A very attractive nudibranch, Janolus cristatus, doesn't have a recognized common name. It is however very abundant some years during the summer on any rocky shore around Britain. It feeds entirely on hydroids whose stings it stores in the white tips of the serrata on its back. These stings stay active and will repel a predator. Grows to about 5cms, semi-transparent, often grey but with a hint of buff.
St Abbs Marine Reserve, Berwickshire. E. Scotland