Re: Onchidoris bilamellata from Puget Sound

December 13, 2006
From: Lowell A Thomson

Concerning message #15949:

Yesterday I dove Harper's Ferry, just south of Port Orchard, Washington. In reading Jan's message, he mentions a "solitary" Onchidoris bilamellata. On the pilings at the pier here, there were definitely swarms of this nudibranch, with multiple specimens present in one picture, and egg ribbons aplenty. The first picture attached has at least seven individuals, and egg ribbon. I am curious about the pale nudibranch in the middle. Is this a color variant? Or is it possibly Onchidoris muricata? One reference (Eastern Pacific Nudibranchs; Behrens, D.W. and Hermosillo, A.) mentions "...rhinophores are yellow-orange tipped with yellow." The pale individual here does not appear to have yellow-orange tipped rhinosphores. A second picture from a different piling is attached, again with at least seven individuals. There is another pale nudibranch in this photo. However, this pale individual appears to have yellow-tipped rhinophores. Is this possibly Onchidoris muricata?

Locality: Harper's Ferry Fishing Pier, Port Orchard, 15-20 fsw, Washington State, USA, Puget Sound, 02 December 2006, On pilings.. Length: Less than 1 inch. Photographer: Lowell Thomson.

Thank you,
Lowell A Thomson.

Thomson, L.A., 2006 (Dec 13) Re: Onchidoris bilamellata from Puget Sound. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Hi Lowell,

This species is amazingly common around the Sound. Populations come and go with the settlement of barnacle larvae. These guys feed on the juvenile barnacles before they form their hard volcano shaped shell, then when the food resource is exhausted, lay hordes of eggs. On my first intertidal collecting trip in the Sound, we came across literally thousands of this dorid, huge masses of them and the egg masses were everywhere.

Concerning the light colored individuals - I have noted this also and have collected animals for genetic analysis. These lighter specimens are definitely O. bilamellata, but for some reason they are not capable of metabolizing the color pigments of their brethren. What is interesting to me is that co-occurring populations of another barnacle predator, the gastropod snail, Nucella lamellosa also have a mix of brown pigmented and white specimens. There must be something here. I hope to learn more soon.

Dave Behrens

Behrens, D.W., 2006 (Dec 13). Comment on Re: Onchidoris bilamellata from Puget Sound by Lowell A Thomson. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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