What is a nudibranch or sea slug? Although the term 'slugs' conjures up all kinds of unpleasant ideas, nudibranchs (pronounced 'noo-dee-branks), more commonly known as sea slugs, are renowned for their boundless variety and beauty. Nudibranchs are gastropod molluscs.

Description: Has a long oval, dorid-shaped white body that is covered in orange spots (papillae) and orange rhinophores. Orange papillae gather on the dorsal oral flap above the nudibranch's mouth. Its gill plume (middle of dorsal side) is also tipped with orange and is not retractable like other dorids.

But don't touch them. While the hydroid stingers housing in the nudibranch's back likely won't be able to penetrate your skin, curious hands could easily damage nudibranchs. And nudibranchs are nearly impossible to keep in aquariums, which is why you rarely see them on display.

Opalescent nudibranchs are one of the prettiest and most colorful species of nudibranchs. Though their colors vary, they always have bright orange areas on their backs and blue lines along each side. Cerrata (fingerlike projections) on their backs are brownish yellow, with white and gold tips.