Nudibranch feeding on a hydroid. Flabellina polaris nudibranch feeding on an oaten pipes hydroid (Tubularia indivisa). Nudibranchs (meaning naked gills) are sea slugs without a shell or true gills. There are over 2000 species, and they are noted for their extraordinary colours and elaborate shapes. They are carnivorous, feeding on invertebrates such as hydroids (aquatic animals related to jellyfish). Photographed in the Russian Arctic, in the White Sea.