Hexabranchus pulchellus, Pease, 1860

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 : 716-718

Hexabranchus pulchellus (Pease, 1860: v.33, pl. 28). Type locality: Sandwich Islands , Hawaii. (new synonym)

Hexabranchus tinkeri (Ostergaard, 1955: v9 (2), pgs. 128– 130, pl. 2, text figs. 14a–e). Type locality: Waikki, Oahu, Hawaiian Islands. (new synonym)

Distribution Restricted to the Hawaiian Islands and Johnston Atoll ( Bertsch & Johnson, 1982; Debelius & Kuiter, 2007; Kay & Young, 1969).

Material examined Five specimens. CASIZ167983 , length 28 mm (preserved), Maui , HI, USA (20° 59′ N, 156° 40′ W), 9 m depth, 12 Sep. 2003 . CASIZ116917 , Oahu Island , HI, USA (21° 17′ N, 157° 57′ W), 10 m, at night, 10 June 1985 . CASIZ166770 , length 3 mm (preserved), Maui , HI, USA (20° 59′ N, 156° 40′ W), 1–4 m depth, 26 Apr. 2003 . UF372683 (dissected and sequenced), length 15 mm (preserved), Napoli Bay , Maui, HI, USA (20° 59′ 39.8″ N, 156° 40′ 05.1″ W), intertidal pool, 12 December 2004 . UF508353 (dissected and sequenced), length 14 mm (preserved), Honolulu , HI, USA (21° 28′ 48.0″ N, 157° 47′ 09.6″ W), 11–16 m depth, 26 May 2017 .

External morphology( Fig. 14 ) Commonly up to 300 mm. 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. The 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. aureomarginatus . 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 forming a circle around the anus. The anus is elevated on a tubular papilla. The kidney pore is on the right side of the anus. The oral tentacles are large, fleshy, oval, elongate, and crenate. The foot is narrower than the body.

Ontogeny, color, and variation ( Fig. 14 ) Juveniles have purple spots in the center of the notum and white rhinophore bases but lack a submarginal white line. Transitional animals lack a yellow band on the front of the head. The purple spots become reddish-purple and increase in number, with growth, while the complete background turns bright yellow. The notum then darkens to red with cream patches. A sharply defined white band develops on the rhinophore collar (in contrast to the diffuse band in H. lacer ). As in H. lacer , “dark” and “cloudy” traits may develop in some animals after sexual maturity. The “dark” trait is fairly common and sometimes even obscures the white band on the rhinophore collar. The “cloudy” trait is rare in large animals. White animals are rare. Mature animals consistently have a red marginal band that is broad dorsally and narrow ventrally. Its border is diffuse on the dorsal side and well-defined on the ventral side. The mantle may have a translucent edge (without white pigment), when spread, and there may be weak lateral striations. The rhinophore lamellae are not edged in white. There is a red line on the outer face of the rachis.

As in H. lacer , during ontogeny, the mantle expands laterally and becomes rolled, the number of rhinophore lamellae H mature, cloudy, I mature, white, J mature, unrolled mantle, K unrolled mantle, thinned margin, L underside, light, M underside, dark, N genital papillae, O rhinophores, mature, P rhinophores, young, Q gills, top, R gills, side, S oral tentacles, T egg mass increases, the gills become more elaborate, and the notum of resting animals assumes the mature texture.

Internal morphology Buccal mass ( Fig. 15 ). The buccal bulb is of similar size to the oral tube. The radula is broad and bi-lobed ( Fig. 15A ) with the center of the ribbon devoid of teeth ( Fig. 15B ). The teeth are simple and hamate. The lateral teeth increase in length toward the center of the row. The outermost teeth are distinguishably smaller ( Fig. 15C ). The inner 6–12 teeth tend to lie laterally. The 1–3 innermost teeth are smaller, degenerate, or vestigial ( Fig. 15C ). The radular formula is 28 × 38.0.38 ( UF 508353 and UF 372683). The jaws are armed with numerous simple, finger-like rodlets ( Fig. 15D ).

Reproductive system ( Fig. 16 ) Unfortunately, all examined specimens were immature. Nevertheless, the larger specimen (28 mm) showed signs of development with the prostate connected to a wide, long, coiled penis surrounding a thin, long, coiled deferent duct ( Fig. 16A ). Kay and Young (1969) provided a description of a mature reproductive system for this species (as H. marginatus ) ( Fig. 16B ).

Natural history and behavior It can be found in shallow water and tidal pools and is most common at moderately protected sites. It is primarily active at night.

Remarks Up to now, H. sandwichensis has been considered a synonym of H. sanguineus , but nudibranch enthusiasts who frequently dive in Hawaii often disagreed with this and tentatively applied the name H. pulchellus to represent the species (e.g., Sea Slug of Hawai’i and MarinelifePhotography.com). The description of H. pulchellus is unmistakably of a juvenile H. sandwichensis . However, H. sandwichensis was described before H. pulchellus and therefore has priority over it. This species is closely related to the widely distributed Indo-Pacific species H. lacer , a fact noted by Eydoux and Souleyet in (1852) (as H. marginatus ) and confirmed by our phylogenetic analysis. Their COI genetic divergence is about 7% ( Table 3). They share similarly pustulate notum, a predominantly mottled appearance and similar reproductive system. Nevertheless, in comparing this species to H. lacer , H. sandwichensis has a red mantle border without striations and the vagina is considerably thinner. Relative to the sympatric H. aureomarginatus , the egg mass is usually higher, more tightly coiled, and darker in color. The gills are typically held in a more recumbent posture than in that species. The notum appears broadly and irregularly pustulate in large, resting animals but the pustules largely “disappear” when swimming. The genital papillae appear to have tapered margins in copulating pairs.

Figures

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. 16 Hexabranchus sand- wichensis reproductive system. A Reproductive system after Kay and Young (1969), B reconstructed drawing of imma- ture specimen (CASIZ167983). Abbreviations: amp, ampulla; bc, bursa copulatrix; dd, defer- ent duct; fmg, female gland; hd, hermaphroditic duct; p, penial bulb; pr, prostate; rs, receptacu- lum seminis; ud, uterine duct; v, vagina