Nudibranchs Hypselodoris nigrostriata feeding a Dysidea dark blue sponge

Hypselodoris nigrostriata

Identified by Bill Rudman :
I am pretty sure this is Hypselodoris nigrostriata, but I find it pretty difficult to distinguish it from H. zephyra.
I have made in messages concerning both species I have some doubts about whether we are dealing with one or two species here.
Of more interest is that these two animals are feeding on the dark blue sponge. The oral tube of the animal on the right of your photo is clearly visible. The sponge is almost certainly a species of Dysidea.
Bill Rudman
Australian Museum, Sydney

Locality: Lambeh strait, 15 m, Indonesia, North Sulawesi, Indo Pacific, 30 april 2006, muck dive. Length: 50-70 mm.