There are three main modes of propulsion for most posthumans on Anthropomundus: cetoid, with cetacean fluked tails; phocapod, propelled by seal-like feet bounded by thin webbing; and, lastly, plectropod, in which the feet are flattened, solid flippers. Although these by themselves do not indicate phylogenetic relation, major clades on Anthropomundus can nonetheless be united by them. Indeed, while far from the only plectropod posthumans, this clade, Plectropoda, is the most diverse one bearing plectropod feet as a basal character, thus lending its name.
Besides their method of displacing the water behind them, they bear a cartilaginous fin projecting from the sacrum, called the sacral fin, which is contiguous with a laterally flattened tail, sandwiched between two flattened legs of moderate length bearing feet usually of crescent shape akin to homocercal caudal fins of ray-finned fish. They are further united through being obligate gill-breathers, facilitating respiration through ventilating with their lungs, the only air-bearing portion of which being an alveolar outpocketing serving as a swim bladder. They do not nurse their young, which typically born in the dozens, typically precocious, though parental care is not entirely lost on them.
Ecologically, they predominantly assume the role of reef fish on Earth, dwelling mostly in shallow, calm waters amid algal reefs and kelp forests, with the open oceans being more dominated by cetoid posthumans, although some prominent plectropodans were yet able to carve out their own place in those perilous seas, where monsters and manchovies reign supreme.
Standing comfortably―but vigilantly―on the sea floor with its prodiguously elongated thumbs and big toes, tall and proud, is the rumplestiltfin. It is adapted similarly to the tripod fish of Terra, propped at just the right height above the sea floor to spring forth tackling with its teeth errant little posthumans hapless enough to stimulate the whisker-like mustache hairs lining its maxilla, fanned out giving an appearance―especially overhead―in the manner of a lion's mane, being pressed back and trailing behind the lips as a true, if feathery, mustache as it swims to a new location.
Despite its name, the shore manchovy is unrelated to the cetoid manchovies of the open ocean, being a plectropod that nevertheless evolved a similar lifestyle but nearer to the shores. A strong sense of smell and enlarged, electroreceptive mustache follicles help it find food. While most plectropods have blunt faces, this species's is elongated, with the nose pointing outward, allowing greater streamlining for its fast-paced existence. Like the cetoid manchovies of the open, this plectropod manchovy can form baitballs in a desperate attempt at defense with its conspecifics, only to be ganged up by other posthumans. It gives birth to several precocious offspring at a time that immediately assume their parent's filter-feeding lifestyle.
These plectropod posthumans' ruddy dark coloration is invisible in the depths they dwell, but with biolumensecent follicles peppering their lower sides, they are also difficult to spot by hungry eyes below due to counterillumination. They and their offspring may still fall for the radiant ruse of the anglerfingers' pinky lures, making up for these and other losses through high fecundity.