Plankton was, according to the Second Doctor, "small plants and animals from the sea". Ben confirmed this to a bewildered Polly, noting that the animals considered plankton were "little spidery ones". Plankton was at the heart of Professor Zaroff's experiments in Atlantis and served as the basis for most food consumed there. The Doctor found plankton-based food very tasty, comparing it to ambrosia. Polly, however, remained dubious. (TV: The Underwater Menace)
Miss Evangelista was once glad when she was told she had the IQ of a plankton. (TV: Silence in the Library)