Hexabranchus sanguineus (Rüppell & Leuckart, 1830)

Tibiriçá, Yara, Pola, Marta, Pittman, Cory, Gosliner, Terrence M., Malaquias, Manuel A. & Cervera, Juan Lucas, 2023, A Spanish dancer? No! A troupe of dancers: a review of the family Hexabranchidae Bergh, 1891 (Gastropoda, Heterobranchia, Nudibranchia), Organisms Diversity & Evolution (New York, N. Y.) 23 (4), pp. 697-742 : 711-714

Hexabranchus sanguineus (Rüppell & Leuckart, 1830) . Abraham (1876: pgs. 103–108) (new combination reference).

Hexabranchus praetextus Ehrenberg (1828: pt1-2, pl. 1a-c). Type locality: El Tur, Egypt (synonymized by Thompson, 1972).

Hexabranchus suezensis Abraham (1876 : v. (4) 18, pgs. 137–138, pl. 6, figs. 3, 3a). Type locality: Red Sea (synonymized by Thompson, 1972).

Hexabranchus petersi Bergh (1878: 2 (13), pgs. 60–564, pl. 64, Fig. 1 ; pl. 67, figs. 7–9). Type locality: Quirimba Islands , Northern Mozambique, East Africa (synonymized by Valdés, 2002).

Albania formosa Collingwood (1881 : v.2 (2), p.133, pl. 10, Figs. 1–5 ). Type location: Ke-lung Harbour , Formosa, Taiwan (synonymized by Thompson, 1972).

margin, D unrolled mantle, white margin, E underside, F genital papillae, G rhinophores, H gills, I oral tentacles, J oral tentacle, pink margin, K transitional in laboratory, L egg mass

Hexabranchus plicatus Hägg (1901: v.6, pgs. 5–7, pl. 1, figs. 4–5). Type locality: Tor, Egypt. (synonymized by Thompson, 1972).

tle, red margin, E unrolled mantle, white margin, F unrolled mantle, violet margin G unrolled mantle, red margin H rhinophores, I genital papillae, J genital papillae, close up, K gills, L oral tentacles

Material examined CASIZ194618 , length 75 mm (preserved), Madagascar, Sud Baie de Lokaro (24° 57′ S, 47° 6.5′ E), 10 m depth, 12 May 2010 . MB28-005033 , length dark, mature E unrolled mante, mature F underside, G gills, H rhinophores, I mature specimen in laboratory

250 mm, Mozambique, Ponta do Ouro (26° 50′ 58″ S, 32° 44′ 54″ E), 39 m depth, 18 June 2016 . MNCN: ADN 110932 (tissue), length ≈ 250 mm, Mozambique, Ponta do Ouro (26° 50′ 58″ S, 32° 44′ 54″ E), 40 m depth, 10 May 2014 . UF455939 Jeddah , Saudi Arabia (21° 45′ 24.1″ N, 39° 03′ 06.5″ E), 15 m depth, 9 October 2012, collected by Gustav Paulay. MNCN: ADN 110934 , length 240 mm, Mozambique, Ponta do Ouro , “The Cake” (26° 50′ 22″ S, 32° 54′ 39″ E), 38 m depth, 12 April 2022 . Other material: MNCN: ADN 110939 (tissue), length ≈ 250 mm, Mozambique, Ponta do Ouro (26° 50′ 58″ S, 32° 44′ 54″ E), 40 m depth, 17 Nov. 2018 . UF449478 (tissue), French Polynesia, Marquesas Islands, Fatu Hiva Island (10° 31′ 58.4″ S, 138° 41′ 05.6″ W), 3 Dec. 2011 ( FLMNH Invertebrate Zoology) . UF449478 (tissue), French Polynesia, Marquesas Islands, Fatu Hiva Island (10° 31′ 58.4″ S, 138° 41′ 05.6″ W), 3 Dec. 2011 ( FLMNH Invertebrate Zoology) . Sequenced but not deposited (tissue), length ≈250, Red Sea, Global Range (27° 40′ 07″ N, 33° 48′ 32″ E), 10 m depth, 29 Oct. 2018 . Sequenced but not deposited (tissue) length ≈ 230 mm, Red Sea, Egypt, Shaab Samadai East (24° 59.144′ N, 34° 59.798′ E), 10 m depth, 1 Nov. 2018 .

Distribution Broadly distributed across the Indo-Pacific from the Red Sea to French Polynesia: Egypt ( Debelius, 1996; Rüppel marginal band, F stranded, red marginal band, G mature pair, heavily flecked, H lighly flecked, detail, I heavily flecked, detail, J gills, juvenile, K gills, mature, L rinophores and oral tentacles

& Leuckart, 1828; Yonow, 2008), Sudan ( Debelius, 1996), Sri Lanka ( Debelius, 1996), southern Mozambique (Stromvoll & Jones, 2019; Tibiriçá et al., 2017), South Africa (Gosliner et al., 2008; King & Fraser, 2014), Seychelles ( Debelius, 1996), Japan ( Atsushi, 2004; Nakano, 2004), and Australia (Marshall & Willan, 1999) including Lord Howe Island ( Coleman, 2001, 2008), New Caledonia (Hervé, 2010), and French Polynesia (Salvat & Bacchet, 2011). On-line sources: Israel, Tanzania (iNaturalist), Indonesia (Sea Slug Forum) including West Papua (iNaturalist), Madagascar, Reunion (South-west Indian Ocean Seaslug site), north-western Australia (Sea Slug Forum, iNaturalist), east Australia (Nudibranchs Sunshine Coast Queensland, Australia), Saipan (Nudipixel archive).

External morphology ( Figs. 8 , 9 , 10 , and 11) Commonly up to 250 mm (with some reports to 400 mm in the Red Sea). The body of resting, mature animals is smooth and more dorsal-ventrally compressed than in other Hexabranchus spp. The extended mantle has an undulating edge that is very thin and delicate on the sides, posteriorly, but shorter and thicker with a smooth edge, anteriorly. The rhinophore sheath is very short with smooth edges. The rhinophores are slightly bent to the back with approximately 40 lamellae in large, mature animals. There are usually six tufts of multi-pinnate gill branches set widely apart and forming a circle around the anus. The anus is elevated on a tubular papilla. The kidney pore is anterior to the anus on its right side. The oral tentacles are large, fleshy, oval, and crenate. The foot is narrower than the body.

Ontogeny, color, and variation ( Figs. 8 , 9 , 10 , and 11) There are four, apparently disjunct, lineages that differ in color (which changes with ontogeny).

Lineage 1 ( Fig. 8 ) Predominantly in the Red Sea with very few records in the western Indian Ocean. No specimens or photos of very young juveniles were available. Gohar and Soliman (1963) provided information on intra-specific and ontogenetic variation in this lineage for specimens above approximately 75 mm, and it is in agreement with the photos reviewed in this study. Transitional animals vary from translucent pink to pink-reddish with variable white marginal bands. As the transition proceeds, lateral red patches (truncated medially) begin to develop, white pigment develops on the outer face of the rachis and often a white line appears on the anterior face of the rhinophore club. With growth, a submarginal red band develops. In large animals, the background usually darkens to blood red, which may obscure the lateral red markings. Additionally, a white marginal band is often present. A small amount of white pigment may appear on the posterior edge of the rhinophore stalk. There are no white markings on the notum or white flecks on the rhinophore lamellae. Lateral striations are uncommon.

Lineage 2 ( Fig. 9 ) Western Indian Ocean (from Tanzania to South Africa). It is similar to lineage 1 in pattern and ontogeny, but the mantle color is predominantly orange. Young juveniles are gray with a marginal white line. The branchia and rhinophores are translucent-gray, tipped with orange-red. Transitional animals are grayish-yellow in color with a yellow-white edge, and a few orange-red lateral patches. The rhinophores are orange. Transitional animals are light orange with a marginal red band and, in some specimens, a white edge. As it grows, the mantle gets darker and the marginal band wider, while the lateral red patches (truncated medially) increase in number and size. The rhinophores remain orange and a white line often develops on the anterior face of the club. As in lineage 1, lateral striations are uncommon, and no white flecks are present on the notum or rhinophore lamellae.

Lineage 3 ( Fig. 10 ) French Polynesia. No specimens or photos of juveniles were available but transitional animals appear to vary from translucent pink to yellow with extensive white pigment clustered in large rosettes. The rachis has dense white pigment. Dark lateral patches and red submarginal or marginal bands develop fairly late. Larger animals may become dark-red with some reduction in the white pigment and a white line may appear on the anterior face of the rhinophore club. The rhinophore lamellae are often flecked with white. The margin in mature animals may be either red-banded or white-banded with the former being most common.

Lineage 4 ( Fig. 11 ) Western Pacific. Regrettably, we could not obtain specimens of this morphotype for DNA analysis but based on extensive photograph review it is likely that a fourth distinct lineage of H. sanguineus exists in this region. This lineage shows an intermediate pattern between the ones from the Indian Ocean and the one from French Polynesia. Juveniles vary from translucent gray to yellow with a white marginal band and scattered white flecks on the notum. With growth, red lateral patches and a red submarginal band develop, white pigment appears on the rachis and the white flecks form clusters on the notum. The clusters of white flecks are present in almost all mature animals while the lateral patches are more variable than in specimens from the Indian Ocean. Dark-red animals occur but they appear to be limited to the northern and southern extremes of its distribution (southern Japan and Lord Howe Island, respectively). A white line usually appears on the anterior face of the rhinophore club. The rhinophore lamellae are often flecked with white. Pale animals seem rare. The margin in mature animals may be either red-banded or white-banded. Rarely, the red band may be interrupted (with associated lateral striations).

In all four lineages, during ontogeny, the mantle expands laterally and becomes rolled, the number of rhinophore lamellae increases and the gills become more elaborate. The notum of resting animals remains smooth.

Internal morphology Buccal mass ( Fig. 12 ). The buccal bulb is oval and slightly larger than the oral tube. The radula is broad and bilobed with 30 anterior raised rows of teeth ( Fig. 12A ). The center of the ribbon is devoid of teeth. The teeth are simple and hamate. The innermost teeth are smaller and degenerate. The lateral teeth increase in length centrally ( Fig. 12B ). The outer teeth are smaller ( Fig. 12C ). The radular formula is 49 × 89.0.89 (MB28-005033) and 32 × 50.0.50 (CASIZ194618). The armed jaws have numerous simple, finger-like rodlets ( Fig. 12D ).

Reproductive system ( Fig. 13 ) As described by Eales (1953) and Gohar and Soliman (1963).

Figures

Fig.1 Bayesian phylogenic hypothesis represented by the collapsed phylogenetic tree (PP≥0.5) of the genus Hexabranchus based on concatenated molecular data (COI+16S+H3). Values on the top of the branches represent Bayesian posterior probabilities (PP, top left) and maximum likelihood bootstrap percentages (ML, top right) and on the bottom bootstrap values for maximum parsimony (MP). Colored

Fig. 2 The haplotype network based on cytochrome c oxidase subunit I (COI) molecular data showing genetic mutations occurring within species of the genus Hexabranchus. Each hatch line represents one mutation and black dots represent hypothetical haplotypes. Each

Fig. 3 Ontogeny, color, and variation in Hexabranchus lacer (morphotype 1: Pacific). Green circles indicate studied specimens and red online sources. A early juvenile, B juvenile, C early transitional, D mild-transitional, E late transitional, F mature, even rosettes, G

Fig. 4 Ontogeny, color, and variation in Hexabranchus lacer (morphotype 2: Eastern and Western Pacific). Green circles indicate studied specimens and red online sources. A early juvenile, B juvenile, C late juvenile, D early transitional, E transitional, F mature, even

Fig. 5 Ontogeny, color, and variation in Hexabranchus lacer (morphotype 3: west Indian Ocean from Oman and Red Sea to the Maldives and south to Madagascar and South Africa). Green circles indicate studied specimens and red online sources. A Very early juvenile, B early juvenile, C late juvenile, D early transitional, E transitional, F transitional, unrolled mantle, G mature, H mature, dark I mature,

Fig. 8 Ontogeny, color, and variation in Hexabranchus sanguineus (lineage 1). Green circles indicate studied specimens and red online sources. A early transitional, pink, B mature, C unrolled mantle, red

Fig. 9 Ontogeny, color, and variation in Hexabranchus sanguineus (lineage 2). Green circles indicate studied specimens and red online sources. A early juvenile B transitional C mature, D unrolled man-

Fig. 10 Ontogeny, color, and variation in Hexabranchus sanguineus (lineage 3). Green circles indicate studied specimens and red online sources. A pink, transitional, B orange, transitional, C light, mature D

Fig. 11 Ontogeny, color, and variation in Hexabranchus sanguineus (putative lineage 4). Red circles indicate online sources. A juvenile, B late transitional, C mature, light orange background, lightly flecked, D mature, red background, moderately flecked, E swimming, white

Fig. 12 Hexabranchus san- guineus (CASIZ194618) SEM photograph of the radula. A General view, B top portion of the radula, C outer teeth, D rodlets of the jaws

Fig. 13 Hexabranchus san- guineus reproductive system (MNCN:ADN 110,937). A photograph, B reconstructed drawing. Abbreviations: amp, ampulla; bc, bursa copulatrix; dd, deferent duct; fmg, female gland; hd, hermaphroditic duct; p, penial bulb; pr, prostate; rs, receptaculum seminis; ud, uterine duct; v, vagina