Oh Noes! Teh Alienz R Heer!
This is an attempt at exploring some new ideas for one of my old alien species, referred to by other civilizations as some local variant of the word “dragon,” as per their reptilian appearance.
They have vents on their backs for a sort of gill-like organ that can extract oxygen from water, and since it doesn’t collapse from gravity on land, can also double as a set of lungs. They retain a separate swimming-bladder like some terrestrial fish.
The Dragons have a bi-modal metabolism: endothermic, or warm-blooded on land, and exothermic, or cold blooded to conserve on oxygen use while underwater.
Not particularly well-streamlined, they sometimes just walk along the bottoms of large bodies of water when there is enough dissolved oxygen at that depth and location.
They are tough, stocky, strong and resistant to great pressures. The tail is vertically flexed, like a terrestrial cetacean’s, for underwater propulsion, and the skin is covered with fine greenish scales.
Their eyes are adapted for both daylight and night-vision use and are particularly well attuned to both blue-green light and ultraviolet, the latter of which their F-Class home star emits in greater quantity than Sol does in our own solar system.
They are a conservative and caste-ridden society, with a meritocratic hierarchy based on childhood personal aptitude tests and continued productive work once assigned to a given caste — ZERO social mobility outside of that, but there’s an incentive to good performance on the job or to be demoted to a lower pay-grade and status in one’s caste.