Hexabranchus sandwichensis (Gray, 1850)

Description

A large dorid nudibranch, commonly reaching about 30 cm and among the largest of the genus. The body is strongly flattened, with a broad mantle whose thin margins are undulated in swimming. In resting mature animals the notum is broadly and irregularly pustulate, though the pustules become inconspicuous when the animal swims. Mature animals are dark red to reddish-brown with cream and white mottling, and the mantle is edged with a red band that is broad dorsally and narrow ventrally — diffuse on its dorsal border but sharply defined ventrally. A sharply defined white band develops on the rhinophore collar. Numerous multi-pinnate gills encircle the anus. Juveniles have purple spots in the centre of the notum; with growth these become reddish-purple and multiply while the ground colour passes through bright yellow and darkens to red.

Distribution

Restricted to the Hawaiian Islands and Johnston Atoll; type locality the Hawaiian Islands. Most common at moderately sheltered sites and primarily active at night.

Etymology

The specific epithet sandwichensis refers to the Sandwich Islands, the former name of the Hawaiian Islands, meaning "from the Sandwich Islands".

Identification

It is readily confused with the sympatric Hexabranchus aureomarginatus. Three characters separate them: (1) mantle-margin colour — red in this species, golden-yellow in H. aureomarginatus; (2) notum texture — broadly pustulate in resting mature animals here, but always smooth in H. aureomarginatus; (3) rhinophore collar — this species develops a sharply defined white band, lacking in H. aureomarginatus. In addition, H. sandwichensis tends to hold its gills more recumbent, and its egg mass is higher, more tightly coiled and darker.

Remarks

Long treated as a synonym of the Spanish dancer Hexabranchus sanguineus, this species is now recognised as a valid species endemic to Hawaii. Hexabranchus pulchellus and Hexabranchus tinkeri, once regarded as distinct, are synonyms. It is known in English as the "Red-margined Spanish Dancer".

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References

• Hexabranchus tinkeri n.sp., Ostergaard J.M. (1955). Some opisthobranchiate Mollusca from Hawaii. Pacific Science, 9(2): 110-136.
• Hexabranchus sandwichensis (Gray, 1850), Tibiriçá Y., Pola M., Pittman C., Gosliner T.M., Malaquias M.A. & Cervera J.L. (2023). A Spanish dancer? No! A troupe of dancers: a review of the family Hexabranchidae Bergh, 1891 (Gastropoda, Heterobranchia, Nudibranchia). Organisms Diversity & Evolution. 23(4): 697-742. https://doi.org/10.1007/s13127-023-00611-0