North-central Abeli, once a wide, bowl-shaped grassland home to Apterra's first large animals, is now the planet's largest example of a new and complex biome, one built on a foundation of a very simple organism. Matkelp, a genus of colonial dinoflagellates, has long since lost any trace of its ancestors' neurotoxic properties. Four million years ago, it was much rarer, not yet having regrown since algal mats collapsed at the onset of the Ice Age. Despite this, a handful of grass species from several different families had already evolved to tolerate the rough environment of this early matkelp ecosystem. This has paid dividends, as new forms of matkelp have expanded this biome by orders of magnitude, creating the world-spanning community seen today.
The Continental Matkelp (Planalgarum expansus) builds colonies that can span hundreds of kilometers. It anchors into the sand at depths of up to five meters, while only reaching about 10cm above the water line. This is possible because it has evolved to secrete an oily mucus that maintains the integrity of the mat even when submerged. As the colony grows, material on the outer edge is gradually pushed out to sea, acting like a biological tectonic plate that drags with it any living or nonliving matter attached to it. Finally, offshore waves and storms begin to tear the algae apart at its margins. Though this destruction is the inevitable fate of every patch of matkelp, the whole mass moves so slowly that any given point can persist for many millennia as it migrates slowly towards its fate. This relative stability is what allows so many plants to live successful lives on the mat, most of which contribute in some way to the survival of the whole.
The seastem group is the most speciose because it was the first to fully colonize Early Arthrocene matkelp habitats. Salinogranus now contains more than 150 species scattered across matkelp coasts worldwide. Its method of reproduction is the main driver behind this endemism-driven diversity; because seastems drop their grains directly below themselves, genes spread from one region to another very slowly. As a group, though, seastems have conquered every locality where matkelp can be found. They provide essential structural support, with their taproots extending deep into the algae. This maintains a connection with the seabed even as the water becomes too deep for the matkelp itself to grow all the way down, while also transporting oxygen to the lower layers. Providing oxygen to their neighbors benefits the seastems by strengthening the very ground on which they grow. Fibrous roots function like blood vessels, creating a fine network that permits algal cells to attach and perform gas exchange. In this way, continental matkelp can grow far thicker than any previous matkelp or red-tide algae - thick enough to support far larger plant life.
The Sailtree (Catenodendron ratiensis) is a species of small chaintree that descends from a beach-dwelling ancestor. About two million years ago, as matkelp was first beginning to form these extensive colonies, it followed the edge of the new "land" many kilometers out over the water. It now grows mostly at the extreme reaches where algae meets open water. It doesn't grow more than three meters high, but its spiral canopy can span five meters and always grows facing out to sea. This was originally an adaptation to catch droplets of water that blew in on the wind; the spray is less salty than the water beneath, so the pseudotree doesn't have to spend as much energy pumping salt out of its cells if it gets most of its water from the air. However, when strong offshore winds batter the matkelp, this growth habit also results in a force that pushes the algal mat towards the shore. If many sailtrees grow in a dense row, this force may be strong enough to prevent the entire structure from drifting away.
Deep in the body of the matkelp, weaving through the sludgy microorganisms alongside seastem fibers, are the thicker, stronger roots of another grass. Seastalks (Thalassosaccharaceae) are one of Apterra's oldest coastal-adapted plant clades, diverging from other skystalks back in the Middle Muricene. They were one of just three skystalk groups to survive into the Arthrocene, outlasting every other genus in the taxon Brevisaccharinae. Despite their success, they were a bit late to the party in terms of joining the matkelp ecosystem, only adapting to live here about a million years ago. Around that time, two different members of the family both independently evolved to suit this biome. One is called the Weaverstalk (Textilorhiza), and it serves a role complimentary to that of seastems. While the latter has deep, anchoring roots, this genus spreads widely but shallowly. Long, tapering roots reach out horizontally up to five meters from each plant, dotted with short, barb-like fibers that hold the amorphous matkelp together. The second type of seastalk is the Pebbleweed (Glareorhiza). Like most skystalks, it hosts nitrogen-fixing bacteria that produce a natural fertilizer for it and its neighbors. Unique among Apterran flora so far, its roots have evolved to form hollow nodules that serve as a perfect home for these microbes. These can reach up to three centimeters wide, and a typical individual has over fifty of them by the time it reaches maturity.
While the pebbleweed grants an essential nutrient to the plants around it, it also has a degree of selfish tendencies. The prokaryotes that perform nitrogen fixation are anaerobic. This means the process is in jeopardy whenever the oxygen-providing seastem roots grow too close. The only way to avoid this is to release chemicals that are toxic to rat-grasses, ensuring the conditions around its nodules remain anoxic. These poisonous alkaloids stunt the growth of many other photosynthesizers, including matkelp itself. This means that the pebbleweed's life cycle is a race against the clock; if it can't reach full size and go to seed within a few months after sprouting, it will erode a hole into its substrate and sink into the water that pools within. Most, however, succeed at fruiting before their time runs out, after which the pebbleweed dies, its leaves, stems, and roots quickly rotting away in the warm, moist environment of the matkelp coast. Only the nodules persist for a time, buried in the goopy mass. No longer producing the secondary metabolites that kept other plants at bay, they now become food or fertilizer for every other species in their ecosystem.
Fruit-bearing grasses also have a presence; depending on the area, up to half a dozen species can coexist, partitioning their niches by relying on different dispersal methods. The largest Fructogranacean in the world is known as the Drunkonut (Thalassofructus pachygramenus). It towers above the algae at up to four meters, yet it remains lighter than chainsails because it has a thin-walled, non-woody stem. It only fruits once every three or four years, bearing a handful of hard, buoyant fruits surrounded by a hard shell. This is derived from the protective skin of the floatseed berry, the group to which it belongs. While it isn't exclusively endemic to the matkelp coast, growing just as abundantly on islands like those of Perinesia, it has another feature that helps it succeed here. Within the tough outer layer, there is a deposit of sugary tissue that ferments as the fifteen-centimeter seed bobs on the waves. By the time it lands on the shore - whether it's one of sand or one of algae - this reaction has built up enough carbon dioxide that the entire drunkonut is ready to burst. When an inquisitive animal comes along, the slightest nudge is enough to make it crack open violently, ejecting the dormant embryo up to thirty feet away. Though many end up falling into the water, a sizable fraction land on the safety of the matkelp, and a lucky minority entangle themselves in the fur or feathers of the creature that disturbed them. If this happens, the seedling can be transported kilometers inland. Here, far from the crashing waves that once carried it, it can live for centuries.
Animal life in this biome includes every major group on Apterra, from rats to birds to (at least where matkelp occurs on Aglirium) several dedicated species of Sentisaurid and Captopsine geckos. The most numerous of all are pill bugs of the genus Thalassohortulanus, a Cursoriarthrid descended from ranger isopods that followed the seastalks when they migrated onto the matkelp. They tend to the woodlouse-grasses that feed them, keeping them free of parasites and ensuring the seeds of competing species don't sprout too close. This is a closer relationship than ranger-type Scansoriarthriforms usually have with their hosts, mostly due to the fact that there aren't any Plague swarms here. This meant they could settle down and establish territories around individual plants, causing a selective pressure that has resulted in them becoming adept gardeners of the matkelp coast.
Mosquitoes, of course, are present in large numbers. The flatness and openness of the terrain makes it a prime hunting ground for all types of Pugilopsids. They feed on Thalassohortulanus woodlice, larger castlebugs that make their homes in the algae, and the young of various vertebrate species. In particular, amphibiinsects find success here, breeding in the short periods after rainfall when tiny pools of freshwater can be found in the hollows of plants. Though still smaller than other members of their family, species like the Matkelp Piperfly (Fodioris epithalassicus) is the largest aerial predator in this biome. In groups, they can even carry off full-grown tiptoe swattermice, which have a significant presence in matkelp environments worldwide.
About a dozen Capiodocine swattermouse species can be found on matkelp rafts across the world, all belonging a genus basal to the rest of their subfamily. This clade originated in southern Ailuropia and descends from a freshwater wetland-dwelling ancestor. The group is characterized by long claws on its forelegs, which it drags through the algae in search of seeds, insects, and pebbleweed nodules. These must grow quickly, as they get worn down constantly by the wiry, abrasive roots they're forced to slice apart every day while the rat forages. A similar behavior can be seen in the Mudtrodder (Latifimbrium epialgarum), a species of mid-sized pillbird descended from the pillplove. Though it has expanded its diet to include nearly all the food sources found here, it retains a particular taste for isopods, feeding on both terrestrial and aquatic species. Its wide, lobed toes that prevent it from sinking into the matkelp also make it a proficient short-distance swimmer, allowing it to chase marine prey. Unlike the rats, the mudtrotter mostly lives within a kilometer or two of the algae's edge, hopping in and out of the water many times per day. It can never travel too far out to sea due to competition with seawis, which continue to rule the offshore waves.