Anatoma discapex, Hoffman & Kniesz & Arbizu & Kihara, 2022

Hoffman, Leon, Kniesz, Katharina, Arbizu, Pedro Martínez & Kihara, Terue C., 2022, Abyssal vent field habitats along plate margins in the Central Indian Ocean yield new species in the genus Anatoma (Vetigastropoda: Anatomidae), European Journal of Taxonomy 826, pp. 135-162 : 142-146

Anatoma discapex sp. nov.

urn:lsid:zoobank.org:act:8662D87B-3AC7-4F7D-8A34-AA8A64965174

Figs 3–4

Etymology

The species name ‘ discapex ’ refers to the flattened apex.

Type material

Holotype INDIAN OCEAN • CIR; 25.47° S, 69.93° E; depth 2628 m; 16 Nov. 2019; stn INDEX19_042RO; spec. I19 _Ma_84, SEM, Fig. 3A–B ; SMF 358976 .

Paratypes INDIAN OCEAN • 1 specimen (paratype 1); same location data as for holotype; SEM, Fig. 3C–D ; SMF 358977 • 1 specimen (paratype 2); same location data as for holotype; spec. I19 _Ma_85, SEM, Fig. 3E–F ; SMF 358978 • 1 specimen (paratype3); CIR; 23.88° S, 69.62° E; depth 3082 m; 11 Nov. 2019; stn INDEX19_031RO; spec. I19 _Ma_86, Fig. 3G–H , SEM; SMF 358979 • 1 specimen (paratype 4); same location data as for preceding; SEM, Fig. 3I –K ; SMF 358980 • 1 specimen (paratype 5); same location data as for preceding; spec. I19 _Ma_80, SEM, Fig. 3L–N ; SMF 358981 • 1 specimen (paratype 6); same location data as for preceding; spec. I19 _Ma_81, SEM, Fig. 3O–Q ; SMF 358982 • 1 specimen (paratype 7); same location data as for preceding; spec. I19 _Ma_79, SEM, Fig. 3R–S ; SMF 358983 .

Other material examined

INDIAN OCEAN • 1 specimen; CIR; 23.87° S, 69.62° E; depth 3032 m; 12 Nov. 2019; stn INDEX19_033RO; spec. I19 _Ma_5 • 24 specimens; 25.47° S, 69.93° E; depth 2628 m; 16 Nov. 2019; stn INDEX19_042RO; specs I19 _Ma_27–50 • 5 specimens; 23.78° S, 69.55° E; depth 3049 m; 2 Dec. 2015; stn INDEX15_49R; specs I15 _Ma_8, I15 _Ma_133, I15 _Ma_135–136, I15 _Ma_144 • 8 specimens; 23.88° S, 69.62° E; depth 3296 m; 8 Dec. 2015; stn INDEX15_62R; specs I15 _Ma_145, I15 _Ma_147–150, I15 _Ma_154, I15 _Ma_158, I15 _Ma_163 .

Shell description

PROTOCONCH ( Fig. 3K of paratype 4). One flat whorl, coarse pitted sculpture; raised flexuous lip with rim at transition to teleoconch, transition clear by change in sculpture; diameter 0.20 mm.

TELEOCONCH ( Fig. 3A–B ). Flattened upper spire and disk-shaped outline with strongly protruding margins of selenizone; flexuous axial ribs; pear-shaped aperture with slit; suture deep. Height 1.3 mm, width 2.1 mm, height of aperture 0.85 mm (60 % of total height); colour opaque grayish white. 2¼ regular whorls with flattened, slightly convex shoulder area and rounded base, selenizone slightly above periphery; suture well below selenizone of penultimate whorl in last half of body whorl. Teleoconch I from protoconch to start of selenizone: ¾–1 convex whorl, in plane with protoconch; about 15 strong axial ribs, flexuous; no spiral cord. Teleoconch II with selenizone: initial whorl about 30 axial ribs, last ¼ whorl of body whorl about 22 axial ribs; bifurcating and closely spaced near lip. Margins of selenizone sharply protruding upwards or sideways near the lip. Axial ribs on external margins of selenizone; inside margins smooth. Frequency of axial ribs below selenizone similar to that above; rough growth lines between ribs, typically 3–6 major stages. Umbilicus open, tortuous and deep; steeply spiralling keel at union of columellar callus with parietal area ( Fig. 3A of holotype and 3C, E, G of paratypes).

APERTURE ( Fig. 3A ). Rounded base, funnel-shaped towards selenizone, pointed at union with penultimate whorl. Columellar lip sharp, protruding, rounded at base, flexuous towards parietal area; parietal callus thin and reclining. Semi-circular lower lip; slit slightly above periphery with concave edge of lip on either side, slit ¼ whorl deep with flattened sharp margins above and below. Callus thin; inside aperture smooth.

VARIATION. Little morphological variation of shell. Variable length of selenizone I and number of axial ribs. Adult height range 1.1–1.4 mm; width range 2.1–2.2 mm.

(SMF 358983), I19_Ma_79, H 0.79 mm, W 1.48 mm, Ha 0.62 mm. Scale bars: 0.1 mm.

Anatomical description

Snout smooth, broad, flattened with one symmetrical front lobe with pointed front margins on left and right sides ( Fig. 4D–E ). No eyes observed. Broad cephalic tentacles, tapered, with eight rounded lobes, deeply folded between lobes, gradually decreasing in diameter towards bluntly rounded tip, nonpapillate. Foot small when contracted, with irregular surface. Four epipodial appendages on right side; anteriorly two smaller, papillate epipodial tentacles and two large ones posteriorly; the most posterior probably epipodial sensory organ (ESO; Sasaki et al. 2010; Haszprunar et al. 2017). Three epipodial appendages on left side; small papillate tentacle anteriorly; second smaller epipodial tentacle or small ESO; most posteriorly a large ESO. Visceral mass with little more than one whorl. Mantle smooth with rounded margin, non-papillate.

OPERCULUM ( Fig. 4 ). Yellowish transparent, thin, corneous, multispiral (about 10 whorls) with central nucleus.

RADULAR TEETH. Configuration 30–4–1–4–30; width 0.10 mm. Central tooth broad cusp, directed anteriorly. Four lateral teeth on either side. Inner marginal teeth with multiple large curved hooks on thick stems changing towards external margin into long and thin stems with fine cusps at external margin.

Distribution

Central Indian Ridge, 23– 26° S, 69– 70° E, depth 2560–3296 m.

Habitat

Living on rocky surface with bacterial mats near active and inactive vents. Shells frequently covered by red bacterial deposit.

Remarks

Shells of Anatoma discapex sp. nov. are similar to those of Anatoma umbilicata (Jeffreys, 1883) , an Atlantic and Mediterranean species from depths of 200–2300 m, Anatoma parageia Geiger & Sasaki, 2009 from intertidal zones in eastern Japan and Anatoma hyposculpta Geiger, 2012 from the tropical SW Pacific at depths of 730–1620 m ( Geiger 2012). All these species show a flattened shoulder but lack the discoid apical outline and the coarse axial sculpture. All other known species in Geiger (2012) have a more elevated spire.

Figures

Fig. 3. Anatoma discapex sp. nov. A–F. INDEX19_042RO. A–B. Holotype (SMF 358976), H 1.31 mm, W 2.11 mm, Ha 0.85 mm. C–D. Paratype 1 (SMF 358977), H 1.15 mm, W 2.20 mm, Ha 0.79 mm, Wp 0.20 mm. E–F. Paratype 2 (SMF 358978), I19_Ma_85, H 0.68 mm, W 1.47 mm, Ha 0.53 mm, Wp 0.19 mm. — G–S. INDEX19_031RO. G–H. Paratype 3 (SMF 358979), I19_Ma_86, H 0.84 mm, W 1.48 mm, Ha 0.58 mm. I–K. Paratype 4 (SMF 358980), H 0.38 mm, W 0.70 mm, Ha 0.32 mm, Wp 0.20 mm. L–N. Paratype 5 (SMF 358981), I19_Ma_80, H 0.86 mm, W 1.34 mm, Wp 0.20 mm. O–Q. Paratype 6 (SMF 358982), I19_Ma_81, H 0.72 mm, W 1.30 mm, Wp 0.19 mm. R–S. Paratype 7

Fig. 4. CLSM images of contracted soft parts of Anatoma discapex sp. nov., Central Indian Ridge, INDEX19_042RO, (I19_Ma_83). A–B. Complete soft parts in basal and left anterior view (A) and apical and right-anterior view (B). C. External view of operculum. D. Left anterior view showing cephalic tentacle (CT), epipodial sensory organ (ESO), epipodial appendage (*), foot (F), operculum (O), mantle edge (M) and snout (S). E. Right view of anterior with same parts as in D. Scale bars: A–B = 0.2 mm; C = 0.15 mm; D–E = 0.4 mm.