Anatoma declivis, 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 : 146-150

Anatoma declivis sp. nov.

urn:lsid:zoobank.org:act:800FCAD4-D68D-43B9-98A9-209D9C0E0B25

Figs 5–7

Etymology

The species name ‘ declivis ’ refers to the sloping shoulders.

Type material

Holotype INDIAN OCEAN • CIR; 23.87° S, 69.62° E; depth 2980 m; 12 Nov. 2019; stn INDEX19_033RO; spec. I19 _Ma_94, SEM, Fig. 5F–G ; SMF 358984 .

Paratypes INDIAN OCEAN • 1 specimen (paratype 1); CIR; 23.88° S, 69.62° E; depth 3082 m; 11 Nov. 2019; stn INDEX19_031RO; spec. I19 _Ma_92, SEM, Fig. 5A–C ; SMF 358985 • 1 specimen (paratype 2); same collection data as for preceding; spec. I19 _Ma_93, SEM, Fig. 5D–E ; SMF 358986 • 1 specimen (paratype 3); CIR; 25.47° S, 69.93° E; depth 2628 m; 16 Nov. 2019; stn INDEX19_042RO, spec. I19 _Ma_89, SEM, Fig. 5H–K ; SMF 358987 • 1 specimen (paratype 4); SEIR; 27.63° S, 73.87° E; depth 2547 m; 5 Dec. 2019; stn INDEX19_102RO; spec. I19 _Ma_87, SEM, Fig. 5L–N ; SMF 358988 .

Other material examined

INDIAN OCEAN • 1 specimen; CIR; 23.88° S, 69.62° E; depth 3082 m; 11 Nov. 2019; stn INDEX19_031RO; spec. I19 _Ma_78 • 4 specimens; CIR; 23.87° S, 69.62° E; depth 2980 m; 12 Nov. 2019; stn INDEX19_033RO; specs I19 _Ma_3–4, I19 _Ma_6–7 • 3 shells; CIR; 25.47° S, 69.93° E; depth 2934 m; 16 Nov. 2019; INDEX19_042RO • 15 specimens; SEIR; 27.63° S, 73.87° E; depth 2532 m; 5 Dec. 2019; stn INDEX19_102RO; specs I19 _Ma_ 51, I19 _Ma_54– 61, I19 _Ma_63– 64, I19 _Ma_66– 68, I19 _Ma_88 • 1 specimen; SEIR; 27.65° S, 73.88° E; depth 2457 m; 11 Dec. 2019; INDEX19_127RO ; spec. I19 _Ma_77 • 21 specimens; SEIR; 27.65° S, 73.88° E; depth 2469 m; 23 Nov. 2018; stn INDEX18_063RO; specs I18 _Ma_256–269, I18 _Ma_273–279 • 8 specimens; CIR; 23.78° S, 69.55° E; depth 3049 m; 2 Dec. 2015; stn INDEX15_49R; specs I15 _Ma_9, I15 _Ma_ 32, I15 _Ma_128– 131, I15 _Ma_137–138 .

Shell description

PROTOCONCH ( Fig. 5C of paratype 1). One flat whorl, pointed nucleus, coarsely pitted sculpture; raised sharp lip with smooth flexuous margin at the transition to teleoconch: transition clear by change in sculpture; diameter 0.20 mm.

TELEOCONCH ( Fig. 5F–G ). Flattened apex, biconical outline with sloping convex shoulders, protruding margins of selenizone; flexuous axial ribs; round aperture with slit; suture deep. Height 1.19 mm, width 1.90 mm, height of aperture 0.81 mm (67 % of total height); colour opaque grayish white. 2½ regular whorls with convex shoulder area and rounded base, selenizone at periphery; distance between base selenizone and suture minute. Teleoconch I from protoconch to start of selenizone: 0.8–0.9 convex whorls, in plane with protoconch; about 12–25 strong axial ribs, flexuous; no spiral cord. Teleoconch II with selenizone: 20–35 flexuous axial ribs on first whorl, 40–65 ribs on body whorl; ribs at regular spacing, decreasing spacing towards lip. Smooth margins of selenizone sharply protruding upwards, axial ribs weaker and more numerous on external margins; inside margins smooth. Frequency of axial ribs below selenizone slightly higher to that above; rough growth lines between ribs. Base of body whorl shows few spiral cordlets. Umbilicus open, narrow, tortuous and deep; steep spiral keel to centre of columellar callus.

APERTURE. Rounded at base, columella, parietal area and shoulder, funnel-shaped at selenizone, pointed at union with penultimate whorl. Lip sharp, flexuous following external ribs, protruding on shoulder area, protruding at columella, thin reclining callus in parietal area. Disk-shaped lower lip; slit at periphery, slit ¼ whorl deep with flattened sharp margins above and below; union of lip aligned with penultimate whorl. Callus thin; inside aperture smooth.

VARIATION. Little morphological variation of shell. Teleoconch I up to 25 axial ribs, teleoconch II up to 65 axial ribs on body whorl. Base of body whorl occasionally with few spiral cordlets, or with one stronger spiral cord. Adult height range 1.2–1.7 mm; width range 1.9–2.0 mm; H/W ratio range 0.6–0.8. Juvenile shells lower H/W ratio.

Anatomical description

Snout smooth, broad, flattened with two symmetrical front lobes ( Fig. 6D–E ). No eyes observed. Pair of cephalic tentacles, tapered, with eight rounded lobes, deeply folded between lobes; blunt tip, nonpapillate. Foot small in contracted form, with irregular surface. Three epipodial appendages on left side

( Fig. 6D ); the two anterior probably epipodial tentacles; ESO posteriorly, non papillate. Four epipodial appendages on right side ( Fig. 6E ); three epipodial tentacles anteriorly; ESO posteriorly. Mantle smooth.

OPERCULUM. Yellowish transparent, thin, corneous, multispiral (11 whorls) with central nucleus.

RADULA ( Fig. 7 ). Largely destroyed in SEM preparation. Central, lateral and marginal teeth arranged 26–4–1–4–26; width 0.15 mm. Central cusped tooth trapezoidal. Four lateral teeth with curved tips. 26 marginal teeth, proximal teeth with 1–3 hooked (curved) denticles ( Fig. 7B ), distal laterals ending in strongly curved cusp with 6–10 symmetrically-placed, curved denticles ( Fig. 7A ).

Distribution

Central Indian Ridge and SE Indian Ridge, 23– 28° S, 69– 74° E, depth 2469–3082 m.

Habitat

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

Remarks

Shells of Anatoma declivis sp. nov. are similar to those of Anatoma finlayi (Powell, 1937) , an IndoPacific species living on the shelf and in bathyal depths, including off Madagascar ( Geiger 2012; Table 1 ). However, the latter species has fine regular spiral cordlets on the teleoconch and its outline is more elevated (H/W 0.8–1.0). All other known species in the Indian Ocean have a stepped outline with a spiral sculpture.

Figures

Fig. 5. Anatoma declivis sp. nov. A–C. Paratype 1 (SMF 358985), I19_Ma_92, INDEX19_031RO, H 1.21, W 1.90 mm, Ha 0.77 mm, Wp 0.19 mm. D–E. Paratype 2 (SMF 358986), I19_Ma_93, INDEX19_031RO, W 0.87 mm, Wp 0.20 mm. F–G. Holotype (SMF 358984), I19_Ma_94, INDEX19_033RO, H 1.19 mm, W 1.90 mm, Ha 0.81 mm. H–K. Paratype 3 (SMF 358987), I19_ Ma_89, INDEX19_042RO, H 1.61 mm, W 1.98 mm, Ha 1.00 mm, Wp 0.21 mm. L–N. Paratype 4 (SMF 358988), I19_Ma_87, INDEX19_102RO, H 0.63 mm, W 1.00 mm, Ha 0.46 mm, Wp 0.20 mm. Scale bars: 0.1 mm.

Fig. 6. CLSM images of soft parts of Anatoma declivis sp. nov., South-East Indian Ridge, INDEX19_127RO, (I19_Ma_78). A–B. Soft parts in basal and left anterior view (A) and apical and right-anterior view; internal tip removed (B). C. External view of operculum. D. Left anterior view showing cephalic tentacle (CT), epipodial sensory organ (ESO), epipodial appendage (*), foot (F), operculum (O) 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.

Fig. 7. Radula of Anatoma declivis sp. nov., South-East Indian Ridge, INDEX19_127RO, (I19_Ma_78). A. SEM image of distal marginal teeth. B. SEM image of proximal marginal teeth. Scale bars: 10 Mm.