Legacy of the Red Tide

Dinoflagellates have defined Apterra from the very beginning. In the absence of diatoms, they were the most common unicellular algae in the sea. Early in the planet's history, some species of Karenia and Gonyaulax adapted to stick together in massive, buoyant biofilms, forming mats and removing carbon from the air, eventually leading to the deadly end-Muricene Ice Age. Those days are now long over, and toxic algal mats are a thing of the past. This is not to say that the dinoflagellates are gone; on the contrary, they've become keystone species in the Arthrocene oceans, fueling diverse ecosystems all across the globe.

Single-celled species are still extant, descending from a Karenia strain that never evolved mat-forming tendencies. These can either be benthic or phytoplanktonic, providing food to filter-feeding invertebrates. They generally outclass the absent diatoms in size, measuring about 250 microns in diameter. This spurs filter-feeders from disparate clades to grow to impressive sizes. Free-floating dinoflagellates are often poisonous like their ancestors, but dense blooms are rare and toxins almost never reach deadly concentrations. 

Rockslime (Petrolubricus) is a genus of Karenia-descended algae that grows on submerged rocks, rough sediments, and craggy shorelines. It too retains a level of toxicity, but only enough to repel non-specialized herbivores. Growing close to the surface, it is very resilient to crashing waves and debris. A common sight in tide pools and on cliffsides, it can recover from being exposed above the waterline for up to six hours. Rockslime provides a food source for Copabinia and other algae eaters, as well as the Dwarf Filthfish (Abyssalichthys ultimus), the only member of its genus to survive after the Muricene algal muck disappeared.

Gonyaulax-derived species have become more complex with time, evolving organized structures as their ancestors' amorphous mats dwindled away. Plumekelp (Pinnalgarum) is one such genus, growing on well-lit, stable seabeds. Unlike rockslime, it cannot survive out of water for any length of time, so it is restricted to areas below the low-tide line. With a maximum height of 30 centimeters, plumekelp is vulnerable to swift currents, but it can generally handle the large, gradual tidal fluctuations common across most of Apterra's coasts. Though much smaller than the seabaskets, its niche is similar, harboring a large assortment of grazing fish, thalassaurs, and seawis. However, because of plumekelp's short stature, many of its inhabitants have also shrunk to allow themselves to hide in the foliage; only a small handful of species weighing more than 10 kilograms are known to frequent the plumekelp meadows.

Matkelp (Planalgarum) is an example of a more basal Gonyaulaceous algae. In fact, out of all the groups discussed here, its lifestyle has changed the least over the past three million years. Like the toxic mats that clung to Muricene coasts, it grows as a solid mass that floats atop the waves, anchored to the sand on the shoreward side. It exists only around the calm waters of the northern and western Medithalassic, as the tides are less extreme there. Having largely lost its poisonous properties, it no longer poses a threat to other species. Many plants actually grow on top of the matkelp, spreading their roots deep into its biofilm. This, of course, still requires extreme specialization for salt tolerance, but the end result is a community rich in life that surrounds the tranquil Medithalassic beaches.