A tidally-locked life-bearing world

Image from Steve Bowers

A view of Zakdakha from orbit. The low level winds converge on the substellar point, and higher level winds return in the opposite direction.

Image from Worldtree

Climate and Geography

Biology

• Dark flora are best known by their array of secondary pigments used to absorb red and infrared light and protect against the higher UV output of the local star. As a result, they appear almost black to standard eyes, although if viewed under artificially bright light, the green tint is apparent. The dark flora are today only found on land, and have never evolved a vascular system, so they are therefore limited in size. They commonly take moss- and lichen-like forms, although a few larger bushy forms also exist, growing new living tissue on a dead woodlike core that provides structure and height. Dark flora dominate the scrublands and savannah, but are only rarely found in the stormforests as they prefer bright light.

• Root flora dominate the large plant-like niches on the planet, as well as many smaller floral niches. They are able to develop extensive root/hyphae systems, for which they are named, and most members have a vascular system as well. Zakdakha's equivalent to fungi evolved multiple times within this group, consisting of a root system with no above-ground portion except a flower or fruit. Many other root flora have evolved flower-like structures as well. To protect against UV, root flora also make use of secondary pigments, which vary by subclade. The largest treelike organisms usually have purple or dark brown foliage, as do many smaller and understory flora. However, understory flora may also be green, olive, or yellowish in color. As photosynthesizers, root flora dominate the stormforests and are the larger flora in the savannahs, and many aquatic forms are reefbuilders; the saprotophic fungi-like forms may be found anywhere.

• Kinetotrophs are a clade of sessile, floral organisms that power their metabolism and fix carbon using the kinetic energy of wind, water currents, river flow, and fauna movement. Specialized cells in the tissues subject to mechanical forces (e.g. at the base of wind-catching 'leaves') use this varying pressure to push hydrogen cations into and out of arrays of membrane-bound organelles by osmosis. This spins enzymes that build the molecule that constitutes the local biota's cellular energy currency. Most kinetotrophs have a vascular system. They are found across most ecosystems, but are especially prominent near the terminator and in the dark oases, where they form entire groves of treelike wind-harvesting organisms.

• Sea vases are ancestrally radially symmetric organisms that are exclusive to marine environments. These generally feed similarly to Earth's sea sponges, and some resemble them, but usually have a straighter and more symmetrical body. Others bear numerous spikes to discourage predators. An innovation of one sea vase subclade are long, thin, sticky tentacles which are instead used to filter feed and which can capture larger prey.

• Fauna occupy the motile animal-like niches, and are found all across the planet. Many body plans as well as intermediate forms have been lost in past severe mass extinctions. Extant fauna are traditionally divided into twelve phyla. Three phyla are most famous and prominent; in addition to these are nine lesser-known phyla, which come in a variety of wormlike, sluglike, and filter feeding forms. Two of these latter phyla were far more diverse in the past, but survive only as relicts today.

Biomes

Stormforest

• The fobek is the largest non-aquatic faunal organism on the planet, a land anemone that can grow up to 4 meters in diameter. Like other members of its family, they mimic the flowers of purple trees, which are borne near the ground. They have a narrow base, atop which are nine turquoise petals surrounding hundreds or thousands of small tendrils, which fade from light blue at the base to deep blue at the tip. The tendrils capture prey (typically thirps that feed on nectar from flowers, and thus instinctively seek out round, colorful blooms of any size). Unlike its relatives, the fobek never stops growing; it bears larger and larger sets of epitokes every 47.3 standard days until it eventually grows too large to sustain its bulk and dies. The organism was named by the first ground explorers of the planet, a group of vecs who were reminded of a glass sculpture at the center of their home habitat.

• False fruits are a unique genus of kinetotrophs that grow on the forest floor in deep shade, such as under logs or in hollowed-out trees, where they cannot be outcompeted by photosynthetic flora. They consist of a round, bulbous "fruit" at their center, surrounded by an irregularly-shaped region of short brown paddle-like tendrils attached to their base by a hinge where the energy-generating cells are. The "fruit" mimicks the fallen fruit of the brown trees, and is usually red or orange. Unlike real fruit, however, it is hard, made of tough cellulose reinforced with minerals from the soil. It attracts frugivorous ground thirps, who approach and attempt to eat the fruit, sometimes circling it to try again, and eventually walk away. In so doing, they press and release the tendrils, thus feeding the false fruit.

Mountains

• Cliff vines are a specialized family of root flora with purple leaves that grow on the sun-facing sides of mountains and cliffs. The stems wind around any nearby protrusions and are attached to the rock, transporting water and nutrients from rain and puddles across the organism. Purple leaves, about 50 cm across, grow from the vines, completely covering the rock face. In the shaded region dwell various endemic thirp species that feed on the vines' fluids and would overheat in the direct sunlight.

Savannah

• Terracing Thirps are a genus of large ground thirps, about the size of an Earth mouse. Their front limbs have evolved into a pair of shovel-like appendages to carry and push soil. Terracing Thirps are eusocial, with up to 500 individuals living in mound-like nests of hardened mud. They derive their name from their habit of damming small creeks with walls of mud hardened with a saliva-derived waterproof cement, creating a series of terraced pools within which live aquatic flora that they eat. In some places, the activity of Terracing Thirp colonies has converted many kilometers of streams into small terraced ponds with small cascading waterfalls. They are effective ecosystem engineers, and these terraces are a refuge for many species of flora and fauna.

Scrubland

• Bur moss is an unusual form of mixotrophic dark flora. Since land anemones cope poorly with the climate of the scrublands, thirp-predatory niches are available to other kinds of organisms. While the bur moss gains energy from photosynthesis like other dark flora, it also traps and dissolves thirps that walk on it. Its tiny leaves have evolved into hooks on which the thirp's legs, wings, and mouthparts can get stuck. While many thirps get away once they feel an appendage is stuck, others end up ensnaring themselves even more firmly. Struggling against the hooks triggers digestive juices to be released, which dissolves the thirp into nutrients to be consumed by the bur moss.

Reef

• Maypoles are members of the sea vase kingdom. They have a flat, wide base up to 40 cm across and a narrow, tall central pole up to 50 cm tall, with a bulb on top. Between the bulb and the edge of the base run hundreds of thin, sticky tentacles that are extruded from the bulb and consumed by the base. These tentacles engage in filter-feeding and also capture the occasional unwary swimming worm or small nektochiton.

Dark Oases

• Galaxy trees are a species of kinetotroph endemic to a single volcanic hotspot on Szarinn. Like their close relatives, they consist of a single tall trunk with two rows of brown, wind-catching leaves that run the full height of the trunk, one on each side, and small flower-like reproductive organs at the top. The galaxy tree is the tallest organism of its oasis and dominates the landscape, reaching up to 3 meters in height. Its most well-known trait, however, is the method it uses to attract pollinating thirps. At the very top, the trunk widens into a disk nearly a meter across. When the tree periodically flowers, the disk develops numerous bioluminescent blue millimeter-sized spots. This lights up the woods and attracts flying thirps to pollinate the organism.

• Scald thirps live in many oasis hot springs and are extremophiles, living in water up to 55 degrees C. They are symbiotic with chemosynthetic microbes in their tissues that live off volcanic hydrogen sulfide in the water, much like Riftia tube worms on Earth. As a result, they do not need to consume food. Their six legs have evolved into paddles they use to infrequently propel themselves in the water, while their mouth tentacles have become intricately branched yellow plumes that hold the symbiotic microbes in their tissues. The scald thirp's body is about 8 millimeters long, while the width of the plumes is nearly 2 centimeters.

Dark Ocean

• Turtlesquids are a diverse and widespread family of nektochitons in Zakdakhan pelagic waters. Their shell pieces have fused into a single solid carapace, covering both the top and bottom of the body with openings at the front and back. At the front, two camera-type eyes sit on either side of two spike-tipped tentacles. Normally retracted into the shell, these tentacles can be launched forward to grab prey, which is then brought to the mouth underneath the tentacles. The organism is propelled through the water by two fins at its rear, which beat individually in sync, moving through the water like a bird's wings. The firefin is often considered the most visually striking turtlesquid and is found in the nightside open ocean during its mating season. Its carapace grows to 46 cm long. Its common name is derived from its spots of bioluminescence along the edges of its fins, glowing red at the base tapering through orange to yellow at the tips. The brightness of this display serves as a signal of fitness and is involved in mate choice.

Hydrothermal Vents

• Vent carpets are several species of derived nektochiton that dominate three hydrothermal vent fields on the planet's nightside. They have lost their shell and their ancestors' ability to swim, and have returned to creeping over the substrate. Their white, flat bodies are up to 10 centimeters long and covered by what appears to be short fur, which is in fact thin extensions of their skin. Like certain Earth crabs, this 'fur' is inhabited by chemosynthetic microbes that grow readily on it and feed on volcanic chemicals in the water. The vent carpets in turn consume the microbes. The carpets are unable to reach their own backs with their mouths; instead, they rely on one another, each carpet grazing from its neighbors' backs.

Geological History

• 3590 mya: Zakdakha finishes forming out of rocky material in a temperate orbit around Tayaksekal. The planet's crust cracks into separate plates and begins plate tectonics. The planet rotates freely for billions of years to come, only very gradually slowing down at first.

• 1393 mya: Water-splitting photosynthesizing microbes evolve a nucleus, giving rise to the most widespread eukaryote-analog group on the planet. These oxygen-producing organisms are highly adaptable and spread widely across the planet, oxidizing it rapidly.

• 1062 mya: The first sea vases evolve macroscopic sizes, making Zakdakha a garden world. Fauna and dark flora evolve shortly thereafter and begin to spread across the oceans and on the land respectively. The planet's rotation has begun to significantly slow down by this point, taking nearly 70 hours to complete a day-night cycle.

• 920 mya: Root flora evolve, and other kingdoms continue to diversify. A faunal phylum resembling a slug wearing a sea urchin begins to colonize land and graze on flora there. The planet's rotation continues to slow down at a greater and greater rate, with hours added every few million years. The resulting climate instability causes high rates of extinction but also spurs on diversification.

• 814 mya: Zakdakha becomes completely tidally locked. The nightside forms a permanent ice cap. These changes trigger a severe mass extinction, nearly wiping out all four multicellular kingdoms that have yet evolved. Slug-urchins go extinct.

• 758 mya: The penultimate supercontinent forms, assembling on the dayside under the substellar point. The planet undergoes an ice age, locking much of the ocean away into ice near the terminator. The substellar continent, meanwhile, is largely covered in desert; complex life is largely restricted to an annulus between the desert and the terminator. The harsh conditions however spur on evolution, with the first kinetotrophs apparently evolving during this time. New faunal phyla have colonized the land, including a poorly-known soft-bodied tentacle-bearing group and a worm-like organism with articulated armor.

• 610 mya: The dayside supercontinent begins to break up. Several additional faunal phyla colonize the land, including a caterpillar-like organism, and groups separated on different continents diverge from each other.

• 440 mya: The beginning of a 50-million-year period known by paleontologists as the Zakdakhan Zenith. The planet's biodiversity reaches its highest point in known history, thanks to a warm climate, favorable continental arrangement, abundant rainforests and shallow seas, and a long prior period of stability enabling continuous diversification. The caterpillar-like organism from nearly 200 million years prior has evolved into a dominant group of megafauna, with an internal skeleton and a wide array of forms. This clade's 'bones' are made of a hard organic polymer with only minor mineralization in some larger organisms; as a result the bones rot easily and the clade's fossil record is patchy.

• 337 mya: The planet's most recent supercontinent assembles on the nightside. The resulting scarce dayside land area (only some small islands) and lower oxygen levels (from less floral biomass), together with a hot climate, cause another mass extinction. It wipes out all endoskeletal fauna, as well as most floral phyla and all fauna above a few grams in mass.

• 215 mya: The nightside supercontinent begins to break up. Greater amounts of land gradually begin to return to the dayside, driving a large range expansion and diversification among species of the floral kingdoms. Modern vascular flora evolve. Nektochitons diversify and fill active aquatic niches, displacing polychaete-like relatives of the land anemones.

• 120 mya: The first modern stormforests form as more and more land drifts further into the dayside. All three major modern faunal phyla launch invasions of land within 20 million years of each other. Thirp ancestors resemble a slug with tube feet, while land anemone ancestors still resemble polychaete worms. However, much like the ancestors of tunicates on Earth, they develop a new mostly-sessile life phase, evidently co-evolving with the thirps. A nektochiton clade also moves onto land, though remains restricted to moist environments.

• 33 mya: Period of numerous volcanic eruptions causes the most recent mass extinction. Land-based nektochitons go extinct, as do stem-groups of modern thirps, and various other groups. Flora and fauna closely resemble modern biota.

• 0.007 mya: Terragens reach Zakdakha, transplanting its biota far and wide.

• Garden Worlds
• Vesperian Type Worlds
• Yanqiu