Anatoma megascutula, Geiger, Daniel L. & Marshall, Bruce A., 2012

Geiger, Daniel L. & Marshall, Bruce A., 2012, New species of Scissurellidae, Anatomidae, and Larocheidae (Mollusca: Gastropoda: Vetigastropoda) from New Zealand and beyond, Zootaxa 3344, pp. 1-33 : 24-26

Anatoma megascutula new species

( Figures 18–20 )

Type material. Holotype ( MNHN 24954: Fig. 18 ). 5.03 × 3.36 mm. Paratypes: Norfolk Ridge, 24.867˚S, 168.183˚E, 820–1220 m ( MNHN 24959, 1: Fig. 19 A); Tongatapu, Tonga, 21.083˚S, 175.367˚W, 456–510 m ( MNHN 24955, 1: Fig. 19 B); Fiji, 17.300˚S, 179.550˚W, 820–863 m ( MNHN 24956, 1: Fig. 19 C).

Type locality. E of Rapa, Austral Islands, French Polynesia, 27.618˚S, 144.257˚W, 800–850 m, 7 Nov. 2002, N. O. Alis (BENTHAUS stn CP1891).

Etymology. Mega-, Greek prefix for large; -scutula, Latin for saucer, shallow bowl, referring to the large size of this flat-shelled species. Noun in apposition.

Description. Shell large (to 6.8 mm. Figs 18–19 ), trochiform depressed, discoidal. Protoconch of 0.75 whorl, flocculent sculpture, apertural varix not connected to embryonic cap, apertural margin shallow sinusoid. Teleoconch I of 0.75–0.875 whorl, approximately 20–30 axial cords, no spirals. Teleoconch II of 3.3 whorls, suture lightly impressed, no sutsel except for last quarter whorl with slight descent showing 1 spiral thread. Shoulder broadly convex, many barely perceptible, low axial cords becoming somewhat more noticeable with growth; first low spiral thread after 0.125–0.8 whorl, increasing in strength to become low to distinct spiral cords, approximately a dozen after first teleoconch I whorl, approximately 25 at apertural margin, forming indistinct thickenings at intersection with axials. Base without constriction below selenizone, continuously sloping with umbilicus; axial sculpture as on shoulder if usually somewhat stronger; approximately 22 spirals, distinct threads from selenizone onwards, changing at mid-base into steps. Umbilicus variable; open, bordered by carina, walls straight, smooth; additionally with distinct funiculus; closed by parietal callus [see Remarks]. Aperture oblong D-shaped. Selenizone at periphery, keels moderately strong, moderately elevated (usually low due to erosion); lunules distinct; slit open, margins converging.

Radula ( Fig. 20 ). Rachidian tooth trapezoid, apical cusp largest, 4–5 on each side in continuous arc. Lateral tooth 1 broadened, apical cusp slightly larger than 7–8 distal ones on cutting edge. Lateral teeth 3–4 similar, apical cusp largest, 3–5 along outer margin of cutting edge. Lateral tooth 4 reduced, apical cusp largest, three small ones on each side. Lateral tooth 5 enlarged by moderate broadening, apical cusp largest, about 12 small cusps along inner edge, approximately 8 not reaching apical cusp along outer edge. Inner marginal teeth with triangular tip, apical cusp largest, approximately 8 minor cusps on each side. Outer marginal teeth with indistinct separation of shaft and tip, tip with multiple fine bristles. Radular interlock of central field moderate.

Operculum corneous, thin, round, multispiral, with central nucleus, covering approximately 70% of aperture. Distribution. Western Pacific, 450–1650 m.

Specimen records. Australia. N of Norfolk Island, Norfolk Ridge, 26.432˚S, 167.181˚E, 750–774 m (NMNZ M.301394, 7). Coral Sea. 21.317˚S, 157.967˚E, 970 m (MNHN, 1). New Caledonia. Grand Passage, 18.808˚S, 163.190˚E, 683– 669 m (MNHN, 2). 18.900˚S, 163.317˚E, 525 m (MNHN, 1); Loyalty Basin, 20.983˚S, 166.983˚E, 1600–1640 m (MNHN, 3); 23.050˚S, 167.533˚E, 825–830 m (MNHN, 1); Loyalty Ridge, 24.767˚S, 170.117˚E, 850–855 m (MNHN, 1); Norfolk Ridge, 24.867˚S, 168.183˚E, 820–1220 m (MNHN, 1); Porthos Bank, Norfolk Ridge, 25.333˚S, 168.950˚E, 680–980 m ( NORFOLK 2 DW 2068, MNHN, 1). New Zealand. Graveyard Seamount Complex, Chatham Rise, 42.766˚S, 179.989˚W, 757–875 m (NMNZ M.174849, 1). Fiji. 17.300˚S, 179.550˚W, 820–863 m (MNHN, 1). Tonga. 20.633˚S, 178.050˚W, 476–478 m (MNHN, 1). Tongatapu, 21.083˚S, 175.367˚W, 456–510 m (MNHN, 1). French Polynesia. E of Rapa, Austral Islands, 27.618˚S, 144.257˚W, 800–850 m (MNHN, 1).

Remarks. Anatoma japonica (A. Adams, 1862) from the Indian Ocean and Indo-Pacific has a wider sutsel on whorl 2 (0.5–1× vs. 0× width of selenizone), has stronger axial than spiral sculpture, and has a narrower selenizone. Anatoma maxima from the Indo-Malayan Archipelago has coarser sculpture (14 vs. 25 spirals on shoulder at apertural margin; distinct vs. indistinct axials) in which axials and spirals form points at intersections, a less rapid expansion of the whorls (1.4 vs. 1.8), and a wider sutsel at the apertural margin showing 2–3 spiral threads. Anatoma rainesi from the Eastern and Western Pacific has a sutsel on the early whorls, has strong axial cords on teleoconch I, and the selenizone is closer to the periphery of the shell. Anatoma rapaensis Geiger, 2008 from French Polynesia shares the overall discoidal shape with no sutsel, but the protoconch lacks an apertural varix, the teleoconch I is shorter (0.2 vs.>0.75 whorl, the axial sculpture is more pronounced forming sharp points at the intersection with the spirals. Anatoma regia (Mestayer, 1916) from New Zealand has a narrower but still noticeable sutsel on early whorls, has a narrower selenizone, and its axial sculpture is stronger than the spiral sculpture. Anatoma disciformis (Golikov & Sirenko, 1980) from the boreal north Pacific has an overall taller shell, a protoconch with strong flocculent sculpture, and much stronger axial cords. Anatoma tenuis (Jeffreys, 1877) from the temperate north Atlantic has a shorter teleoconch I (0.4 vs. 0.75 whorl), and has stronger axial than spiral sculpture.

The number of axials on teleoconch I is difficult to count due to erosion of the early parts of the whorl. The condition of the umbilicus is quite variable in the limited number of specimens available. The more open condition is found in smaller specimens, while the callus closing the umbilicus is encountered in the largest specimens, which suggest an ontogenetic change. Other conchological characters all suggest that the specimens belong to a single species.

The radula of A. megascutula shows an unusual arrangement in the central field. Lateral teeth 1–3 are not similar to one another, lateral tooth 1 is much broadened, and lateral tooth 4 is not as reduced as in most other species.

Figures

FIGURE 18. Anatoma megascutula new species. Holotype. 800 – 850 m, east of Rapa, Austral Islands, French Polynesia, 27.618 ˚ S, 144.257 ˚ W (MNHN 24954). Scale bar shell = 1 mm. Scale bar protoconch = 100 µm.

FIGURE 19. Anatoma megascutula new species. Paratypes. A. Norfolk Ridge, New Caledonia, 820 – 1220 m (MNHN 24959). B. Tongatapu, Tonga, 456 – 510 m (MNHN 24955). C. Fiji, 820 – 863 m (MNHN 24956). Scale bars shell = 1 mm. Scale bars protoconch = 100 µm.

FIGURE 20. Anatoma megascutula new species. Radula. Porthos Bank, Norfolk Ridge, New Caledonia, 680 – 980 m (MNHN). A. Central field. B. Marginal teeth. Scale bars = 100 µm.