Favorinus on Hexabranchus eggs

July 12, 2003
From: Rokus Groeneveld


On the 13th of April 2002 I was on a nightdive on the housereef (Torfa)in Wahdi Lahmi (400 km south of Hurghada, Egypt). I found two small (about 10mm) what I think are nudibranches on the eggs of a Hexabranchus sanguineus.
From what I understand of all the pictures of juvenile Spanish Dancers on this site they're not that.

But then what are they? I'd be very pleased if someone could help me out?
Rokus Groeneveld

rosa12@wxs.nl

Groeneveld, R., 2003 (Jul 12) Favorinus on Hexabranchus eggs. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/9863

Dear Rokus,
You are quite right - they are not juvenile Hexabranchus. They are in fact an aeolid nudibranch of the genus Favorinus which specialise on feeding on the eggs of other sea slugs. Although some species of Favorinus apparently specialise on the egg ribbons of particular species, others like Favorinus japonicus, eat a wide variety of egg masses. Because a duct of their gut extends up into each of the cerata on their backs, they chnage colour depending on the colour of the eggs they are eating. As in your photo they are therefore well camouflaged when feeding on and egg ribbon. From the two swellings on their head tentacles or rhinophores I am pretty sure your animals are Favorinus japonicus. Two other species, Favorinus mirabilis and Favorinus cf. mirabilis, feed on Hexabranchus eggs but their rhinophores are quite different in shape.
Best wishes,
Bill Rudman

Rudman, W.B., 2003 (Jul 12). Comment on Favorinus on Hexabranchus eggs by Rokus Groeneveld. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/9863

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