Re: Conga lines of brown nudis in California

November 16, 2001
From: Dave Behrens

Hi Bill:
Concerning Celeste's conga line ... after a couple interactions with her we believe mystery critters to be Onchidoris bilamellata (Linnaeus, 1767) which Gary McDonald and I have on rare occasions observed in large aggregations. Adults of this species feeds on acorn barnacles while juveniles feed on encrusting bryozoans. Gary and I are at a loss to explain the tailing behavior Celeste observed, however. Pretty interesting observation. The attached photo of Onchidoris bilamellata was taken by Hans Bertsch at Hat Island, Washington in 1995.
Dave Behrens

dave@seachallengers.com

Behrens, D., 2001 (Nov 16) Re: Conga lines of brown nudis in California. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5675

Dear Dave,
When I replied to Celeste, I remembered Tom Thompson's 'refutation' of Pelseneer's (1922) observations concerning Onchidoris bilamellata in France but thought that as it was essentially an intertidal animal it was unlikely to be what Celeste was describing. I had forgotten O. bilamellata has been recorded from up to 20m deep. Pelseneer described great swarms of spawning animals migrating up and down the shore with the tide and 'following the leader' in lines like processional caterpillars. Pelseener's observations have never been reported by other observers and Thompson, on the basis of one visit to the site of Pelseneer's observations, published his 'refutation' of this behaviour. From Celeste's observations, and from Bernard Picton's message, Pelseneer did not concoct his observation. Onchidoris bilamellata is apparently swarming from the northeastern Pacific to the northeastern Atlantic.

Reference:
• Pelseneer, P. (1922) Sur une habitude de Doris bilamellata. Annls. Soc. roy. zool. Belg., 53: 28-32
• Thompson, T.E. (1984) Migrations of Onchidoris bilamellata during tidal emersion: a refutation. Journal of Molluscan Studies, 50(2): 123.

Best wishes,
Bill Rudman.

Rudman, W.B., 2001 (Nov 16). Comment on Re: Conga lines of brown nudis in California by Dave Behrens. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5675

Factsheet

Onchidoris bilamellata

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