Cousin Marvin
Marvin[1] is one of the cousins of Sheldon J. Plankton. Unlike Sheldon's other cousins, it is unknown if he is a Plankton or if he is one of Sheldon's maternal cousins. He only appears in the short, "Plankton. A Thank You Would Be Nice."
Description
Marvin appears to be a much more primitive species of plankton; it is likely for this reason that Sheldon considers Marvin stupid. Marvin is notably smaller than some of Sheldon's other cousins.[1]
History
Cousin Marvin briefly appeared in the short, "Plankton. A Thank You Would Be Nice." when Plankton was explaining how plankton are the most important living creatures on the planet. He uses cousin Marvin, who he describes as stupid, as an example, but he then explains how Marvin has planktonic plant brains that make up half of the Earth's oxygen.