Benthic Isopods of the Inland Seaway

Subfluvial woodlice first emerged at the northern tip of Loxodia, populating streams, rivers, and lakes across the area. Today, they've colonized all of Apterra's major landmasses, occupying bodies of fresh water the world over. A few lineages within the family (now known as the Aquarthrids) have even made their way into the oceans, becoming gradually more salt-loving with each successive generation. They are common in the muck that covers the seabeds of the global ocean, sifting through decaying bits of fish, algae, and arthropods day in and day out. This lifestyle is pushed to its extreme in the Inland Seaway, where the shallow waters remain choked with solid algal mats. As oceanic ecosystems stabilize, most areas have seen reductions in dinoflagellate algae coverage. Here, with ideal conditions for algal mat growth, only a minor dent has been made. Copepod numbers are at an all-time high, feeding opportunistically on the red-tide algae and chipping away at it a little more with each passing millennium. A hundred meters below, though, is a different story.

The organic mud is now dozens of meters thick, built from a million years' worth of marine corpses. In these anoxic waters, few species are capable of surviving. Fewer still have the ability to return this slurry of carbon and nitrogen to the surface. For now, that niche belongs to the genus Caenarthrus, a mid-sized isopod that lives nearly its entire life in the depths here. The only time it enters the upper portions of the water column is when it reproduces. Beneath the algae, ocean currents are sluggish, meaning that young, planktonic Caenarthrus larvae wouldn't get the opportunity to disperse very far if their parents gave birth on the seafloor. Therefore, for a few weeks every spring, females of this genus make an ecologically vital migration. Each morning, they ascend from the substrate, climbing through the murky sea until they can see sunlight peeking through the scattered windows above. They each release between a dozen and a hundred young, which are born live but nearly microscopic, being allotted only a tiny amount of their mother's energy. At night, the adults return to the sea floor, mate, eat, and begin the cycle anew the next morning.

The end result is that each female Caenarthrus can give birth to well over a thousand larvae per season, each of which has only a tiny chance of making it to adulthood. Those that survive the onslaught of fish, predatory copepods, and even other Aquarthrids eventually make their way back to the muddy bottom of the Seaway. In the process, though, a few tons of carbon are brought back to the surface waters each year. At the moment, this is not even remotely enough to offset the steady supply of marine snow that falls from the biofilm above. However, if Caenarthrus expands across the planet, it could one day export a globally-significant amount of carbon back to the shallows.