They have a prominent dorsal fin which starts on the crown of the forehead and runs back through the scalp and down the back as far as the small of the back. They also have fins on both of their lower legs and both of their lower arms. Their gills are in their necks.

They can see very well under the water, having developed nictating membranes that allows them as much visual ability underwater as they had on land. They can live at a reasonably great depth underwater, and can swim like fish.

Argoans can live for up to an entire day on land, but must return to water each day for one hour, or they begin to suffer dehydration.

Most Argoans (Aquans) hate airbreathers. The few that do not hate them still dislike them. Argo is not currently a member of the Federation.

Argoans are creatures of science. Once their civilization lived on the land, just like Terrans. A planet-wide catastrophe caused virtually all the land to sink under the waves. Aquan scientists developed a serum that would change their race permanently into water breathers, and administered it to all the population in time to save the civilization. This was hundreds, if not thousands of years ago in Aquan history. Now they are all at home in the sea, and blame air breathers (in essence, their ancestors) for the planetary calamity that overtook them.

The Federation has had only one contact with Argoans, and have placed a quarantine on the world to protect both the citizens of the world from unwanted contact from air breathers and to protect their world from unscrupulous traders wishing to capitalize on the Argoan's advanced scientific techniques. The Aquans, on their part, are quite glad not to have any unnecessary contact from air breathers.