Opistobranchs, and especially the group of the Nudibranchs, include some of the most colourful creatures on earth.
The Opisthobranchs are invertebrate animals (Mollusca-Gastropoda) commonly known as sea slugs. They are characterized by a small or absent shell. Among the best known Opisthobranchs are the nudibranchs, invertebrate animals with spectacular colour. They generally are small (some millimetres long) but larger species can reach 40 cm. Having no mechanical protection, their only defence relies on their camouflage, and on their toxicity, secreted by them or accumulated from their food.
It is possible to find these species all year round in Catalonia, from shallow water to depths of well over 700 m.