Metaruncina setoensis - external shell?

July 28, 2003
From: Jun Imamoto

Dear Bill.
Here are some more detailed pictures I have taken of Metaruncina setoensis than the ones I sent before. I am happy if these reports are useful for your research.

date: 26 Jul 2003
place: Manazuru-coast Sagami Bay, Japan
size about: 4mm
depth: 0.3m (Intertidal)
temperature: 24 C

Best Regards,
Jun Imamoto

Imamoto, J., 2003 (Jul 28) Metaruncina setoensis - external shell?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Jun,
Thanks for these useful photos. I am particularly interested in the white patch at the back of the body. This is where the internal shell of Metaruncina setoensis is situated, but since it is described as 'internal' it should not be visible. Looking at your earlier photos I can see the same white spot. I missed it earlier because I thought it was another white particle in the water floating over the animal when the photo was taken. In most published illustrations I have seen of this species there has not been a white patch like this. However I have just checked the photo in Marine Mollusks in Japan [Pl. 378, f. 2] and can see a white spot in the same position as in your photo. Hamatani, who wrote this section of the book, places Metaruncina setoensis in the Family Ildicidae which he defines as "Body with external shell..". This is quite contradictory to earlier descriptions of Metaruncina setoensis in which the shell is described as internal.

I am not sure from your photos whether the shell in your animal is external or whether it is internal but so close to the surface that it is easily seen. If it is external then either the position of the shell in Metaruncina setoensis is variable or there are two very similar species, one with an external shell and one with an internal shell. In the lower left close-up, the external gill can be seen to the right of the 'shell'. If you can help us with any more details on this puzzling white patch, I would be very interested.

• Hamatani, I. (2000) Runcinacea: 759, Pl. 378 figs 1,2. [In} Okutani, T. Marine Mollusks in Japan. Tokai University Press: Tokyo.

Best wishes,
Bill Rudman

Rudman, W.B., 2003 (Jul 28). Comment on Metaruncina setoensis - external shell? by Jun Imamoto. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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