Dugong. The dugong is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest modern relative, Steller's sea cow (Hydrodamalis gigas), was hunted to extinction in the 18th century. The dugong is the only strictly marine herbivorous mammal, as all species of manatee use fresh water to some degree. It has been designated a vulnerable species. Illustration by Carl Lumholtz, 1889.