Hexabranchus aureomarginatus Ostergaard, 1955

Description

A large dorid reaching well over 20 cm in length. The body is much depressed and oval in outline when the cloak is fully extended; the cloak is wide and very thin at the edges, where it serves as a swimming membrane. The median dorsal area corresponding to the body region is bright red, with groups of fine white flecks, some concentrated into large irregular spots. Lateral to this area is a pale rose zone, interrupted anteriorly but continuous posteriorly across the midline. The marginal region consists of three uninterrupted colour zones — innermost bright red; middle white with radially disposed muscle bands; outermost golden yellow, the diagnostic golden margin from which the species takes its name. The rhinophores are long and stout, with about 40 golden laminae and reddish-orange basal stems, issuing from elevated collars variegated with bright red and white and bordered by a narrow golden band. Six golden retractile branchial plumes with violet stems surround the low anal papilla. Distinguished from the sympatric Hexabranchus tinkeri by the golden marginal band and the morphology of the oral tentacles.

Distribution

The type locality is Waikiki, Oahu, in the Hawaiian Islands, where individuals can be seen swimming or resting on coral within wading distance of shore.

Etymology

The specific epithet aureomarginatus is a Latin compound of aureus ("golden") and marginatus ("bordered, edged"), in reference to the diagnostic outermost golden marginal band of the cloak.

Remarks

The sympatric Hexabranchus sandwichensis, also reported from Hawaii, is distinguished from this species by coloration and gill branching mode. Swimming is achieved by vigorous flexing of the body and undulation of the cloak margins; at rest the cloak margins are rolled up against the sides of the body.

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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