Astyris H. Adams & A. Adams, 1853, 2017

Chen, Chong, Watanabe, Hiromi Kayama & Araya, Juan Francisco, 2017, First columbellid species (Gastropoda: Buccinoidea) from deep-sea hydrothermal vents, discovered in Okinawa Trough, Japan, Zootaxa 4363 (4), pp. 592-596 : 593-594

Family COLUMBELLIDAE Swainson, 1840

Genus Astyris H. Adams & A. Adams, 1853

Astyris thermophilus n. sp.

http://zoobank.org/urn:lsid:zoobank.org:act:43FA9CFA-3756-4CBC-9861-41FAF66397B9

Type specimens. Holotype ( Fig. 1A–D ) [SH 8.5 mm, SD 4.3 mm, LW 4.1 mm], UMUT RM32644. Paratypes. #1 ( Fig. 1E ) [SH 8.3 mm, SD 4.5 mm, LW 4.6 mm], NSMT Mo 78990. #2 ( Fig. 1F ) [SH 7.4 mm, SD 4.2 mm, LW 4.1 mm], periostracum removed with diluted bleach to reveal fine sculpture, operculum and radula removed for SEM, UMUT RM32645. #3 [SH 7.1 mm, SD 4.0 mm, LW 4.0 mm], NSMT Mo 78991. #4 [SH 7.7 mm, SD 4.1 mm, LW 4.3 mm], NSMT Mo 78992. #5 [SH 6.5 mm, SD 3.7 mm, LW 3.9 mm], JAMSTEC 1140053525. #6 [SH 3.0 mm, SD 1.7 mm, LW 1.6 mm], juvenile with intact protoconch, UMUT RM32646. All type specimens fixed and stored in 99% ethanol.

Type locality. Natsu hydrothermal vent site, Iheya North field, Okinawa Trough, Japan; 27°46.843’N, 126°54.024’E, 1074 m deep; 2014/Jan/24, collected by slurp gun, ROV HYPER- DOLPHIN Dive #1614, R/V KAIYO cruise KY14-01, principal scientist: Ken Takai.

Additional material examined. Two specimens from type locality, fixed and stored in 99 % ethanol, used for genetic barcoding (tissue dissolved). Five specimens, live collected, fixed and stored in 99% ethanol. Aki hydrothermal vent site, Iheya North field, Okinawa Trough, Japan; 27°46.130’N, 126°54.159’E, 1087 m deep; 2014/Jan/25, collected by slurp gun, ROV HYPER- DOLPHIN Dive #1614, R/V KAIYO cruise KY14-01, principal scientist: Ken Takai.

Etymology. From ‘ Thermós ’ (Greek), warm or hot, and ‘ philíā ’ (Greek), love or affection; combined to mean heatloving, referring to its habitat in hydrothermal vent fields.

Japanese Name. ‘ Yomotsu-mugi-gai ’, meaning ‘mitrid from the underworld’.

Diagnosis. A moderate-sized (up to SH 8.5 mm) Astyris with rather tall-spired, uniformly white, thin, semitransparent shell; smooth including base, except 25 to 30 very fine spiral striae. Columella lacking columellar fold. Periostracum thin, greyish brown. Protoconch paucispiral. Radula stenoglossate with acuspate centre plate, three sharp cusps on each lateral.

Description. Shell ( Fig. 1A–F ) moderate-sized for genus (up to SH 8.5 mm), rather tall-spired. Apex always decollate in adults, leaving at most three whorls remaining. Teleoconch thin, semi-transparent, uniformly white in colouration. Periostracum thin, greyish brown, earlier whorls slightly darker coloured. Often covered by further sulfide deposits. Teleoconch largely smooth except approximately 25 to 30 very fine, shallow, evenly spaced spiral striae present across entire whorl (see Fig. 1F ), increasing in strength anteriorly towards siphonal canal. Striations on posterior half of shell usually too fine to detect when covered by periostracum. Whorls elevated, slowly expanding, slightly convex but not angulated. Suture distinct, shallowly constricted. Aperture entirely lacking dentition, semi-circular in shape, siphonate, approximately twice as tall as wide, posteriorly acuminate. Outer lip simple, not thickened, completely smooth on inside. Columella straight, simple, with slightly thickened callus. Siphonal canal short with weakly constricted but distinct siphonal notch. Protoconch ( Fig. 1G ) paucispiral, about 1.5 whorls, smooth, lacking velar sinus, sculpture or distinct growth lines. Transition edge between protoconch and teleoconch clearly marked by a varix. Suture of protoconch shallow, slightly higher than teleoconch.

Operculum ( Fig. 1H ) corneous, small, length about half of aperture height. Lamellate with lateral nucleus on right often eroded away.

Radula ( Fig. 1I ) stenoglossate, typical of columbellids with one lateral on each side separated by an acuspate centre plate instead of rachidian. Laterals sigmoid, well-supported, with three strong cusps. Two distal cusps sharper, longer, closer spaced compared to basal cusp. Centre plate rectangular, slightly wider posteriorly, without sculpture.

Distribution and ecology. Only known from Natsu and Aki sites of the Iheya North hydrothermal field ( Nakamura et al. 2015), mid-Okinawa Trough, Japan. Found in tubeworm bushes attached on the tubes, presumably a predator of other animals inhabiting the same habitat or alternatively it may be ovophagous and feed on eggs of other animals.

Remarks. The present new species is assigned to genus Astyris as it matches well with the diagnosis for the genus given by McLean & Gosliner (1996), most notably by having a small, high-spired shell with smooth surface and a paucispiral protoconch ( Garcia 2009). Of those species currently in Astyris , the new species most closely resembles A. permodesta (Dall, 1890) from methane seeps at Monterey Canyon and whale falls in California ( Smith et al. 1989; Bennett et al. 1994) and A. atacamensis Araya, Catalán & Aliaga, 2016 from northern Chile. Although A. permodesta has been reported also from off Callao, Perú ( Levin et al. 2002), this is likely in fact another record of A. atacamensis ( Sellanes 2017) . Both of these species are easily separable from A. thermophilus n. sp. as they have a thicker, broader shell with wider aperture and less constricted siphonal canal, as well as having spiral grooves on the base. The pronounced columellar fold seen in A. atacamensis and the lirae inside the outer lip are also lacking in A. thermophilus n. sp. Until the discovery of A. thermophilus n. sp., Astyris permodesta was the only other columbellid species known from deep-sea chemosynthetic ecosystems.

Astyris thermophilus n. sp. is conchologically similar to Zemitrella cera Okutani, 1964 from 1000–3000 m deep off Izu Islands, Japan ( Okutani, 1964; 2017), but differs by having spiral striation across the entire teleoconch whorl (sculpture is only present on the base in Z. cera ) and a slightly longer siphonal canal. As the protoconch of Z. cera is unknown, it is not transferred to Astyris at this point. Furthermore, the new species is also very similar to a yet undescribed species reported from deep-water trawls and dredges carried out in northern Pacific coast of Honshu, Japan ( Mitrella sp. 1 sensu Hasegawa, 2009: figs. 162–166), but differs from that species in the spiral striae on the teleoconch being less distinct and that the columella is always straight. At this point we cannot entirely dismiss the possibility that Mitrella sp. 1 (sensu Hasegawa, 2009) may be conspecific with Astyris thermophilus n. sp. and the morphological differences may be due to environmental factors.

The phylogenetic reconstruction ( Fig. 2 ) confirmed the placement of the present new species within Columbellidae . Its generic placement cannot be tested at present because sequences from other Astyris species were not available.

Figures

FIGURE 1. Astyris thermophilus n. sp. A–D. Holotype (UMUT RM32644). E. Paratype #1 (NSMT Mo 78990). F. Paratype #2 (UMUT RM32645), periostracum removed to show spiral striae. G. Protoconch of paratype #6, a juvenile specimen (UMUT RM 32646). H. Operculum of paratype #2 (UMUT RM32645). I. Radula of paratype #2, UMUT RM32645). Scale bars = 2 mm (A–F), 500 µm(G, H), 20µm (I).

FIGURE 2. Phylogenetic reconstruction using the COI gene (618 bp) showing Astyris thermophilus n. sp. nested within Columbellidae. Node values indicate Bayesian posterior probability, those lower than 0.7 are omitted. The DDBJ/EMBL/GenBank accession numbers for each sequence used are shown inside brackets (LC329027 and LC329028 for the two sequenced specimens of A.thermophilus n. sp.).