Undersea Savannahs, Planktants, Snail Reefs, and Chainjelly Colonies

Across tropical seas worldwide lie some of Serina's most ecologically diverse and productive biomes: aquatic grasslands, stony reefs, and a unique new biome produced by floating colonies of photosynthetic jellyfish known as chainjellies.

The Sea bamboos which were the only members of their once abundant lineage to come through the end of the Thermocene have come to dominate the shallow seas of the Pangeacene to a scale never before seen, where they cover the sea floor in a thick, green underwater grassland rife complete with its own pollinators, grazers, and predators. Whereas on Earth similar biomes - inhabited by the seagrasses - can develop only on coastal shallows, on Serina where the sea is typically substantially shallower, such plants can grow successfully many hundreds of miles out at sea, where the water may still be only as deep as 200 feet, but still thrive best in slightly more coastal waters less than 100 feet in depth.

Tropical oceans are extremely abundant in light and warmth, but normally are severely lacking in nutrients and at any distance from shore can often be considered virtual deserts. This has not stopped the expansion of the sea grass savannah, however, which survives and even thrives in these warm, calm seas through an extremely efficient recycling of organic material. As the earliest beds of sea bamboo spread out from shallow coasts, they brought with them biological communities of fishes and small invertebrates which fed upon them. Such creatures, living within the tangles of the bamboo, ensure that no organic matter is wasted; the nutrients in the foliage which is eaten are promptly returned to the grasslands in the form of animals' waste products and rapidly re-absorbed by the rest of the plants. Furthermore, the small organisms living in the grasslands draw in larger shoals of predators from the open sea in search of food which in turn deliver more nutrients into the system. Sea bamboo habitats are thus extraordinarily fertile, providing islands of refuge between areas of deeper water that may in isolated places extend down to depths of 6,000 feet - still far shallower than the larger seas of Earth, but much too deep for sunlight to penetrate and allow the growth of the bamboo.

Sea bamboo grows as a spreading rhizome across the sea floor, which produces leafy canes which can vary in growth habit from just a few inches in height to vines that tangle together and float in mats on the surface hundreds of feet in length, providing welcome cover to thousands of species of fish, mollusks, crustaceans, marine insects and neotenic birds. Shrimp, crabs, copepods and snails feed on either the leaves of the grass itself or on the layer of microorganisms that cover them - the "biofilm" - and in turn provide food for larger animals. Enormous dugong-like descendants of the water snuffle glide through the glades, some feeding on buried mollusk prey they unearth with an electrosensitive snout and others browsing the grass itself with a flexible tentacled upper lip. Nimble sea turtle-like descendants of mucks "fly" over the "plains" with wing-like flippers, cropping the bamboo with their beaks, and returning to shore only to lay their eggs in the sand. Shoals of fish seek shelter between the tangles of foliage from predators in the form of barracuda-like ray-finned fishes which descend from the small surviving saltwater minnows of the Thermocene-Pangeacene boundary, agile seal-tribbets which have taken to the water from a dog-like ancestor, and sea birds that dive below the surface like missiles in their pursuit of prey. Though now wholly aquatic, sea bamboo is still a flowering plant and reproduces by seed in addition to spreading vegetative growth. To do so, its inconspicuous flowers, almost hidden in the axils of its leaves, are normally pollinated by the currents, but in the absence of strong water movement also benefit from the actions aquatic invertebrates act like bees on land, attracted to the pollen as a food source but inadvertently transferring some of it between the flowers as they swim from flower to flower. Species which the undersea savannah recruits as its pollinators include copepods, shrimp, crab larvae, and even a family of tiny, entirely marine ants which have become efficient free-swimmers with paddle-like limbs and feathery hind legs - actually masses of thousands of tiny cillia branching off the exoskeleton - that serve as gills to absorb oxygen from the water with no need to surface for air.

Known as planktants, these species are the direct descendant of the sea ants of the Thermocene, which survived the mass extinction event along with the sea bamboos they depended on in a handful of shallow isolated coastal estuaries and pockets of oxygenated water that persisted throughout the stagnant ocean. As the seas recovered in the Pangeacene, the surviving ants diversified substantially and spread across the tropical seas. Some species of planktant are solitary while others are social, but none of the truly aquatic gilled ants are eusocial; rather, flashing along through the water in search of edible tidbits, female carry broods of their eggs under their own bodies which will hatch as miniature versions of themselves. They have abandoned their larval stages completely, giving birth to fully-formed nymph-like young in a reversal of their ancestral metamorphosis and removing the need for a nest at all. In the case of the female, the only changes the nymph will undergo after birth involve an increase in size over several molts. In males, however, after a short period of life as an aquatic animal much like the female, they molt into a very different form which, though still gilled, also sports folded, stunted wings. The young males then rise to the surface in immense numbers to complete a second molt almost immediately where their wings fully unfold. Now a mature adult, the male of the species retains gills on its hind legs but has now also developed the open spiracles of its ancestors which are adapted to atmospheric respiration. It thus has a short period of time where it can breathe both under and below the water, giving the males time to surface and take flight. After just an hour in the air, however, its gills dry and are shed and it will now drown if it falls back in the water and cannot escape quickly. Living for just a day or two, the male does not feed as an adult and dedicates its entire life to depositing microscopic pouches of sperm into the water, which the planktonic females will locate by scent and collect to fertilize their eggs and start a new generation.

above: a typical female planktant. Most planktants are semi-transparent, with reflective exoskeletons that give them a bluish cast and serve to hide their form in open water. Though a few forms may reach up to an inch in length, the vast majority are smaller than one centimeter and some just a few millimeters in size. In general, they are inconspicuous but super abundant along reefs and sea bamboo grasslands, where they feed on phytoplankton and small organic particles, such as pollen. Locomotion occurs with flaps of the second and hind leg pairs, while the first pair have adapted into grasping arms to procure food. The gills can be seen as hair-like filaments growing out from the inside of the hindmost legs.

The antennae of planktants are very long in order to better sense water vibration to avoid predators and improve their sense of smell.

Illustration by TrollMans

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The dispersal of flying males is dependent entirely on weather conditions, and if the males molt at the wrong time, when the water is too turbulent as a result of stormy weather, an entire generation can be lost. This is not a deadly blow to the planktants in general, though, for the females can successfully reproduce through parthenogenisis, cloning themselves for many generations in the absence of a male and biding their time until a flush of males can successfully take flight and renew the genetic diversity of the population. Though this is obviously ideal for keeping the population genetically fit, some species have nonetheless emerged in which males seem not to exist at all, whose lines can be traced back entirely through cloned females for at least the past 500,000 years. In stable environmental conditions, as have returned in the Pangeacene, it is not always necessary to be capable of rapid change to survive, though as soon as conditions change markedly it is likely these species will be displaced by those which still use sexual reproduction to mix and match their genes and stand a better chance of adapting.

Though the bamboo ant trees which once dominated Serina on land are now gone and all but forgotten, as long as the planktants are still swimming about, pollinating the flowers of the sea bamboo grasslands, the ancient ant/bamboo symbiosis which has been occurring on Serina for more than two hundred million years lives on.

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Where waters are too deep for sea bamboo to take root, another exotic ecosystem thrives in the tropical seas of the Pangeacene. Formed by photosynthetic box jellyfish only a few centimeters in size which have evolved to link together in floating colonies, they produce a biome which is, in effect, a floating reef that can extend for tens of miles. The jellies, which live in symbiosis with algae in their tissues, either feed only via passive filtration or have lost their digestive systems altogether, relying solely on their partners to provide them energy. Most jelly reefs develop as a thin, single layer of jellies, which an appearance a sheet of bubble wrap draped on the surface of the water, each bell containing a pocket of air for bouyancy. In very ideal conditions, however, the colonies may stack upon each other to depths of several inches. Fish shelter underneath the colonies for shelter from sea birds, feeding on microorganisms living within the colony or on the jellies themselves. Numerous large grazers feed on the edges, taking a significant toll on the reef and limiting its spread. Each individual jelly is neotenic, retaining the ability to bud off clones of itself even in its adult medusa state, and this is the manner in which the colony reproduces most often, able to to double itself in size in seven days under good conditions but functionally growing much less as it is constantly under assault from the actions of predatory fishes, sea mucks and invertebrates that gain sustenance by feeding on the reef. Flotsam in the form of floating seaweed, driftwood and feathers collect in the reefs over time, providing further hiding places and food sources for aquatic life.

Because these "reefs" are entirely planktonic and at the mercy of the currents, they tend to collect in patches where numerous ocean currents meet, developing occasionally into massive refuges a hundred miles or more long in the middle of the ocean, like an oasis in the desert for marine life. They also readily wash ashore, sometimes in huge numbers, where the bells of the jellies deflate and leave the beach covered, for a time, in what appears to be thousands of gallons of snot. Unlike other jelly species, however, these beached colonies lack any harmful stings and are entirely harmless to the touch.

The last of Serina's reef-like habitats is a rocky one formed by the distant descendants of freshwater pond snails which have largely adapted to fill the niches vacated following the nearly complete extinction of most bivalve groups at the end of the Thermocene. Some have then gone on and evolved into colonial filter-feeders much like Earth's coral and Serina's extinct stone-hydra before them. Each individual snail is typically only a few millimeters in length at adulthood and begins life as a planktonic, free-swimming larvae which immediately begins searching out a place to anchor itself with sticky threads and become sessile, their holdfasts eventually hardening into cement and locking them forever in their chosen location. Over time, large numbers of such snails will amass along the most favorable places, and as new generations adhere themselves to the shells of those which colonized the rock before them, over many centuries enormous stone reefs develop. Reef snails presumably evolved along rocky coasts, where they filtered food from turbulent crashing waters like clams, but today forms have also adapted to calmer tropical waters as a result of mutually beneficial symbiosis with algae carried in their tissues, as most reef organisms have evolved to do many times over history, both on Earth and Serina, and it is these species in their varied forms which produce the most notable and remarkable reefs home to countless colorful fishes and other animals. Like the chainjellies, such a relationship allows the reef snails to survive in nutrient-poor tropical waters. Nonetheless, the snails retain their digestive systems, emerging from their calcium shelters at night and extending feathery fan-shaped radulas into the water column to capture microscoptic plankton.