Gambusia of the High Seas, Part 2: The Red Tide

Beneath the surface of the Medithalassic, in a nice, warm, tropical region just off the coast of a small volcanic island (itself home to several interesting gecko and kiwi species), is an example of the greatest threat facing Apterra today. A dense mat of dinoflagellate algae sits on the beach at low tide and rises when the waves roll gently in, secreting poisonous defensive chemicals like gonyautoxin and forming a solid mass that excludes light and oxygen from reaching the waters below. The top two or three centimeters is where all the growth happens; the biofilm is so dense that light cannot penetrate through to its lower layers. As such, the dead cells on the mass's underside routinely slough off, sinking to the seabed. 

Mercifully for the inhabitants of these waters, this means the most toxic portions of the algal mat are above the water line, resulting in a just-barely-livable environment below. Further from the coasts, however, the situation changes. Here, solid biofilms cannot form due to the constant motion of waves. In fact, there are often long periods where no red-tide-causing algae can be found here at all, their non-toxic counterparts filling the role of photosynthesizers on the open ocean. The marine copepods are more than happy to snack on this food source, and they in turn are eaten by local Gambusia populations. Ten kilometers from the Gecko Isles coast, four different mosquitofish are commonly seen: the first, G. polymaculatus, is a dedicated algivore. It is one of the smallest fish on Apterra so far, measuring under a centimeter on average. This adaptation is a necessity due to the small size of its targets; without solid algae mats, the algae-eater has to search for individual phytoplankton suspended in the water column. To make up for their tiny size, individuals swim in schools of thousands, sweeping an area clean of green algae before moving along.

Not far behind them is a small school of G. magnops, a predatory species that specializes on its own congenerics. The 30-centimeter giants can easily swallow their smaller cousins whole, living in loose groups of up to a dozen. Given the abundance of prey and the fact that it has no predators of their own, this species' coloration can be quite magnificent. The average male bears a pattern of alternating black and iridescent red stripes along the length of its body, tipped with solid-black fins. Dark red scales lie beneath the eyes and along the jaw, with scattered patches of a similar color arcing along the gills. 

Another predator lives here as well, half the size of its fish-eating cousin, but this one poses no danger to its fellow vertebrates. G. bifurcauda is a speedy and specialized hunter of copepods. With its well-developed eyesight, it can spot its prey from many meters away, attacking from below before the crustacean gets a chance to spot it. Too large to fall victim to G. magnops, it too is free to experiment with extreme color displays. Males of this species are black on top, a form of countershading that prevents them from being seen when approaching their prey. On the underside, however, they sport a patchwork of mossy greens, browns, greys, and even the occasional orange scale. The fins, like those of their sister species, are black, while the eyes are bright green. 

Finally, if we look a bit deeper into the waters, we will catch a glimpse of an ominous-looking creature. This solid-black scavenger, which goes by the name of G. bradymelanocisor, swims lazily in the depths, biding its time until it catches a whiff of a recently-deceased creature. They aren't picky; they'll feast opportunistically on any of their cousins, even taking advantage of the occasional rat or kiwi corpse that washes out to sea. Wounded, ill, young, and old mosquitofish are also on the menu, as this species is happy to take advantage of sufficiently weak but still-living individuals.

Today, though, the everyday lives of the members of this budding ecosystem will be rudely interrupted. Deep on the seabed beneath them lies an oceanic ridge. Every year, the seafloor moves a fraction of a millimeter, pushing the Isles a bit closer to mainland Abeli. This morning, the built-up tension of over two decades of this motion was relieved in the form of a mid-magnitude earthquake. This in turn triggered an undersea landslide, stirring up years' worth of accumulated organic material. By evening, this flood of nutrients and minerals had reached the surface, triggering a red tide of immense proportions. The effects of this eutrophication are biblical; the waters turn from blue to red, poisoning the millions of unsuspecting mosquitofish caught in the plague. All the old predator-prey dynamics mean nothing now, as the various species crowd together in the scarce pockets of well-oxygenated water that remain. Around the periphery of the bloom, millions more can be seen making an exodus into other parts of the sea. If there's a silver lining here, it's the fact that these forced dispersal events are quite conducive to speciation.

In the moment, though, there is little hope for any of those caught in the toxic cloud of algae. The red tide continues to sustain itself for several weeks, first using up all the nutrients it can glean from the upwelling, then feasting on the masses of rotting fish corpses that dot the ocean's surface. As soon as it shows signs of slowing, the cleanup crew arrives. This comes in the form of trillions of copepods, each of which does its part in consuming the deadly sludge. In under a month, this patch of sea is once again habitable for vertebrate life. The four minnow species make their way back home, jostling for the territory against the various other Gambusia lineages that make their homes in neighboring waters. In a sense, the return trip is just as dangerous as the red tide itself, for if a species can't quickly become reestablished in its old home, a newcomer might just outcompete it, leaving the loser doomed to extinction.

This time, though, all four aforementioned species have weathered the challenges, reconquering their old home and beginning their normal life cycles anew. They are joined, however, by another variety. This group of immigrants, named Gambusia abyssalis, is a close relative of the scavenging G. bradymelanocisor, one which ventures even further into the depths of Apterra's oceans. It scours the seafloor for scraps of organic debris, picking off any sunken morsels of food. It doesn't know it, but its actions significantly reduce the available biomatter for the next red tide to feast on. Whenever this occurs - maybe in a year, maybe not for several centuries - the oceans will run a slightly lighter shade of pink. The cycle of red tides is far from over; in fact, the last of these horrific blooms won't fade away until the Muricene itself does. However, the arrival of deep-sea organisms is good news for the long-term survival of Apterra's ocean ecosystems. When a stable marine biosphere finally arrives, it will have the humble G. abyssalis to thank for paving the way toward this bright future.