Marvin appears to be a much more primitive species of plankton. Marvin is notably smaller than some of Sheldon's other cousins.
Cousin Marvin appears in the short, "Plankton. A Thank You Would Be Nice." when Plankton is 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.
It is unknown if he is a maternal or paternal cousin of Sheldon.