Nudibranchs are a group of soft-bodied, marine gastropod mollusks which shed their shells after their larval stage. They are noted for their often extraordinary colors and striking forms, and they have been given colorful nicknames to match, such as "clown", "marigold", "splendid", "dancer" and "dragon". Currently, about 3,000 valid species of nudibranchs are known. Nudibranchs are often casually called sea slugs, as they are a family of opistobranchs (sea slugs), with the phylum Mollusca (mollusks), but many sea slugs belong to several taxonomic groups which are not closely related to nudibranchs. A number of these other sea slugs, such as the photosynthetic Sacoglossa and the colourful Aglajidae, are often confused with nudibranchs.