A frozen ocean moon circling a giant planet, sealed under a kilometres-thick ice shell that blocks nearly all sunlight. Tidal flexing keeps a dark, salty sea liquid beneath the ice, warmed and fed by mineral-rich vents on the seabed. Gravity is light and the cold water oxygen-poor, so life clings to the under-ice ceiling above and the vent fields far below.
A pale, antifreeze-blooded fish that hangs beneath the ice ceiling, rasping chemosynthetic frost-films from its underside.
You choose the situation, not the result. Selection acts on Frostgill; the descendant appears above.
Climate
Resources & space
Predators & life
Habitat & extremes
Mate Frostgill with another branch of the same lineage to make a hybrid.