More than 2.4 million years after shifting to an aquatic lifestyle, the subfluvial woodlice (members of the order Aquarthriformes) are an extremely species-rich collection of several families, together containing well over 3,000 species Apterra-wide. They were the first terrestrial organisms to make this change, predating the semelparous mosquitoes by hundreds of millennia. New competition from insects has sparked a second huge wave of diversification, specialization, and niche partitioning, and the aquatic pill bugs are now on their way toward great things. Those in the ocean especially have been relatively unharmed by Apterra's changing climate, and deep-sea forms are among the most successful. 

Slogging their way through the decaying algae beneath the Medithalassic ocean, Mudeater Crabs (Caenarthridae) leave a cloud of reddish-brown murk in their wake. Heavy and slow-moving, these twenty-centimeter crustaceans number in the tens of thousands per square kilometer. With their food source forming a layer as deep as the height of a skyscraper, their population is limited more by the bacterial illnesses that proliferate in the sludgy seabed than by any resource scarcity. None go hungry, and they can collectively eat through a layer of sludge a centimeter thick per year. In that same time, though, an equal or greater amount of algae will have fallen, and despite their best efforts they've so far been unable to make any overall progress in cleaning the seabed. While this buildup is one of the primary factors contributing to the current glaciation event, the mudeaters don't know or care about this, and they are content to live a life of abundance on their never-ending diet. 

Higher up in the water column, swimming species in the family Pelagarthridae patrol the surface and twilight zones, filter-feeding on non-mat-forming varieties of algae. Since diatoms were never introduced to Apterra, their niches are instead taken by single- and multi-celled Volvocines, dinoflagellates, and numerous species of cyanobacteria. Due to the abundance of large, often multicellular algae, very small filter-feeders are at a disadvantage, finding the majority of phytoplankton far too large to fit in their mouths. This is a major concern for rotifers, copepods, and water fleas, but even the smallest Pelagarthrids have no trouble ingesting their food. Rather, their increase in body size has been primarily driven by the evolution of large predatory fish. With hunters like the stoutling relying on massive suction forces to gulp down anything that fits in their mouths, prey species experience a strong slective pressure to become too big to be swallowed. This has culminated in forms such as the thirty-centimeter Pleotesta, a fast-swimming algivore that was, until recently, inedible to even the largest fish in the sea. Unfortunately for the woodlice, the rapid evolution of enormous body masses among carnivorous fishes in the past few hundred millennia has greatly outpaced their own rate of adaptation, and now none are large enough to fully avoid danger. A slow but steady decline has followed, with pelagic isopods being gradually replaced by faster-swimming forage fish and tailtube worms. 

Freshwater Pelagarthrids are faring quite a bit better; though smaller, these more basal members of their family have a distinct advantage over their marine cousins: they are the only subfluvial group that retains the ability to walk on land and breathe air for short periods of time. All their relatives have, in the many years since taking that first dive below the waves, independently become fully aquatic, never venturing above the surface. Lake-dwellers like these, however, find it more useful to maintain an amphibious lifestyle, crawling from one body of water to another whenever conditions become unfavorable but never venturing so far away that their gills dry out. They are especially fond of reproducing in ephemeral pools where predators are rare, after which the mothers return to their permanent homes. This method is employed in dramatic fashion by the genus Tellotraversus, also known as the Puddle-Plague. These are born en masse in small puddles that form during the late summer in eastern Loxodia. Too small to survive in larger year-round ponds and rivers, they spend the first few weeks of their lives feasting on algae, detritus, and any small flies that end up stuck in the water. After reaching a suitable size (usually about half a centimeter) they all emerge together, traveling downhill as a single swarm that superficially resembles the Plague - so much so that they often cause alarm among arboreal woodlouse colonies until the pseudotree-dwellers notice that they lack the Plague's distinct odor and the fact that they eat nothing during their, on average, three-hour trek. Predators pick off up to 90% of the traveling youngsters; the greatest dangers are pillbirds, Flagelloculicids, and certain small rat species, depending on the local area. This social behavior is a one-time gathering, and the few that survive quickly disperse into the water to live solitary lives. They then shift to their adult diet, which incorporates freshwater basket-grasses and small amounts of carrion.

While several subflvial clades contain members that have convergently evolved the ability to survive in marine environments, one lineage that remains exclusively freshwater is the Hamotarsidae, a family of sharp-clawed riverine predators. While unable to compete with carnivorous fish in open water or with scuttlebugs on calmer lakebeds, their many legs allow them to grip smooth underwater rocks, holding fast against rapids that would sweep away any other creature caught in their currents. The Stoneclinger (Hamotarsus) uses this to its advantage, facing into the flow and allowing prey to come to it. With a pair of long, hook-tipped front legs, it can snag floating morsels in the blink of an eye. Despite its small size (no more than five centimeters), it can use this method to catch prey up to double its own size, including other isopods, larval and neotenic mosquitoes, and small fish. Their most pressing threats are large Scabognathus, a genus of Ailoxichthyids that have expanded beyond the borders of their ancestral lake to inhabit many other waterways across the Ailuropian-Loxodian northern regions. While normally algivorous, their strong, scraping jaws also lend themselves to a cracking the tough, flat shells of Hamotarsids, and they often cannot resist snacking on the stoneclingers they find atop the algae-covered rocks they regularly scour.