Hexabranchus aureomarginatus (Ostergaard, 1955)

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 : 719-721

Hexabranchus aureomarginatus (Ostergaard, 1955)

( Figs. 17 , 18 and 19 )

Hexabranchus aureomarginatus (Ostergaard, 1955) (original combination): v.9 (2): 132–133, pl. 2, text figs. 15a-f. Type locality: Waikiki, Oahu, Hawaiian Islands.

Material examined CASIZ182737 , approx. 65 mm, Kauai Island , Kiohuna Beach , HI, USA (22° 04′ 11.6″ N, 159° 18′ 50.5″ W), depth 1–3 m, 05 Oct. 2000 . UF444681 , (tissue), Kauai Island , Kiohuna Beach , HI, USA (22° 04′ 11.6″ N, 159° 18′ 50.5″ W), 13 May 2010 . CASIZ142942 , Maui, Kapalu Bay , HI, USA, 1–5 m, 05 Oct. 2000 . CASIZ74634 (dissected), 70 mm (preserved), Kauai, Hawaii Islands, USA (22° 04′ 11.6″ N, 159° 18′ 50.5″ W), intertidal, 24 Feb. 1986 . CASIZ 074271 (dissected), length 45 mm (preserved), Kauai Island , Kiohuna Beach , HI, USA (22° 04′ 11.6″ N, 159° 18′ 50.5″ W), intertidal, 24 Feb. 1986 .

mature, frosted, H unrolled mantle, yellow margin, I unrolled mantle, white margin, J underside, K rhinophores, L gills, M oral tentacles, N egg mass, O–P mature in laboratory

Distribution Endemic to the Hawaiian Islands ( Bertsch & Johnson, 1982; Kay & Young, 1969).

External morphology ( Fig.17 ) Commonly up to 200 mm. The notum is smooth. The body appears pyriform when the mantle is rolled and elongate-ovate when it is extended. The mantle extension is wider laterally but shorter on the head. The rhinophore sheath is slightly raised with a smooth edge. The rhinophores are slightly bent posteriorly with approximately 40 lamellae in large, mature animals. There are four to six multi-pinnate gill branches (usually five). 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, and oval to elongate and crenate. The foot is narrower than the body.

Ontogeny, color, and variation ( Fig. 17 ) No photos or specimens were available of very young juveniles, but early transitional animals are translucent gray with a yellow marginal band, cloudy sub-dermal rosettes, and the beginning of red lateral patches. With growth, a submarginal red band develops. In mature animals, the background is typically dark red, and the sub-dermal rosettes are variably replaced with irregular patches of opaque white pigment (usually concentrated between the lateral patches). The dorsal margin of the innermost dorsal band is straight but not sharply defined. Rare animals may remain pale or be “frosted” with cream flecks when mature. The marginal yellow band is retained in mature animals, only rarely being replaced with white. The foot sole is lighter than the background color and has a yellow margin. The rhinophore lamellae lack white flecks and mature animals often have lateral striations. There does not appear to be any geographic variation within the archipelago.

As in H. sanguineus , 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 ( Fig. 18 ) The buccal bulb is oval and slightly larger than the oral tube. The radula is broad and bilobed. The middle of the ribbon is devoid of teeth ( Fig. 18A ). The teeth are simple and hamate. The lateral teeth increase in length centrally and the outer teeth are smaller. ( Fig. 18B ). The innermost teeth are smaller or degenerate ( Fig. 18C ). The radular formulae is 32 × 57.0.57 (CASIZ 074,634). The jaws are armed with numerous simple, finger-like rodlets ( Fig. 18D ).

Reproductive system ( Fig. 19 ) Similar to Kay and Young (1969)’s description; however in the specimens examined by us, the female gland opened in a distinct genital atrium.

Natural history and behavior Hexabranchus aureomarginatus can be found in shallow water and tide pools and is most common at more exposed sites. It is a nocturnal feeder but appears to remain in the open by day more frequently than H. sandwichensis .

Remarks The yellow margin is a distinctive character in H. aureomarginatus . Relative to the sympatric H. sandwichensis , the egg mass is lower, more loosely coiled, and usually lighter in color. The gills of H. aureomarginatus are typically held in a more erect position than in H. sandwichensis . The notum appears smooth in both resting and swimming mature animals and the genital papillae appear to have parallel margins in copulating pairs.

Figures

Fig. 17 Ontogeny, color, and variation in Hexabranchus aureomarginatus (Hawaii). Green circles indicate studied specimens and red online sources. A early transitional, B late transitional, C mature, D mature, little white, E mature, extensive white, F mature, pale, G

Fig. 18 Hexabranchus aureo- marginatus (CASIZ074634) SEM photograph of the radula. A general view, B central por- tion of the radula, C Inner teeth, D rodlets of the jaws

Fig. 19 Hexabranchus aureo- marginatus (CASIZ074271) reproductive system. A Photog- raphy, B reconstructed drawing. 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