Pteraeolidia semperi

Widespread in Indo-Pacific.

Very long nudibranch, making it difficult to photograph. Perfoliate rhinophores and oral tentacles. Although known as the Blue Dragon - not all individuals are blue. Many have cerata that are more brown than blue. The oral tentacles always have blue to purple bands.

Originally described as Flabellina semperi and then for many years was mistakenly known as Pteraeolidia ianthina (which is now recognized as a separate Australian species).