Initially, the character concept was gaseous format for the Hydrans until the found a habitable planet that was not yet home of intelligent life.<br/><br/>I can work around this and add that when needed that they do take corporeal from to interact with others, but with a proviso.  When they do so, they basically stand out in whatever format as a beacon to the evil entity that destroyed their universe, kind of like Frodo using the one ring in Lord of the Rings.  Like a bear that has killed humans, it will continue to do so and thus has a nose for it.