In taxonomy, Watanabea is a genus of algae, specifically of the Trebouxiophyceae.Algae (Latin for "seaweed") are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many distinct organs found in land plants. The largest and most complex marine forms are called seaweeds.