Gastropteron bicornutum Baba & Tokioka, 1965

Valdés, Ángel, 2008, Deep-sea “ cephalaspidean ” heterobranchs (Gastropoda) from the tropical southwest Pacific, Mémoires du Muséum national d'Histoire naturelle 196, pp. 587-792 : 777-778

Gastropteron bicornutum Baba & Tokioka, 1965

Figs 95C, 97A, 98, 99

Gastropteron bicornutum Baba & Tokioka, 1965: 364 , text figs 1-6, pl. 25, figs 1-9.

TYPE MATERIAL. — Not examined .

TYPE LOCALITY. — Tomoika Bay , Amakusa, Japan .

MATERIAL EXAMINED. — Vanuatu. MUSORSTOM 8: stn CP 963, 20°20’S, 169°49’E, 400-440 m, 1 lv .

DISTRIBUTION. — Previously recorded from shallow waters of Japan and Papua New Guinea (see Gosliner 1989). Material herein constitutes the first record from deep waters, collected in Vanuatu (Fig. 99), in 400-440 m.

DESCRIPTION. — External morphology. Length 6 mm (single specimen examined). Animal oval, with 2 large and thick ventro-lateral expansions (Figs 97A, 98A) that are probably used for swimming, as in other species of the genus. The cephalic shield is oval with a conical, dorsal protuberance and 2 blunt posterior expansions. The dorsum is smooth. On the posterior end of the body there are 2 conical processes or flagella. On the right side of the body there is a small, unipinnate gill, with 7 pinnae (Fig. 98B). The gill is surrounded by the genital aperture anteriorly and the anal papillae posteriorly.

The colour of the living animals is uniformly whitish with bright red spots on the centre of the dorsum, around the ventro-lateral expansions, and on the cephalic shield. Posterior end of the cephalic shield edged by a bright yellow line. Flagella bright yellowish with red spots. Gill uniformly white.

Anatomy. The radular formula is 18 x 5.0. 5 in a 6 mm long specimen (MUSORSTOM 8 CP 963). The innermost lateral teeth are hamate, with a long, curved cusp, and 5-7 sharp denticles (Fig. 95C). The denticles vary in size and shape among teeth. The rest of the lateral teeth are also hamate but smooth, with no denticles, and considerably smaller in size.

The reproductive system is monaulic (Fig. 98C). The ampulla is very long and convoluted. It connects to the narrow, short post-ampullary duct. The post-ampullary duct continues distally to the common general atrium. Also entering the common atrium are the female glands and the bursa copulatrix. From the gonopore an open seminal groove runs anteriorly to the protrusible cephalic penis and the prostate. The prostate is elongate and connects proximally into the penial sac. The penial opening has a retractor muscle attached.

REMARKS. — The material here examined differs from shallow water specimens of Gastropteron bicornutum in the external colouration. The deep-water specimen lacks the black and opaque white pigment present in the material examined by Gosliner (1989). In addition, the orange-red and yellow spots of the specimen from

FIG. 97. Photographs of living animals. A, Gastropteron

bicornutum Baba & Tokioka, 1965, specimen from

Vanuatu, MUSORSTOM 8 stn CP 963. B, Philinopsis

cyanea (Martens, 1879), specimen from Vanuatu,

MUSORSTOM 8 stn CP 963.

Vanuatu are restricted to small areas of the dorsum and cephalic shield. However, the morphology of the radula and the reproductive system are extremely similar to those described by Gosliner (1989) and therefore the material here examined is assigned to this species.

This species differs externally from Gastropteron sibogae Bergh, 1905 in having 2 posterior flagella and a dorsal protuberance on the cephalic shield. Internally the seminal receptacle of G. bicornutum is more elongate and the prostate is shorter than in G. sibogae .

FIG. 98. External morphology and anatomy of Gastropteron bicornutum Baba & Tokioka, 1965 , specimen from Vanuatu, MUSORSTOM 8 stn CP 963. A, dorsal view of the preserved animal, scale bar = 1 mm; B, detail of the gill, scale bar = 0.5 mm; C, reproductive system, scale bar = 0.5 mm.Abbreviations:ag, albumen gland; am, ampulla; ap, anal papillae; bc, bursa copulatrix; cs, cephalic shield; esg, external seminal groove; f, flagellum; gl, gill; mg, mucous gland; pn, penial sac; pr, prostate; rm, retractor muscle; sr, seminal receptacle.