Peltodoris atromaculata abnormality

September 9, 2009
From: Antoni López-Arenas i Cama

Dear Bill,

We found this specimen of Peltodoris atromaculata on a day the current was flowing strongly. I think that is the reason I could take photographs of its gills expanded and the undulating mantle, but I'm not really sure...

Do you think that the protuberance surrounded by a yellow ring is a reproductive organ in a wrong place or another abnormality?

Locality: Costa Brava, Tossa de Mar., 23 metres, Spain, Mediterranean, 29 August 2009, Pre-coraligen. Length: 5 cm. Upper Photos: Antoni Lopez-Arenas i Cama. Lower Photo: Marc Bosch

Thank you and best wishes,
Antoni Lopez-Arenas i Cama.

López-Arenas, A., 2009 (Sep 9) Peltodoris atromaculata abnormality. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Antoni,

I first thought this might be an example of a parasitic crustacean, but the damage to the mantle is quite extensive so I suspect that it is the result of a predator's bite. Interpreting anatomical peculiarities from photos is a bit risky, but I suspect the  thin-walled 'balloon' which is poking out is part of the pericardium, which is a blood-filled sac surrounding the heart. The heart is just beneath the surface and just in front of the gills. The heart is very close to the gills because one of its main functions is to pump oxygenated blood around the body - and the gills is where oxygen  from sea water is transferred to the blood. If you look at a dorid, you will often see a small swelling in  front of the gills which is pulsating -  that is the heart at work. I suspect the orange ring around the 'balloon' is the kidney - although I am puzzled by its colour. The kidney forms a layer just below the mantle in the same area. Its function is similar to our kidney. The other possibility is that the orange tissue is the blood gland, which is a thin flattened gland which is often over-looked in dissections. Various functions concerning the blood have been suggested for this organ, but unless I have missed some recent work, we really don't seem to know what its real function is.

So in summary, I think the  thin-walled balloon is part of the pericardial sac which has popped out when something bit away a part of the slug's body wall.

Best wishes,
Bill Rudman

Rudman, W.B., 2009 (Sep 9). Comment on Peltodoris atromaculata abnormality by Antoni López-Arenas i Cama. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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