Aeolidiella alderi and parasite

February 20, 2001
From: Jean-Pierre Bielecki

Dear Bill
Here are some pictures of an aeolid I suppose to be Aeolidiella alderi found :

in Mediterraean Sea, December 2000
Dive site : étang de Thau, Sète, southern France. Depth: 3m , Size: 1 cm.

This is the first time I have seen this purple extension on its back which I suppose is its digestive gland. What is its exact function. Is it a permanent or temporary structure and is it normal?
Best regards.

Bielecki, J-P., 2001 (Feb 20) Aeolidiella alderi and parasite. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Jean-Pierre,
As you will see in a separate message, Bernard Picton has confirmed this is A. alderi. The purple extensions are very interesting. They are not part of the aeolid but are the egg sacs of a crustacean parasite which is living in the body cavity of the aeolid. I am planning to put some information on the Forum concerning crustacean parasites when I get a moment.

There are a number of examples of parasites and 'commensals' already illustrated on the Forum. I have added an enlargement here [on right] of Daniel Geiger's photo of Spurilla neapolitana from Banyuls sur Mer, Mediterranean Coast, France, 1990. Four purplish white egg sacs can be seen amongst the cerata. Also have a look at the Symbiosis & Commensalism page and the Parasite page for some general information. See also the page on Ismaila, a common nudibranch parasite in South America. There are a number of photos on the Forum of egg masses protruding from aeolids in a very similar manner to the ones in your photo. There is also a photo in a message from Erwin Koehler, which also shows these egg masses in S. neapolitana from Spain.

Best wishes
Bill Rudman

Rudman, W.B., 2001 (Feb 20). Comment on Aeolidiella alderi and parasite by Jean-Pierre Bielecki. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Aeolidiella alderi

Related messages

  1. Aeolidiella alderi from Thau, French Mediterranean
    From: Dominique Horst, November 28, 2007
  2. Aeolidiella alderi from Croatia
    From: Mat Vestjens, July 13, 2006
  3. Aeolidiella alderi from Bembridge, UK
    From: Rebecca Cooksley, September 29, 2005
  4. Aeolidiella alderi from UK
    From: Judith Oakley, August 8, 2005
  5. Aeolidiella alderi from sthn England
    From: Judith Oakley, May 13, 2005
  6. Guernsey opisthobranch
    From: Richard Lord, November 20, 2003
  7. Aeolidiella alderi - defensive behaviour
    From: Marina Poddubetskaia, August 25, 2003
  8. Aeolidiella alderi + eggs
    From: Marina Poddubetskaia, August 10, 2003
  9. Aeolidiella alderi from Tunisia
    From: Anis, April 22, 2003
  10. Aeolidiella alderi or A. glauca
    From: Mauro Doneddu, January 21, 2003
  11. Aeolidiella alderi from Strait of Gibraltar
    From: Alma Sánchez, January 29, 2002
  12. Re: Cuthona granosa ? from the Mediterranean
    From: Bernard Picton , January 9, 2002
  13. Cuthona granosa ? from the Mediterranean
    From: Jean-Pierre Bielecki, December 19, 2001
  14. Aeolidiella alderi
    From: Jean-Pierre Bielecki, March 17, 2001
  15. Aeolidiella alderi or A. glauca
    From: Bernard Picton , February 20, 2001

Show factsheet and all related messages