Risbecia imperialis
(Pease, 1860)

Order: NUDIBRANCHIA
Suborder: DORIDINA
Superfamily: EUDORIDOIDEA
Family: Chromodorididae

DISTRIBUTION

Hawaii, and possibly Marshall Ids (see message below.

PHOTO

Honolulu, Hawaii. Photo: Scott Johnson

Risbecia imperialis, first described from Hawaii, and R. godeffroyana, originally described from Tahiti, and more recently reported (Rudman, 1987), from tropical Eastern Australia and Papua New Guinea), are very similarly coloured and probably closely related species. Unfortunately specimens from Hawaii are still unavailable for anatomical comparision.

Risbecia imperialis is creamy white with a dark blue mantle border which has a series of 'fingers' which extend in towards the center of the mantle. The fingers have a row of bright yellow spots. Inside the blue border there is a band of large yellow spots. The rest of the mantle is patterned with slightly smaller yellow spots which are somewhat paler in colour than the outer band. The foot is bordered with dark blue and has a scattering of large yellow spots. The rhinophore club is dark blue with some white spots along the edge of the lamellae and a white line up the anterior and posterior edges of the club. The gills are white with blue edging.

The preferred food of R. imperialis in Hawaii is Dysidea fragilis. It is known only from Hawaii and from a single record from the Marshall Ids. [See Scott Johnson's message below].
The East African R. pulchella is clearly closely related, the main external difference being the purple border to the foot in that species, and the smudged pale purple reticulate pattern on the dorsum.

Reference:
• Rudman, W.B. (1987). The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Chromodoris epicuria, C. aureopurpurea, C. annulata, C. coi and Risbecia tryoni colour groups. Zoological Journal of the Linnean Society, 90: 305-407.

Note added 9 Jan 2001: R. imperialis and R. godeffroyana have previously been considered on the Forum to be colour forms of one species with all records being placed on the R. imperialis page. They are now separated as two species. See accompanying message.

Authorship details
Rudman, W.B., 2001 (January 8) Risbecia imperialis (Pease, 1860). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/risbimpe

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