Bathyacteon aequatorialis (Thiele, 1925)

Valdés, Ángel, 2008, Deep-sea “ cephalaspidean ” heterobranchs (Gastropoda) from the tropical southwest Pacific, Mémoires du Muséum national d'Histoire naturelle 196, pp. 587-792 : 609-610

Bathyacteon aequatorialis (Thiele, 1925)

Figs 10A-D, 12

Acteon aequatorialis Thiele, 1925: 224-225 , pl. 30, fig. 9.

TYPE MATERIAL. — Nineteen syntypes MNHB .

TYPE LOCALITY. — Sombrero Channel , Nicobar Islands, Indian Ocean , 7°48.8’N, 93°7.6’W, 805 m, Equatorial Channel , Maldive Islands, Indian Ocean , 0°2.3’S, 73°24’W, 2253 m, Zanzibar Channel, Indian Ocean, 5°27.9’S, 39°18.8’E, 463 m, and southern Somalia, 1°40.6’S, 41°47.1’E, 693 m [ Valdivia : stns 211, 219, 245, and 251] .

MATERIAL EXAMINED. — Philippines. MUSORSTOM 3: stn DW 739 , Passe de Boulari , 22°35’S, 166°27’E, 465-525 m, 2 CP 106, 13°47’N, 120°30’E, 640-668 m, 13 dd & lv, one des- dd .

troyed to dissect the radula (Fig. 10D). Loyalty Ridge. BIOGEOCAL: stn CP 238, 21°28’S, 166°23’E, Indonesia. KARUBAR: stn DW 28, Kai Islands, 05°31’S, 1260-1300 m, 1 lv.

132°54’E, 448-467 m, 1 dd. Vanuatu. MUSORSTOM 8: stn DW 1045, 16°54’S, 168°20’E, New Caledonia proper. BATHUS 1: stn DE 697, Passe de Hien- 459-488 m, 2 dd.

ghène, 20°34’S, 164°58’E, 570-650 m, 1 dd. — BATHUS 2 : stn Fiji. MUSORSTOM 10: stn CP 1361, SE of Viti Levu , 18°00’S ,

178°54’E, 1058-1091 m, 2 dd. — BORDAU 1 : stn DW 1485 , Tonga. BORDAU 2 : stn DW 1531 , Eua, 21°12’S, 174°56’W, 19°03’S, 178°30’W, 700-707 m, 2 dd; stn CP 1391, S of Viti Levu , 970-983 m, 2 dd; stn DW 1553 , S of Nomuka Islands, 20°42’S, 16°49’S, 179°54’E, 600-660 m, 1 dd. 174°54’W, 650-676 m, 2 dd (Fig. 10A); stn DW 1554 , 20°38’S, Wallis Island. MUSORSTOM 7: stn DW 601 , 13°19’S, 176°17’W, 174°58’W, 482-498 m, 1 dd .

350 m, 16 dd (Figs 10B, C).

DISTRIBUTION. — Originally described from the western Indian Ocean, in 463-2253 m. Material herein collected from the Philippines, Indonesia, New Caledonia, Vanuatu, Fiji, Wallis Island and Tonga (Fig. 12), in 350-1260 m, live in 668-1260 m.

DESCRIPTION. — Shell morphology. Length 4.5 mm, width 2.5 mm (largest specimen examined). Shell thin, oval, with convex sides (Fig. 10A). Body whorl large, about 4/5 of the shell length; spire short with 2-3 whorls. Suture slightly channeled. Protoconch globose, partially eroded or missing in all specimens examined (Fig. 10B); appears to have about 0.8 whorls, 500 Μm in diameter. Umbilicus closed. Aperture wide and long, wider anteriorly, about 4/5 of the body whorl length. Columellar margin thickened, short, with a small fold, not visible in all specimens. Sculpture of a few punctuated spiral grooves (Fig. 10C). The punctuations are small, oval, and clearly separated from each other within each groove. The grooves are separated by gaps that increase in width towards the posterior end of the spire. Colour uniformly whitish.

Anatomy. The radular formula is 34 x 3.1.1.1. 3 in a specimen from the Philippines (MUSORSTOM 3 stn CP 106). The rachidian tooth has 5-6 sharp cusps, some of them larger than the others (Fig. 10D). In most teeth the 2 central cusps are larger, but some other teeth have a smaller cusp intercalated. The innermost lateral teeth are broad, with 1-2 median cusps situated near the inner edge of each tooth. On the inner and outer sides of the cusp/s there are 2- 3 denticles. The rest of the lateral teeth are hamate, with long cusps bearing 5-10 denticles. The teeth become more elongate and with a less robust cusp towards the outer edge of the radula.

REMARKS. — Examination of the type material of Bathyacteon aequatorialis shows that the syntypes, collected from the western Indian Ocean are indistinguishable from the material from the southwest Pacific Ocean here examined. Therefore, the newly collected material is assigned to this species until complete specimens from the Indian Ocean become available for study.