The meandering mountains of the Abelian interior traverse a variety of environments, beginning just south of the fertile floodplains where vertebrate life entered this world. The range then begins an eastward journey deep into the center of the continent. They then turn north, looping around a sizeable desert before gradually petering out as they run along the north coast of Abeli. In the middle of this mess, there is a large valley that forms a pass through the hills. This is a crossroads for all sorts of organisms, as it is the quickest way to cross from the founding floodplains to the similarly productive environment of the mid-Abelian savanna. Any creature hoping to migrate between these two habitats has no choice but to brave the mountains in this region. A lucky few have managed to establish populations on both sides, while less well-adapted animals regularly succumb to the cold, dry conditions of the area. However, a tiny percentage of wanderers meet a third fate, one from which escape is impossible and survival poses unique challenges, but which serves as a refuge in its own right for those without a home anyplace else.
The reason this area is passable in the first place is due to the collapse of an ancient cave system just prior to Corvolea II's arrival in the system. While the mountains on either side of the gorge retain their full height, the one that once stood here is now a pile of rubble filling an immense and ancient sinkhole. The caves, however, did not collapse in their entirety; the sturdier sections remain intact and interconnected to this day. With every passing year, a few more creatures happen to stumble into its various entrances, find themselves lost in its web of tunnels, and - for those lucky enough not to perish in some deep cavern or impale themselves on a stalagmite - earn a living in these dark, quiet corners.
The very first group to colonize these caves were, of course, the pill bugs. A small collection of adventurous Amadillidium individuals set up camp here in the early days, feeding on the bits of grass that fell into the cave openings and establishing a stable population below the surface. Each generation ventured further and further into the dark, stopping only where there was no more food to be found. However, one subgroup of these opportunists eventually began to supplement its diet with another food source: the abundant chemoautotrophic archaea that dwelt in the deepest crevices of the cave system. With this diet, they were no longer tied to a diet sourced from the outside world, and could expand as far as they pleased into the caverns. Without a hint of sunlight, this new species soon lost its eyes, but compensated with massive antennae. Armed with their exquisite senses of touch and scent, these are now the masters of the subterranean world. As they are the only organisms capable of subsisting on the non-photosynthetic autotrophs that dwell here, they are the main food source for every other species that calls these caves home.
Once there was a stable population of primary consumers in this budding ecosystem, the predators soon followed suit. During a freak desert storm several dozen millennia ago, a few weary rats sought shelter under a boulder. To their surprise, beneath this rock was a narrow opening into an exposed section of cave. Realizing this was their best chance at weathering the storm, the group spent the night here. While the majority of them continued on their way the next morning, a handful stayed behind, finding themselves a hearty breakfast of isopods and going about their day within the dimly-lit entryway they'd slept in. Their population grew, and the gene pool deepened a few years later when a few dozen more individuals wandered into the caves. As the centuries progressed, the cave-dwelling rodents became a unique group, distinguishing themselves from their aboveground counterparts though their small size, white fur, and long snouts. Like the woodlice's antennae, the latter trait served to amplify their already-prodigious olfactory and tactile senses, allowing them to easily traverse the dark recesses of their native cave system. Unlike the archaeophagous isopods, though, these rats did still occasionally venture into the light, crawling out to explore the surface under cover of night during the short wet season when food was plentiful aboveground. As such, their eyesight has not been entirely lost after all their generations of underground life, and they retain small but functional eyes in addition to their other senses.
An odd offshoot of the ever-diversifying Aedes made their way here around the same time. Of all the newcomers, these are the most changed from their basal form. This species does not descend from Aedes semelparosus, and indeed takes nearly the opposite approach in its life history. In these caves, standing water is surprisingly hard to come by. While mountainous meltwater forms streams that course through the stone, rarely does the flow stop for more than a moment as it continues on its journey towards the more porous rock below, where it quickly seeps into the local aquifer. In the few small pools of stagnant water, tremendous swarms of larvae jostle for the scraps of food that happen to fall in from the caves above. These must grow fast; indeed, they reach only around half their adult size before pupating. Upon emerging, their exoskeletons do not harden for several days, for this would prevent further growth. Down in the caves, without access to floral food sources, both sexes must consume a blood meal as soon as possible after taking flight, following the muffled sounds of rats as they scurry across the cave floor. Upon feeding, they crawl to a safe place to complete their growth. If the individual is a male, he will require only one or two additional meals to sustain him for his three-month life, while a female may feed ten or more times despite having a similar lifespan. Each drop of rodent blood will sustain her long enough to lay a clutch of several hundred eggs.
The most recent immigrants who've found themselves here are the mosquitofish. The cavern-minnow, Gambusia troglodytes, looks about how one might expect for a cave-dwelling fish of its size: pale white, eyeless, and possessing a rudimentary pair of barbels to feel its environment. It is a strong swimmer, careful not to be swept away in the currents of the underground streams. Feeding as its ancestors did on mosquito larvae, it congregates in pools where currents are slow enough to permit its prey to breed.
As time goes on, the fauna of these caves will continue to flourish, and all of these founding species have bright futures ahead of them. We will see more of the Abelian mountain caves some day, but for now we must return to the surface world.