A Tour of Littoral Biomes, Part 1: The Tideslopes

It's a cool autumn afternoon on the southern coast of Post-Abeli. This subcontinent, formerly the eastern half of Sub-Abeli, broke off about two million years ago. A combination of rising sea levels and tectonic movement has created a wide chasm between the two sides, separating Post-Abeli from Panapterra completely. Highland areas have, as a result, already begun to see high rates of endemism, as terrestrial plants and animals can no longer interbreed with their mainland cousins. On the coastline, things are quite different. Seeds from far away wash ashore regularly, creating a melting pot of littoral flora originating from around the globe. The margins of this massive island are almost entirely Tideslopes, a type of beach unique to Apterra.

As a result of Apterra's moons orbiting so close to the planet, tides around Post-Abeli are extreme. High tide can be as much as fourteen meters above low tide, causing broad areas of coast to oscillate between fully submersed and totally exposed. Tideslopes form in regions where the land is relatively flat as it approaches the sea. On average, the ground in Post-Abeli rises only thirty centimeters for every kilometer one travels inland. This allows the tide to flow deep into the subcontinent, often traveling over forty kilometers into the lowlands. This wide swath of intertidal land is exceptionally fertile, with nutrients being deposited by freshwater rivers and with each incoming tide. Animals here face a deadly challenge. For those that breathe air, being stranded when the tide rolls in is a death sentence, while aquatic species must avoid becoming beached at all costs. Local plants, unable to flee these cyclical changes, must withstand both extremes of life on the tideslopes, adapting to thrive both when underwater and when high and dry. 

The basket-grasses are by far the most dominant group across the tideslopes, with many being pre-adapted to survive here. In the areas farthest from the sea, Giant Pennant-Grasses (Vexillophyllum) are common. Strong winds that blow in from the ocean batter these hills. The pennant-grasses tower up to four meters high, forming a windbreak that protects inland regions from extreme weather. Beach-barrels (Aestodolius), a descendant of buttress-barrels, are found dotting sandbars closer to the edge of the tideslope, where dunes may shift dramatically from one day to the next. The large, deep rhizomes of the beach-barrel help prevent this, allowing the grass to keep itself anchored. Another genus with a similar strategy is the Littoral Wellstalk (Oramorhiza), whose long, tapering roots can extend many meters below the sand, reaching more stable sediments below.

The vast majority of the tideslope, however, is composed of lower-lying mudflats. Lying just barely above the low-tide line, the waterlogged soil supports Kegweed (Thalassodolius), a type of giant keg-grass with a seawater-resistant root system. Excess salt is expelled by the leaves, forming a crystalline crust that dissolves when the water rises. Because it needs air exposure to remove salt, kegweed can only survive about 20 hours underwater, so the genus is restricted to the tideslopes - any deeper, and seedlings would be submerged even at low tide, dying as their salt levels rose uncontrollably. Older plants can reach as tall as 16 meters, with thick, columnar blades that can measure a meter across at their base. Each leaf has a crescent-shaped cross section, adding strength and preventing it from snapping under its own weight. 

Kegweed meadows are dense, with little space to maneuver between plants. Large animals consequently find great difficulty navigating the tideslopes, facing an ever-present risk of becoming stuck in the thick foliage. Small birds and rodents find more success, weaving through the leaves in search of the plentiful food that lies in every nook and cranny. When the water rises, kiwis and rats of all types cling to the tips of every blade, which may only barely peek above the waves. Yet this is not simply a time to await the safety of low tide; many species use the downtime as an opportunity to find mates. The Mudflat Mouse (Doliosaltus), a member of the swattermouse family, is adept at crossing from one plant to another, using its tail as a rudder to maneuver in midair. The chittering calls of females can be heard across the tideslopes, drawing in potential mates from all around. The ability to jump safely across long distances is a sign of fitness, so females often position themselves in the rare patches of territory where kegweeds are sparse, forcing males to attempt leaps of two meters or more. Whichever can achieve the longest distance will win the right to mate, while the many losers must wait to try another day.

While this drama plays out above the waves, fish cruise through the jungle below the water. The Post-Abelian Grumblegulper (Ichthyogyps postabeliensis), a mid-sized scavenger descended from the wandering grumblegulper, is a common sight here. Any land creatures that fail to escape the rising tide are picked off, as are unlucky mudflat mice that miss their marks. The Estuarine Jaguarfish (Postabeliichthys lutensis) is the top predator, with sleek scales and a thin profile that allows it to weave between the kegweeds easily. Instead of retreating to deeper waters like most tideslope fish, this species makes its home in freshwater rivers when the tide goes out. Despite weighing only thirty kilograms, estuarine jaguarfish are by far the largest animals that travel into the depths of this biome; while bigger oceanic fish may venture into the periphery, they cannot navigate the kegweeds quickly enough to access the landward regions.

Tideslopes are not an exclusively Post-Abelian ecosystem; they can be found on every continent and many smaller islands as well. However, their greatest scale and diversity can be found here. They insulate the young landmass from the outside world, preventing non-endemic species from reaching the Post-Abelian interior. As a result, as the Arthrocene progresses, it's likely that increasingly strange and derived animals will arise on this continent, isolated from the rest of the planet.