Northern New South Wales and central Victoria, Australia
Size to 17 mm when fully extended. Body extremely flattened, less than 3 mm in height.Mantle large and circular, flattened dorsally except for two humps in the midline, one immediately behind the rhinophores and the other, the pericardium, mid-centrally. The mantle margin is is thin and produced into numerous, short, dorso-ventrally compressed, simple gills all round. The number of gills can vary but adults tend to have around 100. The head is expanded into a trapezoidal oral veil anteriorly. The anterior margin possesses ten to 14 short, simple papillae that are irregularly arranged. The rhinophores are set well apart on the sides of the head. Their sheaths are exceedingly low and almost circular. The clavus is elaborate, consisting of a tal, flat-topped peg at the rear and 11-13 tall, laterally compressed, branched papillae anteriorly, none of which exceeds the spike in height.
The mantle and foot sole are translucent with visecera (particularly th dark, Y-shaped digestive gland) can be seen. The mantle is pale fawn made up of a dense peppering of microscopic reddish brown and smaller white granules. the uniform shade is interrupted here and there by darker or lighter patches and is slightly darker on the mid-dorsal area. there are often a small number of irregularly shaped hyaline patches on the flanks. These are ringed in white and are arranged roughly in parallel lines, one on either side of the midline. The rhinophoral stalk and clavus are darker than the mantle. The underside of both mantle and oral veil are translucent with a very pale reddish brown shade.
• Willan, R.C. (1988). The taxonomy of two host-specific, cryptic dendronotoid nudibranch species (Mollusca: Gastropoda) from Australia including a new species description. Zoological Journal of the Linnean Society 94: 39-63.
Rudman, W.B., 2003 (June 5) Marioniopsis platyctenea Willan, 1988. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/mariplat