Serrata tuii ( Cossignani, 2001 ) Boyer, 2008

Boyer, Franck, 2008, The genus Serrata Jousseaume, 1875 (Caenogastropoda: Marginellidae) in New Caledonia, Mémoires du Muséum national d'Histoire naturelle 196, pp. 389-436 : 418-419

Serrata tuii ( Cossignani, 2001) comb. nov.

Figs 41-43, 57

Hydroginella tuii Cossignani, 2001: 14-15 .

TYPE MATERIAL. — Holotype (lv) MNHN 1430 , 5 paratypes MMM, 2 paratypes in T. Cossignani collection.

TYPE LOCALITY. — Northern New Caledonia, Grand Passage.

MATERIAL EXAMINED. — Northern New Caledonia. LAGON: 923, 18°52’S, 163°24’E, 470-502 m, 2 dd; stn DW 927, 18°56’S, stn 475, 18°36’S, 163°11’E, 415-460 m, 2 dd . — MUSORSTOM 163°22’ E, 444-452 m, 5 dd (Fig. 42); stn DW 929, 18°52’S, 4: stn DW 181, 18°57’S, 163°22’E, 350 m, 3 dd; stn DW 196, 163°23’E, 502-516 m, 6 dd. — PALEO-SURPRISE: stn DW 1391, 18°55’S, 163°24’E, 450 m, 3 lv, 1 dd; stn DW 197, 18°51’S, 18°29.8’S, 163°02.8’E, 365 m, 1 dd. — TUI 2 (private expedition): 163°21’E, 550 m, 14 dd (Fig. 43). — BATHUS 4: stn DW 914, Grand Passage , 250-270 m, 1 lv (holotype; Fig. 41) .

Grand Passage, 18°49’S, 163°15’E, 600-616 m, 1 dd; stn DW

DISTRIBUTION. — Northwestern tip of New Caledonia, live in 270-450 m, shells in 270-600 m.

DESCRIPTION. — Shell rather oval-subpyriform, solid, opaque. Protoconch paucispiral, small, low, smooth. Spire rather short, conical, whorls weakly convex. Aperture moderately open, very oblique. Base moderately tapering. Outer lip thick, reflected, shoulder rounded, outer edge moderately arched, outer margin faintly thickened, bevelled, very slightly stepped, inner edge straight, bearing 16 prominent denticles, of similar size to the interspaces and becoming larger towards base. Four prominent columellar plaits and small fifth one within aperture, 2 anterior columellar plaits long, sinuous, very oblique, 2 posterior ones shorter, simple, less oblique.

Ground colour opalescent pale amber, with 3 orange spiral bands on last whorl. Base of siphonal canal tinged with pale orange. Outer lip and base of columella intense white. Apex dark, suture zone whitish.

Dimensions: 9.70 x 5.20 mm.

Radula from 1 specimen (Fig. 57): uniserial, 13 plates exceeding 85.80 Μm in width, bearing at least 26-28 unequal strong cusps (left border of ribbon is folded down at back and full width of plates not observed), about 1 out of 4 cusps larger than the others.

REMARKS. — The shell of Serrata tuii varies noticeably in its general outline, from squat pyriform (Fig. 42) to slender subcylindrical (Fig. 43). The spire can be very short (Fig. 42) to tall (Fig. 43). All possible intergrades are found. The shape of the aperture is more constant. The inner labrum can be slightly convex (Fig. 42) to slightly concave (Fig. 43) in its lower part. The number (14 to 16), shape and position of the labial denticles are rather constant, as are the shape and position of the columellar plaits. However, a fifth well-pronounced plait occurs in most shells, unlike in the holotype. The length of adult shells ranges from 8.0 to 11.3 mm.

Serrata tuii was described originally in the genus Hydroginella , characterized among other features by square radular plates with few resorbed cusps, and small columellar plaits packed over the anterior quarter of the columellar border. The comb-like radula with many cusps and the prominent columellar plaits occupying more than half the length of the columellar border both indicate that H. tuii should be placed in Serrata .

One shell referred to S. tuii (PALEO-SURPRISE stn DW 1391) was erroneously attributed by Boyer (2001) to the much smaller species Serrata [as Haloginella ] boucheti Boyer, 2001 , which is restricted to one of the seamounts of the northern Norfolk Ridge.

Despite some similarities with the S. hians group, S. tuii does not seem to belong there but seems closer to the S. boucheti group. Serrata tuii shows superficial similarity to S. dentata in its general outline but is much larger, more solid and more inflated, the shape of its labrum differs in being straight, thicker in its anterior part and thinner in its posterior part, and its teeth are more produced. The differences from S. boucheti are more marked.

Serrata tuii is known from a rather restricted range off northern New Caledonia, at the latitude of the Grand Passage, off the tip of the northern lagoon, in depths between 250 and 616 m, and seems to be rather common.

Because of the irregular “saw blade” shape of its radular cusps (Fig. 57), S. tuii is provisionally considered to belong to the same series as the two species described below, which share the same kind of radula pattern despite their noticeably different shell morphology. Despite the fact that the “saw blade” radula pattern may prove to be a convergent adaptation or to persist randomly as an ancestral feature within various lineages of radulate species with subequal cusped radulae, we cannot rule out that it could be a derived feature and therefore used appropriately as a grouping criterion.