Far from the deep waters where the tide runs red once in a lifetime, another oceanic biome exists on Apterra. Here, the deadly algae spread across the water's surface, their solid mass blotting out the sun from reaching the waters below. Every few dozen meters, a small hole in the algal mat offers a window into the sea below. This is the work of copepods, descendants of those that first began feeding on the red tide back in the Dawn Muricene and close relatives of the forms that control the algal blooms on the open sea. Here, though, they fight a losing battle; they can only keep the algae at bay for a short while before it regrows and closes the openings they've made. During this brief period, life-giving green algae grows in these scattered oases of sunlight, breathing precious oxygen into the waters below. Only through the actions of these microbes can Gambusia survive in these waters at all. That's not to say life is easy here. The world's most resilient and adaptable little fish make their living in these environments. The region that most perfectly typifies this ecosystem can be found in the shallow sea between Loxodia and Abeli, an expanse of water almost entirely covered by biofilm. Only three fish have what it takes to survive here.
The first is Gambusia verticalis, an interesting species of grazer that scoots along the underside of the algal mat. It scrapes off flakes of dead algae before they have a chance to sink to the seabed. The seven-centimeter minnows swim in an upright position, propelling themselves with both their dorsal fins and tails. Their body is ringed with stripes that appear horizontal in this position, with males displaying a deep blue hue on their faces and fin rays. Only rarely does a female actually get the chance to see this pattern, though; this species spends most of its life in the dark beneath the algae and only ventures into the rare columns of sunlight when courting. In these moments, though, they lock eyes with one of their relatives.
G. fenestrophilus mosquitofish dwell in these windows, spending their entire lives in rare ephemeral pockets of habitability. Their life cycle is short, reaching maturity in just a matter of weeks. Soon after being born, fry scatter into the surrounding waters, frantically searching for a new home. They must find a spot where copepods have recently formed a new window, as their older conspecifics will violently defend their established territories. They have only hours to find a place to live before the energy stored in their yolk sacs runs out. The luckiest individuals will find a cozy little hole in the algae, rest for a while, and get to know the dozen or so other young fish who've set up shop in the same refuge. As they grow, they begin to develop aggressive tendencies; fights break out between males, then between females, and soon only a single pair is left alive.
Their only goal now is to gorge themselves. They feast on the copepods and green algae that form their home, devouring everything they can fit in their mouths. This very behavior will soon bring about their demise, but they don't know or care. As they decimate their own habitat, growing larger every day, they soon notice the edges of the window growing inward around them. Only a half-meter or so wide to begin with, this opening soon closes to less than half its original size, tightening around its inhabitants, who by now have reached nearly fifteen centimeters themselves. They are soon so restricted by their shrinking home that they are forced to jostle for access to the well-oxygenated waters they rely on. They writhe and slide over each other, and this physical sensation triggers a hormonal change in their bodies. They are suddenly overwhelmed with the urge to reproduce; the female produces thousands of eggs, which are fertilized internally by her mate. When she gives birth, the fry swim off, each picking a direction and swimming constantly until it either finds a home or dies of exhaustion. The parents, though, are finished. They've engineered the destruction of their sole lifeline, exhausted themselves during the breeding process, and run out of room to avoid brushing against the ever-growing poisonous dinoflagellates. They'll live perhaps a day longer, their purpose fulfilled, before drifting down to the seafloor.
Like most ecosystems, this one has a predator. A close relative of the open-water G. magnops, this slightly smaller and solitary species terrorizes the slow-moving prey species in its range. Going by the name of Gambusia poliops, it is also happy to consume carrion when it comes across some. Of all the Apterran mosquitofish, this one has the greatest resistance to algal toxins. It is also well-adapted to survival in low-oxygen waters, venturing far deeper than the other fish it shares a range with.
These algal wastes are one of the most obvious signs that Apterra is still a young planet. Despite everything life has achieved here so far, the plague of toxic algae persists. For many centuries to come, these subalgal barrens will remain the dominant shallow-water oceanic biome on this planet.