Nudibranchs are marine gastropod molluscs. They belong to the same family as your garden-variety slug, having no shell at maturity. This is why they are often called sea slugs.
The word 'nudibranch' literally translates to 'naked gills' because their gills are located on their back or along the side of their body. Nudibranchs inhabit marine environments ranging from the cold waters of the arctic, to the warm waters of the tropics. Their primary mode of propulsion, like other gastropods, is the single foot extending the entire length of their body. Other species like Hexabranchus sanguineus (Spanish dancer) use their magnificently coloured mantle to both propel themselves through the water and also to startle predators.
Nudibranchs are carnivores. They feed on other soft-bodied sea creatures including soft-corals, anemones and sponges. It is from these tasty morsels that they extract toxins and the amazing colours that decorate their bodies. The colours and toxins form part of the nudibranch defense system. The toxins being used as poisons or in stinging cells, and the colours to warn predators or to blend in with surroundings.
To feed, the nudibranch has a few options. Some use their mouth, others use their cerata; finger like protrusions which are extensions of their digestive systems. The cerata may also contain stinging sells and, in some cases, are used for breathing. A few species harbour green algae inside their bodies to provide them with food. The algae, in-turn, obtain its energy from the sun, making this nudibranch species solar powered.
Nudibranchs are both male and female (simultaneous hermaphrodites), possessing both sperm and eggs. They exchange these with another of the species in a process that may take seconds or a whole day, depending on the species. Both then go their separate ways to lay swarms of eggs, which ultimately hatch to become the amazing creatures known as nudibranchs.