Berthella medietas
Burn, 1962

Family: Pleurobranchidae


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.
Authorship details
Rudman, W.B., 2005 (September 7) Berthella medietas Burn, 1962. [In] Sea Slug Forum. Australian Museum, Sydney. Available from

Related messages

Berthella - reproduction and larval development?

August 1, 2007
From: Erin Smith

Hi guys,
I am undertaking a short assignment as part of university zoology on Berthella (more specificallyBerthella medietas ) and am hunting information on reproduction and larval development. If anyone could provide any info or point me in the direction of some good journal articles. ( I have already looked at Jeff Goddard's 2004 piece in Can. J Zoology but of course it doesn't talk about Australian species) I would really appreciate it.

Smith, E., 2007 (Aug 1) Berthella - reproduction and larval development?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Erin,
I wish it were so easy! I often wonder how it is that university students end up with impossible projects like this - did you teacher give you this topic?  I am pretty sure the only information available on this species is what is available on the Forum or in the references on the species Fact Sheet. As far as I know there have been no studies on reproduction and larval development in this species. It is just one of the vast majority of marine animals we know almost nothing about. The only information I think we know on it 'biology and natural history' is that it feeds on a plakinid sponge of the genus Oscarella  [see Jeff Goddard's message #14715] .

If I am wrong and you do find more information I would be grateful if you could let us know on the Forum. Otherwise I would go back to your lecturer and ask for a possible, rather than impossible, task.

Best wishes,
Bill Rudman

Rudman, W.B., 2007 (Aug 1). Comment on Berthella - reproduction and larval development? by Erin Smith. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Berthella medietas from New Zealand

September 7, 2005
From: Jeff Goddard

Hi Bill,
While in the Wellington area I found intertidally two specimens of Berthella that Richard Willan kindly identified from the accompanying images as B. medietas Burn 1962.

Locality: Tarakena Bay, Wellington, New Zealand. Intertidal. Length: 10 mm
21 August 2005. Rocky shore, low intertidal pool. Photographer: Jeff Goddard

One of the specimens was grazing on the sponge Oscarella sp., which was the most abundant sponge in the under-rock habitat I was searching in. This record, combined with the record of B. ocellata feeding on Plakina trilopha and Bernard Picton's report [message #5894] of B. plumula feeding on Oscarella in the British Isles, suggests that sponges of the family Plakinidae may be a common dietary component of species of Berthella. However, because Oscarella lacks skeletal elements, it would be missed in visual examinations of gut contents. With this in mind, I would like to make another attempt at determining the diet of B. californica from the NE Pacific Ocean. The diet of that species eluded me during my time in Oregon (the guts of the few specimens I examined never had anything recognizable in them), but I have since learned that one of the sponges I had found intertidally in Oregon is indeed an Oscarella.

Best wishes,

Goddard, J.H.R., 2005 (Sep 7) Berthella medietas from New Zealand. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Thanks Jeff,
Another valuable find - and another species to add to the Forum. We can also add Richard Willan's report (1984) of another New Zealand species, Berthella ornata, with it stomach full of the spicules of Plakina monolopha. As well as Picton's report of Berthella plumula feeding on Oscarella, there is also Delaloi & Tardy's study (1977) showing it fed only on Oscarella lobularis, excluding all other sponges it was tested with.

  • Delaloi, B. and Tardy, J. (1977) Regime alimentaire et ethologie predatrice de Berthella plumula (Montagu, 1803), mollusque opisthobranche. Haliotis, 6: 273-280. [for 1976].
  • Willan, R. C. (1984) A review of diets in the Notaspidea (Mollusca: Opisthobranchia). Journal of the malacological Society of Australia, 6: 125-142.

Best wishes,
Bill Rudman

Rudman, W.B., 2005 (Sep 7). Comment on Berthella medietas from New Zealand by Jeff Goddard. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from