Phyllidiella zeylanica from Mayotte Island

November 26, 2009
From: Matthias Deuss

Dear Bill,

While reviewing my Phyllidiella pictures I found a slug that I think is P. rosans, since it has 14 or maybe 15 lamellae on the rhinophores (for a length superior to 30 mm). If indeed you agree it is P. rosans, then it is very hard to distinguish it from P. zeylanica by comparing the size of the tubercles !
Nonetheless, this variation with only one broad tuberculate longitudinal band on the dorsum is uncommon in Mayotte.

Locality: Passe en S, 5 metres, Mayotte Island, Comoros, Indian Ocean, 15 August 2009, Coral reef. Length: 30 mm. Photographer: Matthias Deuss.

Kind regards,

Matthias Deuss

Deuss, M., 2009 (Nov 26) Phyllidiella zeylanica from Mayotte Island. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Matthias,

I would consider this to be Phyllidiella zeylanica. I know that Brunckhorst states that P. zeylanica should have a few more rhinophore leaflets but I woud consider the highly nodulose ridges in your animals to be typical of P. zeylanica.  In P. rosans the ridges are much smoother.

The species of Phyllidiella are difficult to differentiate and I suspect we still have some work to do. I am always suspicious of characters which rely on small differences such as these rhinophore leaflets - 20-23 lamellae in P. zeylanica (specimens greater than 30 mm), but only 12-15 in P. rosans. It gives little room for variability and no room for error in measuring the length of the specimens which stretch considerably when alive and contract to varying degrees when preserved. If one species has 5-6 rhinophore lamellae and another species 20-25, then that is a difference which has practical application, but unlike arthropods which apparently show remarkable precision in bristle numbers, I know of no studies on nudibranchs which give me confidence in using precise counts to separate species.

Best wishes,
Bill Rudman

Rudman, W.B., 2009 (Nov 26). Comment on Phyllidiella zeylanica from Mayotte Island by Matthias Deuss. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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