Shallow Seas
The oceans of Serina in the early Ultimocene, like the ecosystems of the land, are thriving. Far shallower than Earth's oceans, the seas of Serina rarely reach depths of more than 6,000 feet at their lowest. The average depth of the Serinan ocean just around 2,500 feet in comparison to 12,100 feet for Earth's oceans, and wide continental shelves frequently extend hundreds of miles along coastal shores where the water may be just ten to a couple hundred feet deep, with the sediment occasionally drifting to form sandbars shallow enough for a man to walk upon and small islands that rise out of the water entirely. This demonstrates that the total amount of water on Serina is far less than on Earth. While this means they can be more susceptible to changing climate conditions, overall it results in more fertile seas that are able to support much more life, mile per mile, than Earth's oceans because the sea floor is so much closer to the sunlit surface waters and subsequently mineral nutrients found there are more readily able to dissolve into the water column and be utilized by plants. Microscopic plant growth is denser in the open ocean, plankton levels are higher, and a larger number of animals further up the food chain are able to survive. This has the effect of changing the ocean's color; in the tropics - and in the summer in polar oceans - algae in the open sea grows densely enough that the sea is not so much clear and blue as murky and green, making it difficult for predators to hunt by sight. Yet along the coasts, the waters are crystal clear - and from above more apt to still reflect a bright blue coloration. Why would this be so?
This is because the coastal waters extending up to several hundred miles along continental shelves are often shallow enough to permit sunlight to reach the sea floor. This allows a different but perhaps even more fertile ecosystem to occur - the undersea savannah. Along the shallowest coastal seas across equatorial Serina, sea bamboo continues to flourish, forming marine grasslands that support incredible biodiversity. Wherever this plant can thrive in abundance, it largely out-competes free-living algae in the water and so the water still appears clear and blue - and in a positive feedback loop for the bamboo the better it grows, the less algae can survive, the clearer the water becomes, and the more sunlight reaches the sea floor, letting the bamboo grow even more efficiently.
Little changed since the Thermocene, sea bamboo is a fully aquatic grass that cannot survive out of water. Even so, it still produces small, inconspicuous flowers, the pollen of which is the preferred food of planktants that inadvertently transport some of the granules on their bodies while moving from blossom to blossom and thus serving the fertilize the fields like underwater honeybees. Once fertilized the bamboo will set seed, and it is in this way that Ultimocene forms have developed new adaptations. In order to better distribute their seeds across the sea, some sea bamboo evolves large seed pods which are edible and nutritious - a form of fruit-like organ about as large as an olive - that serve to attract animals to eat it and later deposit the seeds later on some distance from the parent plant.
Unlike most fruits on the land these are not sweet, for few if any fully aquatic animals can readily taste sugar. Rather they are rich in fat and protein and savory in taste and so attract animals that would otherwise feed on animal matter. Large fish glide over the groves and engulf the berries in their jaws, but the bamboo's primary targets to do its distributing are diving seabirds that splash into the water and weave through the vines to collect their unconventional food source. It is these creatures that most efficiently spread their progeny, for after filling their gullets they return to the sky and will have likely traveled miles before they defecate, dropping the undigested seeds of the bamboo far away from its parent colony. While some highly mobile fishes also serve to transport their seeds far and wide, the inevitable result of such generous food provision has also resulted in the adaptation of animals that exploit the bamboo's fruits without spreading them to any worthwhile degree at all. Many crabs, shrimps and other crustaceans merely tear the soft flesh apart and eat it bit by bit on the spot, letting the seeds drop down right into the parent plant's foliage where they are unlikely to take root.
Nonetheless, at least some of the bamboo's seeds are guaranteed to reach a suitable place to germinate, and through a combination of animal dispersal, transport by current and the ability for plants to break apart and take root in new areas vegetatively allows these aquatic plants to form the dominant habitat type along coastal seaswherever the water is shallow enough for sunlight to reach the sea floor and where said substrate is sandy and soft - the bamboo's root systems are poorly suited to colonize rocky sea floor. Rocky areas without soft sediment support a different but comparably biodiverse biome, the snail reef which is made up of small filter-feeding snails. Each just a centimeter in length or less, these tiny molluscs are locked in symbiosis with algae in their tissues which like our coral build large stone structures over countless generations that in turn house countless other sea creatures large and small. Like corals, the snails - which have lost their eyes and all of their tentacles and are little more than a mouth and a shell, are almost invisible by day when they hide away. The reef at this time, formed by their collective shells, resembles instead a varied and multicolored collection of stone spires and monuments. It is only at night that the molluscs release their delicate feather-shaped radulas to sweep the water for plankton - the collective motions of billions of these tiny swaying feathers, which are usually red, make the whole reef at night appear to sprout plumage and dance, like a bird attracting a mate.
Sea bamboo and snail reefs do not compete under most conditions, for while both need shallow, sunlit water near the surface, they need different substrate components to establish. Sea bamboo requires soft sand or mud to put down roots, while wherever the substrate is too coarse for the growth of sea bamboo the slower-growing snail reefs will take hold, particularly far out to sea. They are most abundant as atolls or platforms away from shore along eroded volcanic islands, where sea bamboo cannot grow at all, though they can also take hold in shallow coastal seas by establishing on rocks above the height of the sea bamboo - and it is here that the two communities may directly coexist. Over many years, such small colonies may eventually form a raised stony island that slowly spreads up and out from its nursery stone in a mushroom-like shape above the underwater grassland.
Snail reefs and sea bamboo savannahs are both largely restricted to shallow waters less than a hundred feet deep, though some species of sea bamboo can survive at greater depths thanks to air-filled stems that float toward the surface. Both habitats experience their greatest presence and most vigorous growth occurring in crystal-clear water under thirty feet deep. In equatorial waters too deep to support either of the other communities, another sort of floating reef formed by the photosynthetic jellyfish called chainjellies occurs. Composed of colonies of small, highly reduced jellyfish with algae in their bodies, some species have forgone feeding entirely while others still catch prey that passes by their down-hanging tentacles. Completely at the mercy of wind and tide, they float in scattered patches across all seas as they have since the Pangeacene. These clumps may reach several miles across where the sea is calm, and provide shelter for small groups of fish and other small animals in the open sea, but most frequently are broken into smaller clumps of a few meters around by wind and waves. In especially calm areas however, including inland bays and the secluded interiors of atolls where waves are blocked on all sides by snail reefs, chainjellies - which are capable of asexual budding to reproduce - can cover the ocean's surface completely in a layer of what resembles translucent green bubblewrap. Harmless to larger animals, these jelly colonies pose no threat except to the tiniest plankton, but they are also extremely low in any nutrients. Baitfish nibble them and small invertebrates graze their edges, but as far as larger creatures, only a few birdles - aquatic mucks which resemble sea turtles - consume them to any great extent. It is so that their main limiting factor in much of the ocean is their delicate nature; they are easily torn apart by storms and waves, which normally prevent them from taking over the water's surface.
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Deadwood Floats - A Reef in Miniature
Sea bamboo savannahs, snail reefs and aggregations of chainjellies are the major reef-comparable biomes in the Serinan ocean. On a smaller scale, however, another sort of miniature reef also occurs. Deadwood floats are simply tree trunks or other large portions of wood that are washed into the ocean from inland environments. These floats provide a holdfast for filter-feeding animals that take advantage of the abundant phytoplankton found in Serina's open oceans where otherwise the water is too deep for them to be able to establish.
One of the most common types of creature that colonize deadwood floats are molluscs, particularly bivalves, but also relatives of the reef-building snails called feathertongues. Sharing the same basic anatomy as reef snails, they are blind and have reduced their bodies markedly as adults. They are planktonic and mobile as larvae, able to swim and to see, but as soon as they find a suitable perch to settle down their rudimentary foot secretes a cementing mucous and they become sessile. Their bodies elongate into worm-like stalks up to ten feet in length, ending in a mouth from which projects a large flower-shaped radula covered in feathery projections that serve to filter plankton from the water. They lack a shell but protect their lengthy bodies with small, sharp scales. The feathertongue is able to retract its radula rapidly into its armored body if it is threatened by a nibbling predator but is otherwise capable of little movement on its own once it adheres to a float, instead drifting wherever the currents take its host log and passively filtering food from the water. Large logs are usually colonized by many feathertongues all vying for a place to settle near the surface and pulsating their tongues to snare food as if in a competitive dance. Feathertongues are most frequently observed on deadwood floats, but different types also colonize snail reefs and rocky coasts anywhere food can be filtered.
In addition to such sedentary animals, small fishes and crustaceans also find refuge on deadwood floats and many species can spend generations along a single particularly large one, only dispersing when one float passes another in the open sea. The floats thus operate as miniature free-floating reefs. The largest ones, heavily colonized as they are by molluscs, may be comparable to a large filter-feeding vertebrate in how much plankton they can consume in a given day.
In the seas of Earth today, driftwood doesn't last long enough to develop such a diverse colony of hitchhikers. Shipworms, a sort of specialized wood-eating clam, destroy wood rapidly and prevent the formation of a similar floating community as occurs on Serina. This is, however, a relatively recent change. In the Jurassic era before any marine life had evolved which could exploit the hard-to-access nutrition in the cellulose of wood, fossils show that free-floating driftwood was often colonized by filter-feeding crinoids that behaved in very much the same way. No comparable creature to the shipworm has yet specialized to feed on driftwood on Serina, still a much younger world, and so such material lasts decades floating on the high seas and at times centuries before it is finally decomposed. Though at times a deadwood float will wash ashore, stranding its inhabitants, many last for years without ever washing ashore, allowing this novel ecosystem in miniature to continue to flourish.