Marseniopsinae Fassio, Bouchet, Schiaparelli & Oliverio 2023, SUBFAM. NOV.

MARSENIOPSINAE FASSIO, BOUCHET, SCHIAPARELLI & OLIVERIO SUBFAM. NOV.

(FIGS 2, 3, 4A, B, 6A, 9A, 10A, 11A, M)

Zoobank registration: urn: lsid: zoobank. org:act: 0FF5505F-49D2-4814-8E81-CA559340917E

Type genus: Marseniopsis Bergh, 1866.

Included genera: Marseniopsis Bergh, 1866, Lamellariopsis Vayssière, 1906 (probably a synonym of Marseniopsis Bergh, 1866).

Description: Body of small to medium size for the family, 0.5–11.5 cm total length. Shell very thin, weakly calcified to membranaceus; ear shaped, high spired, with expanded aperture; smooth or weakly sculptured by axial growth lines; enclosed by the mantle. Periostracum thin to not visible.

Protoconch of 0.76–1.9.0 whorls; protoconch I of 0.25–0.73 whorls, with or without granular sculpture, nucleus diameter 150–875 μm; protoconch II with or without marked longitudinal ribs; protoconch– teleoconch boundary not always distinct.

Mantle flat or dome shaped, outline rounded or polygonal; often thick, but sometimes thin, with or without warts; with anterior siphonal fold; texture smooth to wrinkled or jelly-like; colour highly variable, almost transparent to white, grey, beige, yellow, orange, red or brown, frequently patterned with dots and/or polygonal lines.

Penis to the right of the right cephalic tentacle; with or without a lateral subterminal papilla. Vas deferens with a free loop in haemocoel.

Radula taenioglossate, formula 2: 1:1: 1: 2; rachidian tooth elongated, with rectangular base, elongated; rachidian cusp with three or four small to pronounced external denticles on each side; lateral teeth elongated, with pointed triangular external cusp, with two or three small to pronounced denticles on the internal side and one or two small to large denticles on the external side; marginals narrow, without denticles.

Jaws elongated, composed of scale-like elements.

Distribution: Southern Ocean and south Argentinian waters; 75–668 m deep.

Remarks: Fassio et al. (2019) suggested that the radiation of velutinids in Antarctica might represent a lineage worthy of taxonomic recognition at subfamily level. This is here confirmed, and the geographical range is now extended outside the Southern Ocean to Argentina, with the record of M5 (= new MOTU P) from north of Isla de los Estados (USNM 1137366, 54°27ʹ19.1″S, 63°52ʹ39.4″W, 108 m depth). Marseniopsinae can be diagnosed by the radular formula combined with the elongated shape of the rachidian tooth with a rectangular base, and by an ear-shaped shell, not well calcified, with a high spire.

Lamellariopsis is probably a synonym of Marseniopsis, but the genetic analysis of type or topotypical material is needed to check it. Two types of protoconchs, potentially indicating a difference in the length of the larval phase, have been described for this subfamily (see Fassio et al., 2019).