Author: Rudman, 1984

Order: Nudibranchia  Family: Chromodorididae

Maximum Size: 110 mm

Sightings: Sunshine Coast, Capricorn/Bunker Group GBR

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Ardeadoris egretta Rudman, 1984

Ardeadoris egretta is the type species of the genus that was raised by Rudman in 1984. Only one other species: Ardeadoris scottjohnsoni Bertsch & Gosliner, 1989, was placed in this genus until the molecular sequencing of Chromodorididae by Johnson R. & Gosliner, in 2012. That paper transferred eleven other species into Ardeadoris, ten from Glossodoris and one from Noumea (now known as Verconia). In raising the monotypic genus (at the time) Rudman stated: “This is another monotypic genus but Ardeadoris egretta has a number of distinctive features which separate it from all the larger genera. The wide mantle overlap is most distinctive.” An extremely long oral tube plus differences in reproductive anatomy were also cited.

Ardeadoris egretta is a large dorid nudibranch up to 110 mm in length. The mantle is smooth with an extremely wide overlap. The mantle margin presents with a series of undulations but the edge remains relatively thin compared to some of its relatives such as Ardeadoris averni and Ardeadoris rubroannulata where it appears quite “puffy”. The tail extends past the posterior edge of the mantle. The rhinophores can retract into protective pockets. The gills too can retract into a protective pocket. The gills have a distinctive appearance. Each gill is quite thin and elongate, tapering to a point. The gill is arranged in an arc around the anus, open posteriorly, and with each end of the arc forming an inward spiral. The gills moves continuously with a regular rhythmical motion when expanded.

The colouration is stark, being wholly white apart from the mantle edge that carries a golden-yellow (sometimes orange) band dorsally and ventrally. There is an opaque white band adjacent to the yellow margin that is of a brighter white than the rest of the mantle. Rudman states that this band carries “small ramifying mantle glands”. The yellow margin is not repeated on the foot or tail that instead, carry a narrow band of opaque white pigment. The rhinophores are translucent white with an opaque white line running up both the anterior and posterior faces. The gills too, are translucent white with opaque white on the outer faces.

Ardeadoris egretta is a spongivore but the type of sponge preyed upon has not been identified/recorded.

This is not a common species but distribution is widespread in the western Pacific, from Japan in the north down through the Philippines, Malaysia, Indonesia, PNG, Solomon Islands, the GBR to northern NSW and also tropical Western Australia. Yonow, 2008 lists Ardeadoris egretta from the Red Sea but I am not entirely convinced that those images represent this species.

Originally described as: Ardeadoris egretta.

This is the “Chromodoris marginata (Pease, 1860)” of Willan & Coleman, 1984. Their specimen was labelled prior to Rudman’s authorship and their uncertainty was certainly expressed in the text.

Mike Miller, the Webmaster of the renowned SlugSite, was so taken with the way this species presents itself (“magic regal bearing”) that he adopted it as the website’s logo way back in 1995.

Both the generic and specific epithets are references to the type locality of the genus and also this animal – the type species of the genus.
– Ardeadoris: “The genus name Ardeadoris is from the Latin “ardea”, the heron, a reference to Heron Is, Great Barrier Reef, where the type species is commonly found.”
– egretta: “The species epithet “egretta” is a reference to the similarity in colour of the nudibranch (white and yellow) to the white colour form of the Reef Heron Egrettu sacra, so common at Heron Is, Capricorn Group, Great Barrier Reef, where the species is often seen.”
(Whimsical Note: Technically speaking the birds on Heron Island after which the island was named were originally misidentified as herons and are actually egrets – Eastern Reef Egrets. Having said that the classification between the two is not clear and different opinions abound as to which species should be assigned to the two genera – Ardea and Egretta.)

Above: Ardeadoris egretta with gills and rhinophores moving with the surge.

David A. Mullins – February 2022

References:
– Willan, R. C. & Coleman, N., (1984). Nudibranchs of Australia, Neville Coleman, AMPI: 28-29

– Rudman, W. B., (1984) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: a review of the genera. Zoological Journal of the Linnean Society 81: 115-273.

– Rudman, W. B., (1990). The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: further species of Glossodoris, Thorunna and the Chromodoris aureomarginata colour group. Zoological Journal of the Linnean Society, 100(3): 263-326.

– Rudman, W. B., (January 16, 1999). Ardeadoris egretta Rudman, 1984. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/ardeegre and associated messages.

– Marshall, J. G. & Willan, R. C., (1999). Nudibranchs of Heron Island, Great Barrier Reef: A survey of the Opisthobranchia (sea slugs) of Heron and Wistari Reefs; Backhuys: Leiden, The Netherlands.

– Miller, M., image by Miller, M. (2002). Opisthobranch of the Week, Week: 306. Mike Miller’s Slug Site. Available at: http://slugsite.us/bow/nudwk306.htm

– Debelius, H. & Kuiter, R. H., (2007). Nudibranchs of the World. IKAN-Unterwasserarchiv, Frankfurt.

– Yonow, N. (2008). Sea Slugs of the Red Sea. Pensoft Publishers.

– Coleman, N., (2008). Nudibranchs Encyclopedia. Neville Coleman’s Underwater Geographic Pty Ltd, Springwood, Qld.

– 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.