Skarlix
The Skarlix lives shallowly buried in thin, fairly metal-rich mud or sand at the bottom of its coastal-waters habitat. Skarlixes are equipped to "breathe" iron, manganese, vanadium, chromium, molybdenum, cobalt, and palladium, but cannot "breathe" copper. Thus, while aerobic organisms "breathe" oxygen, it "breathes" metal compounds. Its metabolism is somewhat similar to the Earth organism Shewanella oneidensis. The Skarlix needs no oxygen for its metabolism, though it can tolerate tiny quantities of oxygen. Its source of energy and electrons (its food) are the simple hydrogen compounds ammonia, ammonium, methane, and even hydrogen gas. Skarlixes are sensitive to the oligodynamic effect, by which certain metals kill microbes. It is especially sensitive to anything containing copper, making copper effectively poisonous to it. In some low-metal areas, its populations are sparse and slow-growing. Due to greater ultraviolet levels, in tropical environments its populations are smaller, and live a little deeper under the mud and sand.
The Skarlix has a thin, floppy body, with a distinctively "frizzy" cell membrane. Unlike other amoeboid organisms of Sagan 4, which stick onto something with their pseudopods and then move forward, it uses its frizzy cell membrane to grab and haul itself forward in continuous (if slow) motion. Overall, it resembles a millipede moving. Skarlixes' exact body shape varies, but they often vaguely resemble manta rays.
A Skarlix has two dense organelles next to its nucleus. Though these organelles vaguely resemble eyes, it has no ability to detect light.
Its genetic material is coiled more tightly in its single nucleus than its ancestor's, and many stretches of its DNA (such as cell repair mechanisms) are duplicated. On account of its backups, it is slightly more resistant than other organisms of its time to radiation damage, whether from ultraviolet light or radiation from radioactive materials. It's a handy skill, since it lives fairly close to the surface of the ocean, even in tropical environments, in a world that still has no protective atmosphere.