The direction of egg-mass spiralling

April 1, 2001
From: Yoshiaki J. Hirano

Dear Bill,
I really enjoy the Sea Slug Forum. Thank you very much for everyday effort.

Here is a photo showin the spawning of Pteraeolidia ianthina from Boso Peninsula, central Japan. I took this photo through the glass of aquarium tank. As you see, the animal spawned the egg mass circling counterclockwise. I strongly believe that the direction of spiralling is universal in different species or in different geographic area. I have an idea that the direction of spawn is closely related with the position of female genital aperture in nudibranchs. By spiralling counterclockwise they can touch the newly spawned mass effectively and can continuously press it with their foot. I think both activities are quite important for making neat spiral shape. If the direction were clockwise, both would have been very difficult. The important thing is opposite direction of egg ribbon spiraling against the genital aperture I think.

The very interesting photo of the "pseudodextral" spawn of Doridella obscura presented by Dr Irina Roginskaya reminded me of the egg mass of the littoral pulmonate limpets such as Siphonaria japonica. They always start spawning at the outside towarding the centre. However, I did not know there is the same way in nudibranchs! Yes, as you suggested, we have to go out and start watching animas more carefully.
Best wishes,

Yoshiaki J. Hirano
Kominato Marine Laboratory
MBRC, Chiba University

Hirano, Y.J., 2001 (Apr 1) The direction of egg-mass spiralling. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Thanks Yoshi,
By coincidence, Mike Miller's beautiful photo of Hypselodoris bullocki spawning shows how the sole of the foot is used to attach the egg ribbon to the substrate.

I must look out the photos I have of Pteraeolidia spawning in the southern hemisphere to see whether they spiral the same way down here. Unfortunately I can't make out the direction of spiralling in Akos's photos from Sydney.
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Apr 1). Comment on The direction of egg-mass spiralling by Yoshiaki J. Hirano. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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