Roughly 1.2 million years ago, falling sea levels left a massive stretch of water landlocked deep within Unciolis. This inland sea then gradually shifted to a freshwater lake over the next few millennia and remained at its former height - now nearly 20 meters above the shore, which lies over a hundred kilometers downstream. Measuring over 60,000 square kilometers, the lake appears more like a small sea from a distance, with waves and weather systems similar to those produced by the ocean. It is also one of the smallest bodies of water on the planet with noticeable tides, with a daily range between 6 and 8 centimeters. The river that flows from its northern bank is proportionally massive; though short, it carries thousands of cubic meters of water per second and is over seven kilometers wide at its mouth.
Stoutlings, a genus of large fish that spread across Unciolis's lakes during the Middle Muricene, were among the first to colonize this new waterway. They were well-adapted to navigating long, fast, shallow rapids in search of suitable nurseries for their fry. The slow-moving, deep, and peaceful river that led to this new lake was, by comparison, trivially easy to traverse. Over the generations, the body size of this population increased dramatically; fry in the lake had few predators, while females no longer had to remain small to make their journeys. In the present day, both sexes reach three or four meters long. This new species is known as the Great Red Stoutling (Iridichthys gigas), though its eponymous coloration only appears for about a month each year and does not occur in females, who remain silvery-gray like non-breeding males.
The beginning of spring is their spawning season, with tens of thousands of females swimming leisurely up the river to give birth to their three-centimeter-long fry. Unlike smaller stoutling species, the adults will then make their way back to the sea to live many more years, rejoining the males on the open ocean. They are apex predators, with no other species matching them in size, speed, or power. In one gulp, they can swallow nearly any other fish whole; only a few large seabasket-grazers are too big to be prey. The equatorial Medithalassic is their primary spring and autumn hunting ground, as its undersea forests and tidal flats are incredibly fertile during this time. When summer arrives, they migrate to the arctic regions of the Perithalassic, taking advantage of the short but productive growing season. In winter, they cross to the opposite pole, placing themselves close to Unciolis just in time to reproduce when spring arrives again.
While small in comparison to the adults, newborn great red stoutlings are enormous for livebearers; at three centimeters long, they're nearly the size of the full-grown minnows originally introduced to Apterra all those years ago. Already too big to fear tailtube worms, their only threats are other fish. Cannibalism is as common in this species as it was in the rainbow stoutling, and the fry's primary predators are their own kind. They'll remain in the lake, consuming each other and anything else they can fit in their mouths, until they reach over a meter long. If they were any smaller, they'd be in serious danger of being eaten by their parents when they entered the ocean. They take about a decade to reach their maximum size of about a hundred kilograms, at which point they become not just Apterra's biggest fish but the largest animal ever to evolve on the planet so far.