Ilbia ilbi
Burn, 1963

Superfamily: RUNCINOIDEA
Family: Ilbiidae


New South Wales and Victoria, southeastern Australia.


Port Phillip Bay, Victoria, Australia. March 1986. 4mm long alive. PHOTO: Bill Rudman.

Ilbia ilbi and the closely related runcinids are a group (Rucinoidea) of small herbivorous opisthobranchs with no close links to other cephalaspidean superfamily. Many, but not Ilbia, have a small reduced shell embedded in the skin at the posterior end of the body. The mantle cavity is essentially disappeared and the gill or gills, if present, and the anus, open at the posterior end of the body below the mantle edge.

Their radular teeth are quite distinct and they have four gizzard plates, rather then the usual three. Bulla is the only other cephalaspidean with four gizzard plates.

Runcinoideans are found in both tropical and temperate waters from the intertidal to about 50m. All are small, seldom growing more than about 5mm long. Only three species of the Ilbiidae have been discovered, Ilbia ilbi from southeastern Australia, Pseudoilbia lineata Miller & Rudman, 1968 from New Zealand, and Pseudoilbia mariana Hoff & Carlson, 1990 from the Mariana Islands.

• Rudman,W.B. (1971). Structure and functioning of the gut in the Bullomorpha (Opisthobranchia). Part 1. Herbivores. Journal of Natural History, 5: 647-675.

Authorship details
Rudman, W.B., 1998 (December 18) Ilbia ilbi Burn, 1963. [In] Sea Slug Forum. Australian Museum, Sydney. Available from

Related messages

Ilbia ilbi from Victoria

December 26, 2000
From: John Chuk

Dear Bill,

I seek help in identifying two species of cephalaspideans I photographed in Port Campbell Harbour, here in Victoria, [Australia] in February 1999. Views of both specimens are attached. Unfortunately these are not great photographs as all specimens were 4mm in length and fine detail is lacking.

The first species is a dark purple colour with white patches and yellow lines and spots. The cephalic shield appears joined to the mantle and parapodia are lacking. No gills were visible. Only one specimen was sighted and it was found on an algal stipe in 3m of water. It may be Ilbia ilbi but I'm not at all sure.

Any help in identifying this animal would be gratefully appreciated!
John Chuk

Chuk, J., 2000 (Dec 26) Ilbia ilbi from Victoria. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear John,
Your animal is Ilbi ilbi, one of two quite common runcinids from southeastern Australia. They are seldom noticed however because they are so small.

In case you are wondering what happened to your other photo, I have separated it into a separate message. Just as a general comment to everyone, if you have questions on more than one species it helps me a lot if you send them as separate messages, even if it means repeating collecting data etc. I try and link messages and photos to a relevant species page so if you send combined messages I have to then untangle them into separate messages.

Hope you are having a restful Christmas holiday,
Best wishes,
Bill Rudman.

Rudman, W.B., 2000 (Dec 26). Comment on Ilbia ilbi from Victoria by John Chuk. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from