Chromodoris aureomarginata from northern New Zealand

Thanks Floor,

As the close-up photo alongside indicates, I am pretty sure the aggregation of nudibranchs here is for mating rather than feeding. In rich substrates like this, it is impossible to be sure if an animal is attracted by a particular species of sponge, unless you actually catch it in the act. In fact most of the organisms here are not sponges - I can see bryozoans, ascidians, hydroids and an encrusting algae. The only sponge I can see is the bright reddish one in the lower right corner. Its possible there is a food sponge nearby but it is most probably in a crevice or partially overgrown.

These photos show the characteristic two-tone submarginal yellow band very well.

Best wishes,
Bill Rudman