Dallina lusitanica, Hansen, 2024

Dallina lusitanica View in CoL n. sp.

Figures 52 View Figure 52 , 53 View Figure 53

Terebratula septata – Jeffreys (1878) [partim], p. 407–409.

Magellania septigera – Fischer & OEhlert (1891) [partim], p. 64–71, pl. 4, figs. 9a–9ab; pl. 5, figs. 9ac–9ah.

Magellania septigera – Joubin (1907), p. 7 –8.

Dallina septigera View in CoL – Atkins (1960a) [partim], p. 91–99, 1a–d, 3a–d.

Dallina septigera – Brunton & Curry (1979) [partim], p. 55, fig. 28.

Dallina septigera View in CoL – Cooper (1981a), p. 23 –24, pl. 2, figs. 41–43.

Dallina septigera View in CoL – Anadón (1994), p. 74, pl. 3, figs. 3–5.

Dallina septigera View in CoL – Logan (1998), p. 558, figs. 4h–n.

Dallina septigera View in CoL – Gaspard (2003), p. 297 –299, figs. 5(10), 6(1–10).

Dallina septigera View in CoL – Anad ó n et al. (2022), p. 7 –8, figs. 3d–e.

LSID: zoobank.org:act: 0AB0E914-2A5A-4E48-B671-979AF4B0EC92

Type locality: Southernmost Celtic Sea , off France, 47°11’– 47°14’N , 6°11’– 6°13’W, 1143 m depth.

Etymology: After the Lusitanian marine region of the temperate Northern Atlantic realm, in which the species mostly occurs.

Diagnosis: Moderately large Dallina species. Mature specimens characterised by their typically strongly subtriangular outline with greatest width close to anterolateral corners. Lateral commissure distinctly curved and anterior commissure gently to moderately parasulcate. Dorsal septum extended to approximately 80–90% of valve length.

Material: Holotype: Celtic Sea, 47°11’ – 47°14’N, 06°11’ – 06°13’W, 1143 m depth, one specimen, BMNH PI ZB 4948 GoogleMaps . Paratypes: Celtic Sea, 47°37’N, 07°27’W, 722 m depth, eight specimens, BMNH PI ZB 4955–4962 GoogleMaps ; Celtic Sea, 48°06’ – 48°26’N, 09°18’ – 09°44’W, 977 m depth, one juvenile ventral valve, BMNH PI ZB 4946 GoogleMaps ; Celtic Sea, 48°32’ – 48°33’N, 10°09’ – 10°10’W, 686–896 m depth, 14 specimens BMNH PI ZB 4944–4945 , 4951–4953 , 4963 , 4967–4969 , 4972 , 4973 , 4977 GoogleMaps ; Celtic Sea, 48°38’ – 48°39’N, 09°45’ – 09°50’W, 933–1061 m depth, one specimen, BMNH PI ZB 4970 GoogleMaps ;? Celtic Sea (from study material of D. Atkins), seven specimens, BMNH PI ZB 4953 , 4964–4966 , 4971 , 4976 ; Celtic Sea, 47°11’ – 47°14’N, 06°11’ – 06°13’W, 1143 m depth, one damaged ventral valve kept together with holotype.

Description: Shell moderately to strongly biconvex. Outline variable, but adult shell normally with subtriangular to strongly triangular outline, and widest typically at or very close behind anterolateral corners. L/W ratio 0.94–1.37 with mean of 1.15. Juvenile specimens with more egg-shaped outline. Umbo short and stubby. Anterior commissure changing from rectimarginate in juveniles, over broadly unisulcate, to strongly parasulcate. Lateral commissure usually moderately curved in adults. Anterior part of dorsal valve typically with two very pronounced folds along the flanges. No ornamentation besides growth lines. Shell somewhat translucent, fragile and white with a thin yellowish periostracum often peeling in stored specimens. Shell matrix endopunctate. Egg-shaped subcircular and rather large pedicle foramen. Deltidial plates fused to form a symphytium, often with a raised longitudinal ridge. Deltidial plates well-defined, but not separated from rest of valve by ridges. Pedicle collar (thickening in pedicle tube) very short. No dental plates supporting ventral teeth. Descending lamellae of brachial support thin and gently diverging and joined by broad transverse band to form a long loop. Brachial loop not attached to median septum at least on specimens longer than 17 mm. Long and low dorsal median septum usually extends to 80–91% of valve length (a few dorsal valves below 21 mm length with septum down to 51% of valve length), and always visible as a whitish knife-cut line on dorsal valve exterior. Spicules in tissue mostly absent. Maximum shell length 30 mm.

Depth range: 460–3000 m depth ( Logan 1998; Anadón et al. 2022), but mostly between 600–1500 m.

Temperature range: Based on three stations 7.0–9.5˚C (this study).

Substrate: Typically found on mud- or fine sand-dominated bottoms, very often in connection with hard corals and coral debris, where it is found attached to corals, shells, gravel and stones ( Joubin 1907; Anadón 1994; Anadón et al. 2022; this study).

Geography: Celtic Sea, Bay of Biscay, NE Atlantic, Portugal, Azores and Canary Islands. This species most likely has its northern border at the Celtic Sea, but a systematic examination of the Irish Dallina specimens is required to clarify this.

Remarks: Dallina lusitanica n. sp. is smaller and more fragile, subtriangular to strongly triangular with the greatest width typically at or close to the anteroventral corners, and with distinctly curved lateral commissure in contrast to Dallina septigera , which is more solid, pentagonal to subtriangular with the greatest width mostly located well behind the anterolateral corners, and with the main part of the lateral commissure nearly straight.

Among the 10 species included in the genus, this new species most closely resembles Dallina floridana ( Pourtalès, 1868) and Dallina triangularis Yabe & Hatai, 1934 . However, the approximately 28 mm long species Dallina floridana (known from Florida, the Gulf of Mexico and the West Indies) is generally as wide or wider than long, with the dorsal septum reaching 64–76% of valve length, anterior commissure more strongly parasulcate and a ventral umbo that is more strongly curved. Based on descriptions and five new shells from the Philippines’ Balut Island housed at the Arctic University Museum of Norway, the up to 32.5 mm long Dallina triangularis known from Japan, the South China Sea and Fiji differs by having a shorter dorsal septum reaching 70–80% of valve length, typically being much more strongly parasulcate, with a shell matrix markedly thinner around the foramen, dorsal umbo markedly more acute and lateral ridges on ventral valve diverging increasingly at the front on adult shells, giving the valve a bell-shaped outline ( Yabe & Hatai 1934; Hatai 1940; Bitner 2008, 2019; this study).

Atkins (1960b) redescribed Dallina septigera largely based on the material of the present species whilst assuming the observed variations were intraspecific. Also, Cooper (1981a) and Anad ó n et al. (2022) used the diagnostic external features of the new species to distinguish what they thought were D. septigera from the morphologically close species Fallax dalliniformis , suggesting D. septigera is at least nearly absent from France and N Spain. To date, there appears to be very little, if any, overlap in the geographical ranges of these two species.