Hexabranchus orbicularis Abraham (1877

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 : 705-711

Hexabranchus orbicularis Abraham (1877 : pgs. 261–262, pl. 30, figs. 23–24). Type locality: Mauritius. (new synonym)

Hexabranchus anaiteus Bergh (1878 : v4, p.73). Type locality: New Hebrides Islands, Vanuatu. (synonymized by Bergh, 1900: pg. 225)

Hexabranchus faustus Bergh (1878 : v.2 (13): 550–555, pl. 41, Fig. 3 : pls. 61, Figs. 14 , 15 : pls. 62, figs. 25–28; pl. 63, figs. 1–9; pl. 67, figs. 3–6). Type locality: Aibukit, Palau Islands. (new synonym)

Hexabranchus punctatus Bergh (1905 : v.50, p. 92, pl. 12, Fig. 27 ). Type locality: Pulu-Kebala-dua, Borneo Bank (Sta. 79) W. of Celebes, Indonesia. (new synonym)

Distribution Broadly distributed across the Indo-Pacific from the western Indian Ocean to the central Pacific and French Polynesia: Oman (Debelius & Kuiter,), Tanzania (Edmunds,), Mozambique (Tibiriçá et al., 2017), Madagascar (present study), South Africa (Gosliner, 1987), India ( Apte & Salahuddin, 2010), Thailand ( Chavanich et al., 2010), Japan ( Atsushi, 1999; Baba, 1936; Nakano, 2004), Philippines ( Colin & Arneson, 1995), Indonesia ( Debelius, 1996; Gosliner et al., 2008), Vietnam ( Debelius & Kuiter, 2007), Papua New Guinea ( Coleman, 2008), Australia (Nimbs & Smith, 2016; Thompson, 1972) including Lord Howe Island ( Coleman, 1989, 2001, 2008), New Caledonia (Hervé, 2010), Mariana Islands, Guam (present study), French Polynesia (Salvat & Bacchet, 2011), Tonga (present study). On-line sources add: Kenya, Comores, Mayotte, Emirate Arab, Maldives, Okinawa ( Japan), Malaysia, Taiwan, East Timor, Marshall Islands, Vanuatu (iNaturalist), Reunion, Seychelles, Rodriguez, Mauritius (South-west Indian Ocean Seaslug site, 2011).

flecks, G mature, patchy flecks, H mature, dark, I mature, cloudy J detail on spots on mantle underside K mature, unrolled, white margin L underside M genital papillae N rhinophores O gills P oral tentacles

Material examined Thirty-three specimens. CASIZ217191 , length 35 mm (preserved), Philppines, Visayas, Siquijor Island , Paliton Wall (9° 10′ 12″ N, 123° 27′ 36″ E), 0–2 m depth, 4 Apr. 2016. CASIZ193381 , length 11 mm (preserved), Papua New Guinea. CASIZ191209 , length ≈ 20 mm, Papua New Guinea, Madang Province, 14 Nov. 2012. CASIZ191385 , length ≈ 25 mm, Papua New Guinea, Madang Province, Sek Island , 22 Nov. 2012. CASIZ191017 , length ≈ 30 mm, Papua New Guinea, Madang Province, Tab Island (5° 10′ 6″ S, 145° 50′ 31″ E), 7 Nov. 2012. ZMBN131962 , length 15 mm, Japan, Hachijō-jima (33° 08′ 43″ N, 139° 44′ 18″ E), 5–19 m depth, 03 Oct. 2019. ZMBN131092 , length 10 mm, Japan, Hachijō-jima (33°08′ 43″ N, 139° 44′ 18″ E), 5–19 m depth, 04 Oct. 2019. ZMBN131966 , length 18 mm, Japan, Hachijō-jima (33° 08′ 43″ N, 139° 44′ 18″ E), 5–19 m depth, 04 Oct. 2019. ZMBN131997 , length 105 mm, Japan, Hachijō-jima (33° 08′ 43.8″ N, 139° 44′ 18.6″ E), 7–22 m, 04 Oct. 2019. MB28- 004474 , length 67 mm, Mozambique, Zavora , rock pool (24° 31′ 09″ S, 35° 12′ 25″ E), 2 m depth, 7 Feb. 2012. UL-YT1639, length 80 mm, Mozambique, Ponta do Ouro , Atlantis (26° 50′ 58″ S, 32° 44′ 54″ E), 40 m depth, 16 April 2014. MHNM-0177 ( MNCN: ADN 110935 , tissue), length 220 mm, Mozambique, Nanatha Bay , Nuarro-Enupa (4° 12′ 03″ S, 40° 40′ 30″ E), 5 m depth, 31 Aug. 2017. NMVF253027 , 24 mm length (preserved), Australia, Sunshine Coast , Caloundra , Raper Schoal (26° 23′ 26″ S, 153° 07′ 51″ E), 16 m, 16 Nov. 2018. NMVF253028 32 mm length cloudy pink J mature, cloudy dark K mature, cloudy red, L mature, cloudy white, unrolled mantle interrupted red band, M mature, light dorsum, unrolled mantle, red margin, N intermediary, unrolled mantle, yellowish margin, O underside dark, P underside light, Q rhinophores, R gills, S oral tentacles, T eggs

(preserved), Australia, Sunshine Coast, Moloolaba Gneering Schoals (26° 23′ 10″ S, 155° 18′ 43″ E), 18 m depth, 16 Nov. 2018. CASIZ194337 , length ≈ 30 mm, Madagascar, south Madagascar, ponte Evatra (24° 58.1′ S, 47° 6.1′ E), 3–8 m depth, 30 Apr. 2010 . CASIZ68295 , 38 mm (preserved), Ryukyu Island (24° 19′ N, 124° 10′ E), unknown depth, Mar. 1969 . CASIZ191404 , length ≈ 15 mm, Papua New Guinea, Madang Province (4° 35′ S, 145° 49′ E), 14 m depth, 23 Nov. 2012 . CASIZ194621 , length 60 mm (preserved), Madagascar, South Madagascar, Sud Ponte (24° 60′ S, 47° 6′ E), 18 m depth, 10 May 2010 . CASIZ202307 , length 31 mm (preserved), Philippines, Luzon , Batangas (13° 42′ 36″ N, 120° 52′ 12″ E), 0–2 m depth, 8 May 2014 . MB28-005009 , length 14 mm, Mozambique, Zavora, Area 51 (24° 26′ 28″ S, 35° 16′ 15″ E), 11 m depth, 12 June 2015 . ZMBN131901 , length 20 mm, Japan, Hachijō-jima (33° 08′ 43″ N, 139° 44′ 18″ E), 5–19 m depth, 03 Oct. 2019 . ZMBN92419 , length Mozambique, Nanatha Bay, Nuarro-Enupa (4° 12′ 03″ S, 40° 40′ 30″ E), 5 m depth, Mozambique . ZMBN131926 , length 30 mm, Japan, Hachijō-jima (33° 08′ 43.8″ N, 139° 44′ 18.6″ E), 5–12 m, 03 Oct. 2019 . ZMBN131927 length 75 mm, Japan, Hachijō-jima (33° 08′ 43.8″ N, 139° 44′ 18.6″ E), 5–12 m, 03 Oct. 2019 . ZMBN131928 , length 12 mm, Japan, Hachijō-jima (33° 08′ 43.8″ N, 139° 44′ 18.6″ E), 5–12 m, 03 Oct. 2019 . ZMBN131967 , length 13 mm, Japan, Hachijō-jima (33° 08′ 43.8″ N, 139° 44′ 18.6″ E), 5–19 m, 03 Oct. 2019 . ZMBN131995 , 4 spcs., length 6–17 mm, Japan, Hachijō-jima (33° 08′ 43.8″ N, 139° 44′ 18.6″ ″E), 7–22 m, 04 Oct. 2019 . ZMBN131996 , length 30 mm, Japan, Hachijō-jima (33° 08′ 43.8″ N, 139° 44′ 18.6″ E), 7–42 m, 04 Oct. 2019 . ZMBN 131997, 105 mm, Japan, Hachijō-jima (33° 08′ 43.8″ N, 139° 44′ 18.6″ E), 7–42 m, 04 Oct. 2019 . CASIZ072156 , 3 specimens., up to length 62 mm (preserved), Tonga, Nuku Island (18° 26′ 06″ S, 174° 01′ 12″ E), 1–5 m depth, 24 Jul. 1985 . Other material : MNCN: ADN 110936 (tissue), length 400 mm, Mozambique, Ponta do Ouro (26° 50′ 6″ S, 32° 44′ 5″ E), 1 m depth, 16 April 2022 .

External morphology ( Figs. 3 , 4 and 5 ) Commonly up to 220 mm (with unconfirmed reports of 500 + mm in the Marshall Islands). The notum in resting, mature animals is broadly and irregularly pustulate. The body is pyriform, when the mantle is rolled, and oval when it is extended. Mantle extension becomes gradually wider toward the back but is short and differentiated on the head. The rhinophore sheath is short with a smooth edge. The peduncle is stocky, and the club is broader than in H. sanguineus . There are about 40–50 lamellae on the rhinophore clubs of large, mature animals. The gill branches are complex and multi-pinnate with a variable number of gill tufts (often more than seven) forming a circle around the anus. The anus is elevated on a tubular papilla. The kidney pore is on its right side. The oral tentacles are large, fleshy, oval, elongate, and crenate. The foot is narrower than the body.

Ontogeny, color, and variation ( Figs. 3 , 4 and 5 ) Hexabranchus lacer is abundant, widely distributed, and highly variable. The details of the dorsal banding vary with white marginal bands, red marginal bands, lateral striations, interruptions of the red band, and violet tinting “mixing and matching.” The innermost dorsal band of mature animals is sharply margined, medially, and strongly scalloped. Diffuse white pigment is often present on and around the rhinophore collars (most noticeable in larger animals) and the rhinophore lamellae are usually edged in white. There is a red line on the outer face of the rachis. The overall color pattern of mature animals is “mottled.”

There are two late-appearing traits that emerge after sexual maturity in some animals: “dark” and “cloudy.” In “dark,” the whole animal darkens, sometimes with complete replacement of the initial pattern. When present, the dark pigment is opaque (in contrast to translucent in dark-colored H. sanguineus ). In “cloudy,” the dermis becomes partially opaque, obscuring the underlying pattern. This development is usually patchy allowing the underlying pigment to show through, irregularly, and sometimes creating the illusion of spots that are not inherent to the original pattern. The frequency of both morphotypes seems to vary between populations. Rarely, animals have extensive white pigment. The color of the foot sole is similar to the background color on the sides of the animal but with a pale margin. However, this pattern may be reversed or overridden in some very dark animals.

The maximum COI intra-specific genetic variation between all specimens was 1.98% ( Table 3); no genetic structure or internal differences could be found between morphotypes. Despite that, extensive review of thousands of photographs suggests the presence of three distinct morphotypes that differ in their ontogeny.

Morphotype 1 (m1) ( Fig. 3 ): ( French Polynesia, central Pacific & western Pacific) Juveniles lack purple spots in the center of the notum, purple marginal spots are present and typically larger than in morphotype 2 (m2), lack a white submarginal line, largely lack a translucent yellow marginal band on the front of the head, and appear to lose the white rhinophore bases earlier than in m2. Transitional animals develop a dense covering of white spots on all surfaces. In mature animals, the spots become larger and cluster to form closely spaced rosettes (either uniformly or patchily distributed). A diffuse white ring around the rhinophore collar is usually present, particularly in large animals, highlighting a narrow orange line on the margin of the collar. In “dark” animals, dark pigment fills in the space between the dorsal bands and the central notum but does not fully obscure the rosettes on the notum (or elsewhere). The “cloudy” trait is relatively rare. In mature animals, the mantle margin in all examined photos was white-banded and “tinted,” usually with lateral striations.

Morphotype 2 (m2) ( Fig. 4 ): (from western Pacific to Marshall Islands) Juveniles have purple spots in the center of the notum, a submarginal white line, and white rhinophore base. Late juveniles and transitional animals have a translucent-gold band on the front of the head. In transitional animals, fine white flecks develop on the notum while the underside of the mantle becomes covered with small, diffuse red spots. Those two features appear to be strongly correlated but both may be either uniformly or patchily distributed. Some large animals develop the “dark” trait. First, the space between the lateral bands and the central notum apparently fills in with dark pigment. Then, the central notum fills in with moderately dark to dark pigment, obscuring the underlying pattern. Finally, the underside of the mantle fills in with dark pigment obscuring the red spots. The diffuse white rings around the rhinophore collars often remain visible in older animals but the dark pigment may completely replace other white features or reduce them to a few scattered, minute flecks. Moderate numbers of old animals develop the “cloudy” trait. White animals are rare. In mature animals, the margin may be either white-banded or red-banded, but “tinting” is probably somewhat less common than in the other morphotypes. The red band may be interrupted, and lateral striations may be present.

Morphotype 3 (m3) ( Fig. 5 ) (west Indian Ocean from Oman and Red Sea to the Maldives and south to Madagascar and South Africa) Juveniles are nearly identical to juveniles of m2. They have purple spots in the center of the notum, a submarginal white line, and a white rhinophore base. Late juveniles and transitional animals have a translucent-gold band on the front of the head. In transitional animals, fine white flecks may develop on the notum while the underside of the mantle may become covered with small, diffuse red spots. Mature animals appear to develop more prominent yellow-white patches than in m2, both on the notum and on the underside of the mantle. The patches on the notum appear to form through “coalescence” of the white flecks while the patches on the mantle seem intrinsic to the underlying color. In “dark” animals, dark pigment replaces most of the patches although a few scattered remnants are sometimes retained, and a few even darker patches may develop on the notum. Many larger animals develop the “cloudy” trait. In some animals, red spots on the underside of the mantle may appear to be generated by the uneven distribution of cloudy pigment rather than being intrinsic to the underlying pattern. White animals are rare. In mature animals, the margin may be either red-banded or white-banded. “Tinting” lateral striations and interruptions of the red band are common.

As in other Hexabranchus spp. , during ontogeny, the mantle expands laterally and becomes rolled, the number of rhinophore lamellae increases, the gills become more elaborate, and the notum of resting animals assumes the mature texture. The mantle margin in large animals may become somewhat frillier than in other species (highly dependent on posture).

Internal morphology Buccal mass ( Fig. 6 ). The buccal bulb is oval and slightly larger than the oral tube. The radula is broad and bi-lobed with the center of the ribbon devoid of teeth ( Fig. 6A ). The teeth are simple and hamate. The lateral teeth increase in length toward the center of the row. The outermost teeth are smaller or degenerate ( Fig. 6B ). The inner 4–9 teeth tend to lay laterally. The innermost teeth are smaller, degenerate, or vestigial ( Fig. 6C ). The radular formulae are 32 × 61.0.61 ( CASIZ 193,381), 31 × 47.0.47( CASIZ 202,307), 35 × 55.0.55 ( CASIZ 217,191), 33 × 47.0.47 ( CASIZ 194,621), 30 × 41.0.41 ( UF 253,028), 30 × 36.0.36 ( UF 253,027), and 51 × 69.0.69 (MHNM-0177). The jaws are armed with numerous simple, finger-like rodlets ( Fig. 6D ).

A fresh photograph of the internal morphology from a dissection of this species is illustrated here for the first time ( Fig. 7A ). The large blood gland is dark-brown covering part of the nerve ring. The intestine and stomach are bluish and of similar diameter. The digestive gland is cone-shaped and orange bearing a distinct pink duct.

Reproductive system ( Fig. 7B–D ) The reproductive system is triaulic with the hermaphroditic duct leading to a very long, convoluted, cream-colored ampulla. The ampulla divides into a short oviduct leading into the female gland mass and deferent duct through the prostate portion. The prostate gland is granular, kidney-shaped, orange in color, stocky at its base, and narrowing toward the distal deferent duct. The distal deferent duct is thin and long. The long, thick penis is coiled around the deferent duct. The deferent duct opens into a common atrium with the vagina. The vaginal duct is curved, thick, and enclosed by connective tissue. The vaginal duct bifurcates into ducts leading to the ventral side of the bursa copulatrix and the receptaculum seminis. The receptaculum seminis is short, convoluted, and creamyorange. The bursa copulatrix is black, large, and oval. The short uterine duct emerges between the receptaculum seminis and the bursa copulatrix, entering the female gland. The female gland is oval, creamy in color, and of similar size to the bursa copulatrix.

Natural history and behavior Francis (1980) found that, in Tonga, sightings of this species were affected by tidal period and their habitat preference depended on their stage of development. He also pointed out that “the animals actively avoided live coral” (page 254). Apte and Salahuddin (2010) found that 200 mm specimens were active at night at low tide. In Mozambique, juveniles and intermediary specimens are commonly seen in tidal pools and shallow water. Larger specimens are usually found below 5 m. Particularly in the north of Mozambique where the predominant habitat is coral reefs, which offers more hidden spaces than in the southern rocky reefs, this species is only found at night .

Edmunds (1968) suggested a difference in swimming behavior between H. lacer (as H. marginatus ) and H. sanguineus but, as pointed out by Thompson (1972), this difference is barely discernable. Additional information on the reproduction and feeding behavior of this species is provided by Francis (1980).

Remarks as predicted by Edmunds (1968), our results show that H. lacer and H. marginatus are the same species, which is distinct from H. sanguineus .

No genetic, behavioral, or internal morphological differences could be found to differentiate the three studied morphotypes of H. lacer , suggesting that they are likely variations of the same highly polychromatic species. Nevertheless, the existence of distinct morphotypes may provide a starting point for further investigation. We have found photos of 27 sets of copulating or closely paired animals (two of m1, 23 of m2, and two of m3). Several of these pairs were light and dark animals but none of them cross the boundaries between the morphotypes. That might suggest that m1 and m2 are reproductively isolated even though they are largely sympatric. In addition, Morton (1964) described the swimming behavior of a specimen of m1 which differed from the swimming behavior described by Edmunds (1968) for a specimen of m3. Edmunds (1968) suspected that such a difference might indicate different species, but this could not be confirmed.

Relative to the sympatric H. sanguineus , the egg mass is usually higher, more tightly coiled, and darker in color. The gills of H. lacer are typically held in a more recumbent posture than in H. sanguineus . In H. lacer , the notum appears broadly and irregularly lightly to strongly pustulate in large, resting animals. Nevertheless, the pustules largely “disappear” during swimming. The genital papillae appear to have tapered margins in copulating pairs. Juveniles and transitional animals are commonly seen in tide pools and shallow water, while mature animals are found in moderately protected submerged habitats and on deeper reefs.

Figures

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.14 Ontogeny, color, and variation in Hexabranchus sandwichensis (Hawaii). Green circles indicate studied specimens and red online sources. A juvenile, B early transitional, C mid-transitional, D late transitional, E mature, F mature, dark, G mature, very dark,

Fig. 15 Hexabranchus sand- wichensis (UF508353). SEM photograph of the radula. A General view, B central region and inner teeth, C outer teeth, D rodlets of the jaws

Fig.27 Map of species distributions. Full-filled colors represent specimens studied and colors with empty circles represent photograph records found in the literature and/or on-line. Yellow=Hexabranchus aureomarginatus. Pink=Hexabranchus gigan-

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. 6 Hexabranchus lacer (CASIZ193301) SEM photo- graph of the radula. A General view, B outer teeth, C inner teeth, D rodlets of the jaws

Fig. 7 Hexabranchus lacer. A Photography of internal anat- omy of exemplar MHNM-0177 with details on the reproductive system (ventral view above and dorsal view below). B Recon- structed drawing. Abbrevia- tions: amp, ampulla; bc, bursa copulatrix; bg, blood gland; dd, deferent duct; dg, digestive gland; fmg, female gland; h, heart; hd, hermaphroditic duct; it, intestine; p, penial bulb; pr, prostate; reps, reproductive sys- tem; rs, receptaculum seminis; st, stomach, ud, uterine duct; v, vagina