Torsion & nudibranch gills

October 28, 1998
From: Hilda Norris

I was wondering if you had any information on gill torsion of nudibranchs or if you knew of any books that explain the evolutionary process of it well.

In particular I was just wondering if you knew of how the nudibranchs had evolved external gills. 

Thank you for your time.

Hilda Norris

Hilda Norris, 1998 (Oct 28) Torsion & nudibranch gills. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Hilda,
Have a look at the top of the page for some information on detorsion in sea slugs. It's not really how the nudibranchs evolved external gills, but how they lost their shell. One of the jobs of the mollusc shell is to protect the gills. Once the shell has gone, the gills have no choice - they are nude! Which is where the word NUDIBRANCH (naked gill) comes from.

I have realised in the last couple of weeks that I need to add some more of the primitive sea slugs (cephalaspids in particular) to the Sea Slug Forum so the evolution of the group can be better illustrated. If you can be patient I will add some more examples over the next few weeks.

About good books on "torsion". It really depends on how deep you want to explore. Most good invertebrate text books have a general account of the process. For a fuller review you would need to have a look at a specialised mollusc text. Check out your local university library and see what they have. If you have no luck let me know and I'll look out a couple of useful references. ... Bill Rudman.

Rudman, W.B., 1998 (Oct 28). Comment on Torsion & nudibranch gills by Hilda Norris. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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