In this image, a nudibranch, commonly known as a sea slug, searches for food on the coral reef. Most nudibranchs feed on small sponges, tunicates, and other small invertebrates. They can range in size from a half an inch to about two feet in length. Nudibranchs are among the most colorful creatures in the sea. There are over 2,000 different species of in the world's oceans. They can be found in nearly every color and color combination imaginable.