Dallina tasmaniaensis, sp. nov.

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http://zoobank.org/urn:lsid:zoobank.org:act:10F319A7-641D-44FC-AC2A-D7834415D686

Holotype lodged with the Tasmanian Museum and Art Gallery, Tasmania (TMAG E58673), see specimens examined.

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Fig. 5. Dallina tasmaniaensis, sp. nov. A–D) dorso-ventral aspects with dorsal aspect of Holotype (A; E58673; L=18.75 mm, T=11.0 mm), dorsal and ventral aspects of a larger specimen (B, C; E58675, L=23.7 mm, T=15.7 mm), and ventral aspect of specimen with gonad development (D; E58675, L=17.7 mm, W=16.7 mm). E) Lateral aspect of larger specimen (same specimen as B). F–H) anterior aspects depicting anterior commissure variation, specimen with well-developed folding (F, same specimen as B), reduced folding (G; E58673), and nearly absent folding (H, same specimen as D). I–L) Aspects of cardinalia and brachial loop, interior of dorsal valve, close-up of cardinalia (I), ventral aspect of brachial loop & cardinalia (J), lateral aspect of brachial loop & cardinalia (K), and anterior aspect of brachial loop (L) (E22188; L=23.6 mm). M–O) Beak and delthirium close-up, dorso-lateral aspect of beak (M; E58673), dorsal (N), and antero-dorsal (looking inside the beak) (O) aspects of delthirium and hinge teeth (N, O; E22188). Abbreviations per Methods. Scale bars: 10.0 mm (A–H, J), 5.0 mm (I, K–O).

Fig. 6. Dallina triangularis type material. A–H) Holotype specimen (TUM No. 56221) with ventral (A), lateral (B), Dorsal (C), and anterior (D) aspects of whole specimen, followed by closeup of ventral valve delthirium (E), close-up of dorsal valve cardinalia (F), and interior aspects of the dorsal valve (G, H). I–L) Paratype specimen (TUM No. 56779), dorsal (I), lateral (J), and anterior (K), followed by dorsal aspect with part of damaged dorsal valve removed to reveal the cardinalia still connected at the hinge (L). Scale bars: 10.0 mm. Photography by Nemoto Jun, The Tohoku University Museum (used with permission).

Table 3. Dallina tasmaniaensis, sp. nov. measurements, notes, and indices. All measurements in mm, NA = not assessable as valves damaged, ✝ = contains further specimens too damaged to measure, *=specimen used for dry preparation of brachial loop (given new registration number E22188). ** = dry specimen (others in ethanol).

Diagnosis: Moderately large, thin shelled, and subtriangular Dallina, with relatively long beak, intraplicate anterior commissure (moderate folding), relatively broad symphytium, rectangular plate-like cardinal process, large crural processes supported by a straight-edged crura–descending branch junction, and relatively thick descending branches.

Etymology: Specific epithet ‘tasmaniaensis’ after the island of Tasmania, around which much of the Type material was collected, combined with the Latin suffix ‘-ensis’ forming an adjective from the toponym (i.e., of/from Tasmania), neutral in gender.

Description

Overall valve form: Maximum valve length 24.2 mm (average 20.5 mm, SD 2.0, smallest 17.7 mm long). Valves wide, W%L averaging 88.1% (SD 5.1; range 78.9%−95.7%), and relatively thick, T%L averaging 63.8% (SD 3.0, range 58.7%−70.6%).

Overall outline (dorso-ventrally) subtriangular, the widest point being near the anterior border which is truncated (Fig. 5 A–D). Valves deeply biconvex, the ventral (pedicle) valve being deeper. Lateral commissure dorsally convex (curving towards the dorsal valve) (Fig. 5 E). Anterior commissure weakly to strongly intraplicate (plicosulcate), the dorsal valve with an antero-medial ridge with weak folding towards the dorsal valve flanked by two shallow grooves folding ventrally more strongly, creating an antero-medial groove on the ventral valve (see Fig. 5 F). On some specimens the overall folding (dorsally and ventrally) is weak to near absent (see Fig. 5 G, H). The folding sometimes creates two shallow indentations along the anterior margin (viewed dorso-ventrally, see Fig. 5 B).

Valves punctate, relatively smooth and thin, but with fine growth striae. Radial ribs/costae absent. Colouration beige, though several have a dark yellow or brown coating (see Supplementary Fig. 1 C).

Dorsal (brachial) valve: Cardinal margin angle obtuse (~118–135°). Cardinal process moderately developed, produced as a small rectangular plate, anteriorly folded from the dorsal umbo (with small myophore). Hinge sockets narrow and well defined. Inner socket ridges diverge at approximately 120° (relative to antero-posterior axis), anteriorly widened and steeply inclined towards the ventral valve (see Fig. 5 I, J, and lateral in Fig. 5 K). A gentle groove separates each inner socket ridge from each outer hinge plate, which are narrow and only slightly inclined. Crural bases separate the outer and inner hinge plates (bases narrow, outer edges not continuous with inner socket ridges). Inner hinge plates, inclined more than outer plates and meeting medially as an excavate septalium, supported by the median septum. Anterior edge of septalium forming a V-shape, extending anteriorly well beyond level of crural bases (but not reaching level of crural processes) (see Fig. 5 I). Crura relatively short, diverging only weakly, crural processes angled ventrally (ends directed slightly medial), large and pointed in lateral profile, dorsal edge of crura−descending branch junction straight (see Fig. 5 I, K, for ventral and lateral aspects respectively). Crural process located at ~20% dorsal valve length. Brachidium dalliniform (see Fig. 5 J–L), only attached by the crura (condition confirmed on two specimens, one illustrated), descending branches long, free from ascending branches for most of their length. Ascending branches wide, transverse band with pair of antero-lateral indentations/ridges (see Fig. 5 K, L). Median septum very high and thin posteriorly, dropping off abruptly in height at approximately ⅓ dorsal valve length (anterior to crural processes), then continuing as a low septum (slowly reducing in height), ending approximately ⅔ dorsal valve length (approximately at the anteriormost extent of the brachial loop) (see Fig. 5 J, K for ventral and lateral aspects of septum).

Ventral (pedicle) valve: Beak moderately small (length 14%−16% of overall length), suberect to erect. Foramen subapical (posterior edge is against the posterior border of the valve), moderately large (foramen diameter 11%−12% overall valve length), mesothyridid, beak ridges indistinct (partly visible, see Fig. 5 M). Pedicle collar short (as a ring within foramen). Deltidial plates are partly exposed, conjunct as a symphytium, medial seam where the plates abutted either visible (Fig. 5 N, O) or obscure (Fig. 5 A, B). Symphytium relatively broad (symphytium length 39%−46% its width). Hinge teeth at anterolateral corners of deltidial plates, relatively small compared to symphytium, squared, bases not especially thickened, dental plates absent (Fig. 5 N, O).

Soft tissue: Lophophore plectolophous. Pedicle very short (not extended beyond beak). Gonad development visible in some specimens, small and confined to posterior half of valves (see Fig 5. D).

Attachment substrata and epibiota: Specimens were generally lacking attachment substrata (other than one with a shell fragment) and lacking distinctive epibiota other than thin films of microorganisms perhaps giving a brown colour to some specimens.

Specimens examined

Holotype. 1 specimen, seamounts south of Tasmania, 44.2394°S, 147.2929°E, 1354−1414 m, IN2018_V06 Stn 169, 14.xii.2018, TMAG E58673.

Paratypes. 30 specimens, off Pt Hicks, NE Bass Strait, 38.4799°S, 149.5529°E, 1520−1527 m, SS200001 Stn 154, 18.iv.2000, TMAG E22119; 2 specimens, east of Flinders Is, eastern Bass Strait, 39.9155°S, 149.0884°E, 1557−1608 m, SS200001 Stn 256, 28.iv.2000, TMAG E22120; 15 specimens, off Pt Hicks, NE Bass Strait, 38.4784°S, 149.5566°E, 1426−1509 m, SS200001 Stn 158, 17.iv.2000, TMAG E22160 (one specimen used as a dry preparation of internal structures, given new registration number E22188); 5 specimens, seamounts south of Tasmania, 44.2969°S, 147.1149°E, 1712−1465 m, IN2018_V06 Stn 11, 24.xi.2018, TMAG E58675; 2 specimens, seamounts south of Tasmania, 44.2143°S, 146.2669°E, 1157−1353 m, IN2018_V06 Stn 74, 3.xii.2018, TMAG E58677.

Comparative material. Dallina triangularis Holotype [L=23.5 mm, W=21.0 mm, T=16.0 mm]. Kyushu coast, Japan, 31.2028°N, 129.8292°E, 402 m, S.S. Soyo-Maru (Imperial Fisheries Institute, Tokyo), Stn 419, 15.vii.1929, TUM No. 56221 (Dry shell): D. triangularis Paratype [L=20.0 mm, W=16.5 mm, T=12.7 mm], TUM No. 56779 (Dry shell; collected with Holotype).

Remarks

The specimens examined herein were attributed to Dallina because of their overall shape and folding, the dalliniform brachidium only attached to the crura, fused deltidial plates, minimal trace of beak ridges, and the absence of dental plates.

Comparison to Atlantic species: Dallina contains nine valid species (Emig 2022). D. septigera, D. floridana (Pourtales, 1867), and D. parva Cooper, 1981 are all restricted to the North Atlantic (Zezina 2010). In D. tasmaniaensis, sp. nov. the cardinal process is higher and more developed than in D. septigera but lacks the small flanking elevations on the anterior face of the cardinal process depression noted on that species. The brachial loop also has thicker descending branches (but this may be variable). D. septigera was variable in shape (generally subpentagonal but occasionally elongate triangular to equilateral triangular) (Atkins 1960), whereas all specimens of D. tasmaniaensis, sp. nov. were subtriangular (close to equilateral). D. floridana is triangular with similar folding but is smaller (largest 22 mm long and 25 mm wide), with the beak compressed laterally (rather than broad) and with ‘Deltidium small, in two pieces’ (i.e., deltidial plates disjunct at a valve length of 22 mm) (Pourtalès, 1867). D. floridana is also restricted to the Gulf of Mexico(?) at shallow depths 64–724 m (Zezina 2010). Finally, D. parva Cooper, 1981 from the north-east Atlantic is also triangular (longer than wide), but is sulcate, and very much smaller as an adult (largest 11.0 mm long), the cardinal process not being described (Cooper 1981).

Comparison to Pacific and Southern Ocean species: The Pacific has a much larger number of Dallina species. D. raphaelis (Dall, 1870), D. obesa Yabe & Hatai, 1934, and D. elongata Hatai, 1940 were all described from off Japan, and all differ from D. tasmaniaensis, sp. nov. in having ovoid (quadrate-ovate) forms widest midway along the valves, much larger sizes (38.4, 31, & 38 mm long respectively), much shallower bathymetric ranges (all within 23–402 m depth), flatter lateral commissures (especially D. obesa), and thicker valves (Hatai 1940). D. eltanini Foster, 1974 (as well as D. elongata) is known from the far southern Pacific (Foster 1989), and in addition to differing with an ovoid to subpentagonal shape, it also has a sulcate anterior commissure (vs. variably intraplicate on other Pacific species). D. eltanini, D. raphaelis, and D. elongata interestingly share a moderately-developed cardinal process alike that seen in D. tasmaniaensis, sp. nov. (Foster 1974; Hatai 1940). Two problematic Dallina species (D. simosensis and D. profundis) are commented on in the discussion.

D. triangularis was described from two specimens collected off Japan (402 m depth). These type specimens (Holotype and Lectotype) were partly damaged (brachial loop ascending and transverse bands missing, some sections of valves cracked) but in good enough condition to allow comparison (see Fig. 6). Again, the overall triangular form, size, and ratios were similar. The primary differences between the D. triangularis Type specimens and D. tasmaniaensis, sp. nov. related to the beak, cardinal process, and brachial loop. In the former the beak is proportionally shorter (11%−14% vs. 14%−16%), with the symphytium proportionally narrower (symphytium length%width ~47.5%−57.9% vs. 39%−46%). On the D. triangularis Holotype the cardinal process is much more pronounced than in D. tasmaniaensis, sp. nov. (tapering and projecting more anteriorly, Fig. 6 F), and the anterior commissure folding of D. triangularis is far broader and stronger (especially on the Holotype). The form of the crura and brachidium descending branches are also rather different in D. triangularis, viewed semi-laterally (Fig. 6 H), the dorsal edge of the crura−descending branch junction (facing the inside of the valve) is distinctly indented, vs. straight in D. tasmaniaensis, sp. nov. (Fig. 5 K), with the crural processes also much shorter in D. triangularis. Specimens of these two species were similar in overall size (D. tasmaniaensis specimen with brachidium illustrated with L = 23.6 mm, similar in size to the 23.5 mm long D. triangularis Holotype), and thus brachidium and cardinal process differences were unlikely to be ontogenetic. Finally, the bathymetric range was also very different between the taxa (402 m vs. ~1100–1700 m for the southern Australian species).

D. triangularis is also known from putative material collected from the South China Sea (491−538 m), Lau Ridge off Fiji (416−1226 m), and Tonga Islands (356−367 m) (Bitner 2008; Bitner 2019; Bitner & Romanin 2018), but these specimens were only tentatively assigned to this species, and critical comparison of this material to both D. triangularis type material and D. tasmaniaensis, sp. nov. will be needed. The South China Sea specimen was juvenile and similar to D. triangularis in overall shape and short beak (~10% overall length) (Bitner & Romanin 2018 fig 7A,B). The specimens from near Fiji and Tonga both had proportionally short beaks (beak length ~10%−13% overall length) with the Tonga specimen at 32.5 mm long exceeding any D. triangularis type material or D. tasmaniaensis, sp. nov. material, these specimens also had a much more pointed anterior edge, creating a ‘diamond-like’ shape (Bitner 2008 fig 16B–G; Bitner 2019 fig 7N), and may turn out to represent a distinct species.

Distribution: Continental slope from NE of the Bass Strait, eastern Bass Strait, and seamounts south of Tasmania (Fig. 1 A). Bathymetric range 1157−1712 m.