Sea slug Janolus cristatus
What you see in this photo is a sea slug scientifically named Janolus cristatus (Nudibranchia - Proctonotidae/Zephyrinidae), a large species up to 7.5 cm long, distinctive by having numerous, transparent cerata, that clearly show the digestive gland duct running to the tip where it divides into a number of terminal branches. The tips of the cerata are colored with an iridescent bluish-white pigment.
The species lives in shallow sublittoral conditions, especially on sheltered rocky coasts, and can be found in the Northeastern Atlantic from Norway to Morocco, and western Mediterranean.
Photo credit: ©Miqui Rosa | Locality: Tossa de Mar, Catalonia, Spain (2011)