Japonacteon longissimus, Valdés, 2008

Valdés, Ángel, 2008, Deep-sea “ cephalaspidean ” heterobranchs (Gastropoda) from the tropical southwest Pacific, Mémoires du Muséum national d'Histoire naturelle 196, pp. 587-792 : 604-605

Japonacteon longissimus n. sp.

Figs 6E-G, 7, 8B

TYPE MATERIAL. — Holotype MNHN 20265 and 19 paratypes MNHN 20266-20269, 1 paratype LACM 2978 .

TYPE LOCALITY. — Philippines, 13°47’N, 120°30’E, 640-668 m [MUSORSTOM 3: stn CP 106] .

MATERIAL EXAMINED.— Philippines. MUSORSTOM 3: stn CP Tanimbar Islands 08°19’S, 132°02’E, 457-461 m, 1 dd, para- 106, 13°47’N, 120°30’E, 640-668 m, 17 dd, holotype ( MNHN type ( LACM 2978 ); stn CP 87, 08°47’S, 130°49’E, 1017-1024 m, 20265; Fig. 6E) and paratypes ( MNHN 20266 ; Figs 6F, G, 8B). 1 dd, paratype ( MNHN 20268 ); stn CP 91, 08°44’S, 131°05’E, Indonesia. CORINDON: stn B 213, 00°31’N, 117°50’E, 488 m, 884-891 m, 1 lv, destroyed to dissect the radula, paratype 1 dd, paratype ( MNHN 20267 ) . — KARUBAR: stn CC 58 , ( MNHN 20269 ) .

DISTRIBUTION. — Known only from the Philippines and Indonesia (Fig. 7), in 461-1017 m, live in 884-891 m.

DESCRIPTION. — Shell morphology. Length 9 mm, width 4 mm (holotype); length 13 mm, width 10 mm (largest specimen examined, apex eroded). Shell thin, elongate, with convex sides (Fig. 6E). Body whorl large, about 2/3 of the shell length. Spire elongate, conical, with 3 long whorls. Suture slightly channeled. Protoconch globose, about 1.5 whorls and approximately 500 Μm in diameter (Fig. 6F). Umbilicus closed. Aperture narrow, wider anteriorly, and short, about 2/3 of the body whorl length. Columellar margin thickened, slightly oblique, with a small, simple fold. Sculpture of a number of punctuated spiral grooves (Fig. 6G). The punctuations are conspicuous, oval, and situated next to each other within each groove. The grooves are more separate in some specimens, and occasionally narrower grooves are intercalated. Two of the grooves situated near the spire suture are wider. Colour uniformly cream.

Anatomy. The radular formula is 18 x 5.0. 5 in the holotype. The radular teeth are hamate, with an elongate base and a sharp cusp (Fig. 8B). All the teeth lack denticles and they increase in size towards the outer edge of the radula.

REMARKS. — Acteon longissimus is very similar to Acteon delicatus Dall, 1889 , described from several localities in the western Atlantic, from the Caribbean Sea to Patagonia, in 134-732 m ( Dall 1889). However, the shell of A. longissimus seems to be more elongate.

Among the Indo-Pacific species, A. longissimus most resembles Acteon dolichoroseus Iredale, 1936 in shell morphology. The latter species was originally described from an unknown depth in Sydney Harbour, Australia. Both species have a very elongate spire with long whorls. Differences include the colour of the shell, pinkish with a subsutural white band in A. dolichoroseus and uniformly cream in A. longissimus , and the presence of an umbilicus in A. dolichoroseus .

Acteon longissimus is provisionally placed in Japonacteon because of its radular morphology, with a small number of hamate lateral teeth.

ETYMOLOGY. — From the Latin longissimus (longest), in reference to the elongate spire of this species.

FIG. 7. Collection localities of species of Japonacteon Taki, 1956 in the southwest Pacific: °, Japonacteon sieboldii (Reeve, 1842) . Z, Japonacteon longissimus n. sp.