Antonietta cf. janthina
Locality: Dauin, Lipayo, Negros Oriental, 6m , Philippines, Pacific Ocean, 08 May 2003. Length: 11 mm, 14 mm and 12 mm. Photographer: Erwin Koehler.
I have some doubts about identifying this as Antonietta janthina, which is known at present from Japan. Certainly the non-Japanese animals illustrated here and on NudiPixel have some similarities in colour to Japanese material, especially the colour of the rhinophores and traces of the colours on the head and oral tentacles. However the thinner and longer shape of the cerata in Japanese animals looks rather different, but perhaps animals can change the shape of their cerata - perhaps something to do with how recently they have eaten. Opposed to that is the fact that all the non-Japanese animals seem to have fatter and shorter cerata. Also, one of the photos on NudiPixel shows a coiled egg ribbon which looks rather different from that laid by Japanese animals where the egg ribbon is divided up into short packets.
Interestingly Japanese animals are said to feed exclusively on the hydroid Hydractinia epiconcha which, as its name suggests, is found growing in clusters on snail shells. Erwin Koehler's photo shows an animal feeding on a solitary hydroid, quite a bit larger than any Hydractinia I am familiar with. They are both athecate hydroids so if it is A. janthina it may feed on both.
Until we know more about its anatomy I think it would be better to call this 'form' Antonietta cf. janthina
Further photos of this species or 'form' can be found on Nudi Pixel identified as Antonietta janthina
Rudman, W.B., 2009 (October 26) Antonietta cf. janthina [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/antocfjant