Denizens were a heavily engineered sentient species created on Europa by the Europan Demarchy. They possessed varying levels of intelligence, ranging from animalistic to baseline human capabilities, but were invariably of great physical strength and well adapted to life in the Europan ocean. Their most unusual adaptation was their ability to utilize hydrogen sulfide instead of oxygen in a modified human body -- Europa's oceans containing pockets of high sulfur content. The Denizens later mounted a rebellion, successfully destroying the first Europan Demarchy.