Thieleella (=Anatoma) weddelliana is characterized by having a large, laterally expanded shell, with rounded whorls, wide and deep umbilicus, and strong axial ribs.

Description

Shell large (up to 5.6 mm in diameter), trochiform, laterally expanded. Protoconch of one whorl, about 220 µm in diameter, with reticulate ornamentation; margin of protoconch convex, without varix. Teleoconch I of 3/4 to 5/6 rounded whorls, sculptured with 10-12 axial ribs, one spiral cord in position of selenizone forming strong angulation; teleconch II of up to 2 3/4 whorls. Whorls of spire rounded; last whorl nearly straight; shoulder and base convex. Shell with cancellate sculpture; axial ribs strong: 22-26 on shoulder of first whorl, 27-28 on second whorl, 31-41 on base; spiral threads weak, beginning at start of selenizone, gradually increasing in number with growth: 5-8 on first whorl, 25-30 at apertural margin. Selenizone displaced towards suture, sculptured with low axial ribs, surrounded by narrow, strong keels. Umbilicus open, wide, deep; funiculus present. Aperture rounded.

Anatomy: Animal pale cream, uniform. Cephalic tentacles long, lanceolate, flat,
sparsely papillate; papillae tubular, of about 110 pm long, spirally arranged, slightly ex¬panded distally with a handfull of cilia. Eyes and eyestalks absent. Head separated from foot by wide neck, with one pair of smooth cylindrical tentacles on each side. Three pairs of conical papillate epipopial tentacles; first epipodial tentacle divided basally forming two tentacles. Papillae similar to that of cephalic tentacles, but smaller (42-50 µm long). Columellar muscle thick, elongate in outline. Pallial cavity with enlarged mantle border, deeply sinuated in medial line, one rounded papillate tentacle extending anteriorly in medial line. Two large bipectinate ctenidia; left ctenidium about 10% larger than right. Hindgut very large, sinuous, drawing posteriorly directed loop at left side, before ending in anus.

Radula: Rachidian tooth trapezoidal at base, cusp nearly straight, with one main central denticle and 6-10 narrower, sharper denticles on each side. Five pairs of lateral teeth; laterals 1-3 with rectangular bases, innermost tooth widest; cusp with prominent innermost denticle, 7-11 narrower denticles on outer margin; inner margin smooth. Fourth lateral tooth narrower than lateral 3; lateral tooth 5 claw-shaped, wider at base, with wide, sharply serrated cusp. Marginal teeth numerous, long, narrow, becoming more delicate outwards, with prominent, delicatelly serrated cusp.

Anatoma weddelliana resembles Anatoma amoena and Anatoma conica, from which it differs in having a larger shell, with wider umbilicus and selenizone displaced towards the suture. Furthermore, the last whorl is more uniformely rounded in A. conica and A. amoena than in A. weddelliana. In addition, A. weddelliana has fewer axial ribs in the first whorl of teleoconch II than A. amoena (22-26 vs. 37-39).
Anatoma weddelliana differs from Anatoma euglypta and Anatoma shiraseae, the other two Antarctic species of anatomids, in having a globular instead of a biconical last whorl, and stronger axial sculpture.
Anatoma baxteri (McLean, 1984) from the northeastern Pacific is similar in overall shape, but has more numerous (30 vs. 13) weaker axial ribs on teleoconch I, and lacks a spiral cord in the position of the selenizone on teleoconch I. The differences in the density and strength of the sculpture are similar on teleoconch II. As the spiral threads on shoulder and base are of equal strength in both species, A. baxteri appears more reticulate as spirals and axials are more equal in strength, whereas in A. weddelliana, the axial ribs are much more prominent than the spirals.
The reticulate protoconch sculpture of this species is composed of smaller and more irregular cells than in other Anatoma species. The observed sculpture is best described as reticulate, which is diagnostic for Anatoma. The sculpure of A. weddelliana cannot be described as the flocculent type frequently encountered in Anatoma spp.

Type Locality: 70°31.08'S, 14°34.83'W, Weddell Sea, 4384-4420 m.
Known only from the type locality