Hexabranchus aureomarginatus Ostergaard, 1955

Description

A large dorid, commonly reaching about 20 cm. The body is strongly flattened, with a wide, thin-edged mantle that is rippled during vigorous swimming. The notum is smooth and stays so both at rest and while swimming. In mature animals the background is dark red, overlaid with irregular patches of opaque white. The outer margin of the mantle is bordered all around by a golden-yellow band, the most reliable field character. The rhinophores are tilted slightly backwards and bear about 40 lamellae in large animals, without white flecks. There are four to six (usually five) multi-pinnate gills encircling the anus. The oral tentacles are large, fleshy and crenate; the foot is narrower than the body, its sole paler than the dorsum and edged in yellow. Juveniles and young animals are translucent grey with a yellow marginal band, darkening with age.

Distribution

Endemic to the Hawaiian Islands; type locality Waikiki, Oahu. Most common at exposed shallow reefs and tide pools, where it is chiefly active at night.

Etymology

The specific epithet aureomarginatus combines Latin aureus ("golden") and marginatus ("margined"), after the golden band running around the edge of the mantle.

Identification

It is most easily confused with the sympatric Hexabranchus sandwichensis (the red-margined Spanish dancer). Three characters separate them: (1) mantle-margin colour — golden-yellow in this species, red in H. sandwichensis; (2) notum texture — smooth at all times here, but broadly pustulate in resting mature H. sandwichensis; (3) rhinophore collar — H. sandwichensis develops a sharply defined white band that this species lacks. In addition, H. aureomarginatus tends to hold its gills more erect, and its egg mass is lower, more loosely coiled and paler.

Remarks

Hexabranchus tinkeri, once treated as a distinct species, is now regarded as a synonym of H. sandwichensis. Endemic to Hawaii, it is distinct from the wide-ranging Hexabranchus lacer and from the true Spanish dancer Hexabranchus sanguineus.

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References

• Hexabranchus aureomarginatus, Ostergaard J.M. (1955). Some opisthobranchiate Mollusca from Hawaii. Pacific Science, 9(2): 110-136.
• Hexabranchus aureomarginatus, 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