Hexabranchus sanguineus ( Rueppell & Leuckart, 1828)

Yonow, Nathalie, 2012, Opisthobranchs from the western Indian Ocean, with descriptions of two new species and ten new records (Mollusca, Gastropoda), ZooKeys 197, pp. 1-130 : 20-21

Hexabranchus sanguineus ( Rueppell & Leuckart, 1828) Plates 26, 27

Hexabranchus sanguineus . - Yonow & Hayward 1991: 15, fig. 3D (Mauritius); Valdés 2002: 291, figs. 1A, C, 2-4 (South Africa, Mozambique Channel, Madagascar, Philippines, Hawaii) incl. extensive synonymy.

Hexabranchus flammulatus (Quoy & Gaimard). - Narayanan 1968: 378, fig. 2 (India).

Hexabranchus marginatus (Quoy & Gaimard). - Edmunds 1971: 340 (Tanzania); Richmond 2011: 280 (East Africa).

Hexabranchus sp. - Rudman 1986: 347, figs. 1, 19, 20 (Tanzania and Christmas Island).

Material.

Socotra: 55 mm pres. length (WPU Rostock, MAR 85, RC-N57), Hadibo, late 1980s, leg. W Wranik. - Yemen: 85 × 60 mm pres., distorted and flattened (WPU Rostock, MAR 85, RC-N16), Al Mukalla, late 1980s, leg. W Wranik. - Kenya: two specimens 60 and 95 mm in length, pres., Vipingo, 25 m N of Mombasa, ELW in rock pools on exposed reef, 23 September 1984, leg. J Hognerud. - Tanzania: photographs of one individual, M’Nazi Bay, Msimbati, near Mtwara, May 1994, IM Horsfall. - Maldives: one juvenile specimen 13 mm (MDV/AB/96/17), Yacht Tila, South Malé Atoll, 25 m depth, 09 May 1996, leg. RC Anderson & SG Buttress; photos of 15 mm juvenile individual, 1986-1994, J Hinterkircher. - La Réunion, Mauritius, Mayotte: photographs of numerous individuals, including juveniles similar to the Maldives specimen examined here (http://seaslugs.free.fr/nudibranche/a_intro.htm) and slides of 27 individuals varying from yellow to mottled pink, M Parmantier. - Seychelles: photos of several individuals including juvenile and sub-adult, Lilôt, NW Mahé, 1988-1989, P Kemp. - Sri Lanka: photographs of one large individual, Unawatuna, S of Galle, on sediment-covered rock, 27 December 2011, S Kahlbrock.

Description.

The large specimens and individuals were of the typical Indo-West Pacific colour pattern, blotchy red and cream, especially along the margins. The 13 mm juvenile specimen from the Maldives illustrated here (Plate 26) is well relaxed, and the six gills can be seen to insert into separate openings. The oral tentacles are large rounded lobes, clearly the precursors to the lappets of the adult: these tentacles are huge in comparison to those of a chromodorid (or dorid) of similar size. The rhinophores have 13 lamellae; note that the white edges present in the adults have not yet developed. Another 15 mm juvenile individual was identical in colour pattern, while a slightly larger animal from the Seychelles demonstrated the developing adult colour pattern already had white edges to the 17+ rhinophore lamellae (Plate 27).

The radular formulae of the larger specimens are tabulated below: the sizes listed are of the same dimension of a large lateral tooth - from the tip of the cusp to the flange where the cusp meets the base. There is no relationship between the radular formula and maximum tooth size, nor are they correlated with preserved animal size. A giant Hong Kong specimen is included for comparison, and has the largest teeth but not the largest radula. It had a bubbly texture and was pinkish yellow in life (M Collard pers. comm.; specimen, radula, notes and photographs lodged in the Natural History Museum, London: NHMUK acc. no. 2337 with the "Red Sea Giant").

Socotra 55 mm pres. 41 × 64.0.64 500 μm

Yemen 85 mm pres. 53 × 93-83.0.83-93 550 μm

Kenya 90 mm pres. 47 × 77.0.77 700 μm

Kenya 95 mm pres. 46 × 79.0.79 450 μm

Hong Kong 190 mm pres. 51 × 67.0.67 800 μm

Remarks.

Yonow (2001) suggested that the uniformly red species in the Red Sea should be assigned to Hexabranchus sanguineus ( Rüppell & Leuckart), and that it was distinct from the widespread Indo-Pacific Hexabranchus marginatus (Quoy & Gaimard). The study mentioned in that paper was never published, since the following year a paper analyzing the same problem was published by Valdés (2002): Valdés examined material from the western Indian Ocean and the Pacific, but no Red Sea specimens were included in his analysis, nor were any ‘giants’ but, despite this, he concluded that all Indo-Pacific species were the same, and that only the Caribbean species Hexabranchus morsomus Ev. Marcus & Er. Marcus was distinct.