Author: Rudman, 1982

Order: Nudibranchia  Family: Chromodorididae

Maximum Size: 50 mm

Sightings: Sunshine Coast

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Chromodoris willani Rudman, 1982

This is an intriguing species often mistaken for Chromodoris lochi, unless the white speckling on rhinophores and gills are noticed and the knowledge of what that feature signifies. Both were described in the same paper. I personally find that the contrast of the white speckling, accentuated by reflection of the underwater flash, seems to have the effect of lifting the animal off the substrate.

Chromodoris willani is elongate oval in shape with the tail of the foot protruding posteriorly from beneath the mantle, otherwise the mantle completely overhangs the body and foot. The overhang and thinness of the mantle can cause the edges to undulate, especially in the mid-region when the animal shortens its length for any reason. The mantle colour is pale blue with a matt-like appearance, and carrying a narrow band of white to the margin. The white band is not sharply defined and diffuses into the blue. The blue can turn almost greyish in colour laterally and this may be due to the presence of the scattered white dermal glands visible on the surface in that region.  A narrow black band runs entirely around the mantle enclosing the rhinophores and gills. In some specimens this line may be incomplete anteriorly. Another black line is located on the midline from anterior to the rhinophores back to the gill pocket. This line may be broken and in some specimens there are additional black dashes (infrequently spots), to incomplete lines, found mostly within the encircled region but sometimes outside it submarginally. The black lines often exhibit some diffuse darker blue pigment along their edges. The sides of the body are coloured similar to the mantle, blue with a white margin and bearing black lines, up to three, one of them often in the hyponotal groove. Some run sufficiently posterior to be visible on the tail.

The rhinophores and the gills are most usually a translucent brown or straw colour but are densely covered in small opaque white specks, this being the species’ most distinctive feature. The rhinophores are lamellate and the gills are arranged in a circle around the anus but open posteriorly.

Chromodoris willani is most similar to Chromodoris lochi Rudman, 1982 (sometimes even found feeding together) and Chromodoris dianae Gosliner & Behrens, 1998 however the white speckling on the rhinophores and gills serve to easily distinguish it from those two species. (See Species Profile of Chromodoris lochi for this discussion.)

Like all the Chromodorididae family members (indeed like all the cryptobranchs) Chromodoris willani is a spongivore. Most reports have it feeding upon Thorectidae sponges including Petrosaspongia mycofijiensis and Semitaspongia sp.

The spawn is laid as a flat spiral of up to four whorls. It is translucent white in colour without extra-capsular yolk. The larvae is thought to have planktonic development where the larvae spend significant time living and feeding in the plankton before settling and metamorphosing into the adult. The laying of a flat egg spiral is a feature of all the true Chromodoris species as defined by the molecular sequencing of the Chromodorididae family by Johnson & Gosliner, 2012.

Distribution is widespread in the Indo-Pacific.

Regarding the name, Rudman the author states: “This species is named after Dr R. C. Willan, University of Queensland, who collected the material described here, and who has provided other valuable specimens of chromodorids.”

David A. Mullins – June 2021

References:

– Rudman, W. B. (1982). The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Chromodoris quadricolor, C. lineolata and Hypselodoris nigrolineata colour groups. Zoological Journal of the Linnean Society 76: 183-241.

– Rudman, W. B., (1998, November 29). Chromodoris willani Rudman, 1982. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/chrowill

– Rudman, W. B., (1999, March 4). Chromodoris lochi Rudman, 1982. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/chroloch and associated messages.

– Wilson, N. G. (2002) Egg Masses of Chromodorid Nudibranchs (Mollusca: Gastropoda: Opisthobranchia). Malacologia, 2002, 44(2): 289-305.

– Rudman, W. B., Bergquist, P. R. (2007). A review of feeding specificity in the sponge-feeding Chromodorididae (Nudibranchia: Mollusca). Molluscan Research, 27(2): 60-88.

– Johnson, R. F. & Gosliner, T. M. (2012). Traditional taxonomic groupings mask evolutionary history: A molecular phylogeny and new classification of the chromodorid nudibranchs. PLoS One 7 (4): e33479.

– Gosliner, T. M., Valde ́s, A ́. & Behrens, D. W. (2018). Nudibranch & Sea Slug Identification – Indo-Pacific, 2nd Edition. New World Publications, Jacksonville, Florida, USA.