Saeptibranchiae share a common ancestor with Cetanthropichthyes, as shown by their shared cetoid bodyplans. Where this clade diverges, however, is in the loss of the operculum, with the gill arches being divided by true interbranchial septa as seen in Terran sharks. They further do not have alveolar air bladders to keep them afloat, as one of their lungs becomes solely ventilation devices for when they are still with the other being specialized for fat storage, which partly makes up for the loss of buoyancy an air bladder. In addition, they bear two dorsal fins, enabling greater stability as they dash towards their prey. As a clade consisting mostly of raptorial predators, saeptibranchians are mostly ecological equivalents to small- to medium-sized sharks.
The mansharkpig is serious about finding its favored foods under the sand, and is backed with a battery of molars rooted into a thick mandible to extract it from their bony carapaces. This by no means imply that it is limited to those items, as this saeptibranchian highly-innervated upper lip and electroreceptive follicles lining mouth can detect other posthumans moving through the sand, such as anthroscolicid worm-men. Yet even then, when the opportunity presents itself, it will abandon rooting in the sand for dead or dying prey, whatever their sizes. It usually gives birth to four precocious offspring.
The seadog assumes the role of medium-sized pelagic sharks from old Earth; despite their fearsome reputation by its ancestors, at their size, most of their diet consist of smaller, bite-sized prey. Itself is no exception, feeding mostly on smaller cetanthropichthyan or plectropod posthumans, though it is not above tackling wounded or dead larger prey items. Occasionally, it will even try to steal fresh phocacetanthropid kills. All these feeding strategies are often performed in packs of several individuals. Its litter size often numbers half a dozen.
The ruddy drakeman makes its home at depths where the color red is invisible, making them effectively dark, invisible with most of its prey, which would then be caught in its toothy snare after detection with its large, watchful eyes and powerful electroreceptive folicles lining its lips and upturned nose. It may occasionally root for prey buried in the sand as well, particularly when nektonic prey is scarce.
Saeptibranchians have taken to the dark depths as other posthumans have. Using a crooked extension of the nasal cartilages tipped with a biolumenescent blister, the glow-whale has convergently evolved the adaptations of the angerfingers it shares its habitat with and sometimes devours itself. It also shares the same sexual dimorphism as the anglerfingers, smaller males with features optimized for finding mates in the deep.