Arcadians were distinguished from most humanoids by their broad, enlarged heads with two parallel lines of hair running front to back on the top outer edges of the cranium. In contrast, the bodies of Arcadians were slim and delicate. Their noses protruded very little from the face and their ears were large and pointed. They had wide-set eyes that were particularly brightly colored, and did not have eyebrows. (Star Trek IV: The Voyage Home)
According to the Star Trek IV Sourcebook Update, Arcadia was the fifth planet in the Vellun Gamma system. The Arcadians are described as a pacifistic, aquatic race with skin color ranging from light blue to pale yellow. They feed on plankton, but can derive some of their nourishment from photosynthesis. Arcadians live for 40-60 years and raise their young in marsupial-like pouches during the first 3-6 months of life. Arcadians are credited as the discoverers of protomatter.