It has been found throughout New Zealand and southern Australia (Tasmania, Victoria, South Australia, southwestern Western Australia)
Tesselated Pavement, Pirates Bay, Tasman Penin, Tasmania, Australia. 15 Feb 1984, 8 specimens under rocks, intertidal, AM C141248. Photo: Bill Rudman.
The body is typical for the genus, with a broad mantle extending out on each side to hide the foot, and usually the gill on the right side. The triangularly shaped oral veil, is grooved along each edge and the enrolled rhinophores are relatively large, able to extend in front of the oral shield or back over the anterior mantle. The mantle is relatively thick and is uniformly coloured, the colour ranging from translucent white to a translucent yellowish brown. There can be scattered white spots and there is usually a dark black-brown central patch from the digestive gland showing through the body wall.
It grows to approximately 30 cm in length and has been found in the intertidal and shallow sublittoral regions. It has been found throughout New Zealand and southern Australia (Tasmania, Victoria, South Australia, southwestern Western Australia). It is reported by Goddard [#14715 ] to feed on the plakinid sponge Oscarella sp. It has many similarities to the widespread Berthella stellata, but differs externally in lacking the white cross-like mantle colouration of that species. The name has often been spelt B. mediatas, because of a typographical error in the original publication where the species heading is spelt B. mediatas but elsewhere in the paper it is spelt B. medietas (see Willan, 1987).
Burn, R.F. (1962) On the new Pleurobranch subfamily Berthellinae (Mollusca: Gastropoda); A Revision and new Classification of the species of New South Wales and Victoria. Memoirs of the National Museum of Victoria, 25: 129-148. (Text-figs 1-5, Pls. 1,2)
Willan, R.C. (1983) New Zealand side-gilled sea slugs (Opisthobranchia: Notaspidea: Pleurobranchidae). Malacologia, 23: 221-270
Willan, R.C. & Morton, J.E. (1984) Marine Molluscs Part 2: Opisthobranchia. University of Auckland, Leigh Marine Laboratory, Leigh, New Zealand. 106 pp.
Willan, R.C. (1987) Phylogenetic systematics of the Notaspidea (Opisthobranchia) with reappraisal of families and genera. American Malacological Bulletin, 5: 215-241.
- Willan, R.C. & Bertsch, H. (1987) Description of a new pleurobranch (Opisthobranchia: Notaspidea) from Antarctic waters, with a review of notaspideans from southern polar seas. The Veliger, 29: 292-302.
Rudman, W.B., 2005 (September 7) Berthella medietas Burn, 1962. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/bertmedi