Astyris axicostata Kantor & Zvonareva & Krylova 2023, n. sp.

Astyris axicostata n. sp.

Zoobank registration: urn:lsid:zoobank.org:act: 8199E15C-A73 A-4828-AEED-9339C2B42E00

Astyris sp. nov.: Nekhaev et al., 2022: 9, Fig. 4D.

Type specimens. Holotype (Fig. 2 A–D) [SL 7.3 mm], MIMB 45730. Three paratypes [SL 7.4, 7.0, and 3.0 mm] (Fig. 2 E–I) MIMB 45731; two paratypes [SL 7.3, and 5.3 mm] IORAS INV0000847.

Type locality. Northern summit of submarine Piip Volcano, 55°24.96′N, 167°16.464′E, 373 m, R / V Akademik M.A. Lavrentyev, cruise 82, sta LV82-1/7, 2018, rock with zoantharians (Epizoanthus sp.) and other Hexacorallia.

Additional material examined. 28 specimens from the type locality, one sequenced and examined for radular morphology (Buc298, shell partially destroyed). Northern summit of submarine Piip Volcano, 55°24.996′N, 167°16.488′E, 402 m, R / V Akademik M.A. Lavrentyev, cruise 75, sta LV75-10/1, 2016, 1 specimen, rocks on bacterial mats. Southern summit of submarine Piip Volcano 55°22.938′N, 167°15.696′E, 468 m, R / V Akademik M.A. Lavrentyev, cruise 75, sta LV75-15/2, 1 specimen, in vicinity of Calyptogena bed on soft sediment.

Etymology. The species epithet refers to characteristic axial ribs (‘costa’, L. ‘rib’).

Description (holotype). Shell (Fig. 2A–D) moderate-sized for genus (up to SL 7.4 mm), tall-spired. Apex decollate, leaving 4.5 whorls remaining. Teleoconch whorls evenly rounded, convex, very thin and fragile, semi-transparent, very light yellow. Periostracum very thin, smooth, transparent, and tightly adherent to shell. Spiral sculpture of rather indistinct spiral cordlets slightly varying in width, best pronounced on penultimate whorl (16 on penultimate whorl) and less distinct on last whorl, very obscure on canal. Axial sculpture of very thin but distinct weakly sinuous axial ribs present on all teleoconch whorls, more distinct on penultimate and last whorls, and extending to canal, around 30 on last whorl. Aperture lacking dentition, semi-circular in shape, siphonate. Outer lip simple, not thickened, completely smooth on inside. Columella weakly concave, simple, with slightly thickened narrow callus. Siphonal canal short with weakly constricted but distinct siphonal notch.

Operculum corneous, small, irregularly oval, length about half of aperture height. Nucleus lateral.

Protoconch persists in one juvenile paratype with SL 3.0 mm (Fig. 2 E–F), paucispiral, of about 1.5 whorls, smooth, diameter 0.75 mm, transition to teleoconch is poorly demarcated by a weak varix.

Radula (Fig. 3A) rachiglossan, typical of columbellids with acuspate smooth plate-like central tooth and one lateral tooth on each side. Laterals with narrow bases and three cusps, basal one obtuse and two distal ones sharp and closely spaced.

Distribution. Presently known only from both summits of the submarine Piip Volcano (southwestern Bering Sea) at the depths of 373– 468 m.

Ecology. Most specimens were collected from a single station on a rock among dense population of Epizoanthus sp. in an area without visible hydrothermal vent activity (Fig. 3 C–D), however two specimens were obtained from the hydrothermal vent habitats in close proximity to Calyptogena beds and bacterial mats. In addition, a study of photographs shows that some molluscs located close to bacterial mats may be A. axicostata n. sp.

The value of δ13C of a specimen of A. axicostata n. sp. from a site without bacterial mats indicated prevailing contribution of photosynthetically derived carbon from sinking POM (particulated organic matter) in their nutrition (Kiyashko, Mordukhovich, pers. comm.)..

Remarks. The present new species demonstrates little degree of conchological variation. The spiral sculpture can be more distinct in some specimens than in holotype (Fig. 2 F–G). The new species is most similar to A. thermophila, differing in more rounded teleoconch whorls and in the presence of thin but distinct axial ribs.

Astyris axicostata n. sp. is attributed to Astyris since it belongs to the same molecularly defined clade as Astyris cf. rosacea, very similar to the type species of the genus. Correspondingly the tentative position of A. thermophila is here confirmed. The radulae of all three studied Astyris species are very similar (Fig. 3), although in A. thermophila the basal cusp of the lateral teeth is more distinct and better defined in comparison with A. axicostata n. sp. and Astyris cf. rosacea . Astyris axicostata n. sp. is the fourth species of the genus recorded from reducing environments and the second species found in hydrothermal vent areas (Table 1). As for the feeding type of A. axicostata n.sp., preliminary data on δ13C indicated organic matter mainly of photosynthetic origin in their diet (Kiyashko, Mordukhovich, pers. comm.). Thus, although found in the area of hydrothermal activity, the species is probably not specific for hydrothermal vents but rather has high tolerance to environmental factors allowing inhabiting areas of hydrothermal activity. Astyris thermophila, so far the only columbellid species of hydrothermal vent community, was found in tubeworm bushes attached on the tubes and in washings of Bathymodiolus mussels. It was suggested that A. thermophila is a predator or ovophagous (Chen et al. 2017). It is not clear whether A. thermophila is a species obligate to the hydrothermal vent habitats, since very similar specimens were recently collected at the methane seeps of the Koryak Slope of Chukotka, western Bering Sea (Nekhaev et al. 2022: 9). We failed in sequencing those specimens and therefore their conspecifity with A. thermophila is not presently confirmed. But it may appear that both A. thermophila and A. axicostata n. sp. can dwell in rather wide types of environments.