the siphonal ridge. Interior tan to golden brown, shell lip white with dark chocolate brown rays extending over shell margin and ADM to spatula, aligning under rib interstices; siphonal groove prominent; ADM scar distinct, CMS convex; no thickening of shell lip noted.

Reproductive system ( Fig. 59G; n = 1). Positioned within right coelom under the respiratory cavity, hermaphroditic complex ( HG, AG and MG) to posterior against right foot wall and over foot sole, epiphallic parts to anterior between RAM and BM; AO medium, broad, blunt, weakly centrally bent, side MA, merges to upper part of indistinct GA, singular GP; ED short, wide, compressed in coil, longer than AO, joins to lower side of GA; GA, AO, ED all white muscular fibrous tissue; EG soft whitish, folded, slightly smaller than AO; single twisted thin flagellum (F1) lays over MG; BD and CD join closely but with opposing connections to side of GA between AO and GP; both ducts smooth and pass closely together through RAM ( BD over CD); BD long very narrow with prominent short distal loop with MA to inner body wall, connected to small sized bulbous BC with thin translucent test, positioned in folds of AG; CD short, wider than BD; CD connects into MG; HD short, thick, brown markings, coiled, under AG, links AG to much smaller yellowish granulated HG; MG and AG folded, soft white tissue. Several parasite eggs were found on the EG and AG of holotype (Halafuoleva Beach, S coast Tongatapu, Tonga).

Spermatophore ( Fig. 59H). Thread-like (length = 5.64 mm, n =1), translucent, test thin; head section bluntly rounded, evenly cylindrical, containing a core white gelatinous mass, tapers along the transparent flagellum to a thin tip; both sections smooth, featureless. Head section longer wider than flagellum (head length = 3.36 mm, ~ 64% of SPM length, n = 1; flagellum length = 1.98 mm, head width = 45 μm, flagellum width = 11 μm). 2 SPM tightly coiled, embedded in purple/brown gelatinous mass in one BC ( AM C.585281).

Comparative remarks. In our mitochondrial phylogeny ( Figs 1–4), S. tongatapuensis sp. nov. ( plicata group, unit 69) is a well-individualized lineage that represents the sister group of a clade comprising S. guamensis , an unidentified species from Rarotonga (‘unit 17’ in Dayrat et al. 2014), and S. nusalikensis sp. nov. It differs from other species by COI distances of ≥ 26% (Table S7). We found S. tongatapuensis sp. nov. in sympatry with S. plicata ; for comparative remarks refer to that species. While both species have a similar RS structure, in particular size and shape of the epiphallic parts and SPM, they clearly differ in shell sculpture, colouration, and absence of a prominent multi-ribbed siphonal ridge. Siphonaria tongatapuensis sp. nov. differs from other species of the plicata group, such as S. nuttallii , S. lirata , and S. monticulus particularly in size and shape of AO, ED, and HD. The SPM shape of these species is generally similar. Siphonaria nuttallii has the most similar RS.

Distribution and habitat. Recorded exclusively from Halafuoleva Beach, Tonga ( Fig. 60). In this study, found in sheltered positions (mainly in rock hollows) on exposed rocky shores, mid and upper littoral levels ( Fig. 57P).

Etymology. For Tongatapu, Tonga, where this species is found.