Shallow Sea Grazers
Serina’s oceans have taken millions of years to recover from the events of the end of the Mid-Ultimocene, but now they are stable and host to new forms of life that have taken the torch, risen from the disaster, and gone on in the story of life. No longer running toxic with acid and anoxic and deadly to life, the oceans of the early hothouse are healthy again - but gone are the days of the coast-to-coast sunlit shallows, full of higher plants. Now most of the ocean is again too deep for sunlight to reach the seafloor in sufficient levels to sustain such communities, and the new foundation of the ecosystem is again free-floating phytoplankton. Only along shallow coastal shelves close to shore do similar ecosystems to the once global undersea meadows still survive, yet they have formed anew from different colonist plants than before, some from the land. Aquatic puffgrasses have spread rapidly through sandy sediments, yet have changed little and still produce flowers above the surface that only occasionally manage to set seed - they primarily grow by spreading via runners. Rocky coasts without loose substrate in which to send down roots remain the domain of various large kelp-like algaes, now large and leaf-like but descended from simple colonial species that survived the death of the ocean age in very shallow coastal tidepools and similar refugia.
95% of all living things from the seas of the ocean age died out at its end, and the sea’s plants were just the beginning. Also gone are the overwhelming majority of marine ray-finned fishes as well as the oceanic ‘fishes’ which had evolved from metamorph birds. Species of both groups which did survive were much more likely to be riverine, yet there were few rivers still flowing on Serinarcta and none on Serinaustra at that time, which means freshwater life from the now sunken Meridian Islands - those which could rapidly adapt to raising salinity, at least - were the most successful. Jetguppies have gone extinct, for no species ever infiltrated freshwater refuges, and there join most of Serina’s most derived fish clades, leaving behind mainly ”living fossil” generalist groups resembling cichlids and guppies. These primitive cyprinodonts that still look much like their ancestors have re-entered the oceans along with small shrimp, snails, and crabs and functionally, restarted the evolution of the aquatic biosphere from its original starting point… with one major exception.
Snarks, a clade of extremely derived, intelligent, free-swimming snails have come through this period in far better success than the fishes due to their benthic lifestyles, tolerance for low oxygen as a result of burrowing abilities, and increased parental investment in their offspring instead of producing small vulnerable larvae. Now as the hothouse era truly begins, this alien and bizarre animal group comes into their own as the rest of the ocean is all but reset around them.
But with the oceans so badly depleted a few million years ago, the beginning of the hothouse age has also allowed for the evolution of new aquatic animals from the land, just as occurred after Serina’s last similar oceanic extinction event at the Thermocene-Pangeacene boundary. Some of the first animals to take the plunge are those which had been here before. Surfscooters, a coastal and riverine dolfinch species from the Meridian Islands, were the sole survivor of their entire lineage. Fleeing the breakdown of the marine ecosystem by retreating to newly-formed meltwater rivers in Serinaustra, some have now returned to their ancestral home in the ocean while others remain inland. Those at sea are joined by others: some burdles have also become semi-aquatic again almost immediately after reaching Serinaustra, for even though they had adapted to walk upon the land, they retained the ability to swim well and ability to eat a variety of food sources.
The seastrider is a man-sized, omnivorous - but mostly plant-eating - burdle that now spends most its time in shallow coastal waters where it grazes on vegetation and whatever small animals it can incidentally catch. Though it can swim, it has not merely re-adopted the sea turtle-like shape of its ancestors but has retained the stronger walking legs of the burrowing burdle. With thick, heavy bones and a hard defensive skin, it is naturally not very buoyant and so is able to walk at the bottom of the seafloor seafloor, propped up on the tips of its forelimbs, and graze at leisure without floating away - only with some difficulty can it swim to the surface to breathe, and this limits it to very shallow regions right near the shores of the southern continent. A tendency to consume small rocks to aid in breaking down its plant-based diet and compensate for a short digestive system still not very well adapted to eat that has a secondary advantage of providing ballast, and seastriders can adjust their buoyancy by eating or regurgitating stones to make themselves even heavier to provide stability in fast-moving currents and turbulent water. Like their ancestors, seastrider females must return to shore to lay their eggs and do so in burrows upland and away from the shore where they are safe from predators and from flooding; here females still continue to incubate and guard their eggs until they hatch, when the mother and her young return to the sea together, protecting them from predator birds along the way. She does not provide additional care after their initial escort to the water, however, and the family then parts ways. The babies swim better than adults and are faster, relying on this to flee aquatic enemies. Though males too can come ashore, they do so only to bask and warm themselves, and rarely if ever walk far out of water. With the coastal waters warm and comfortable, even basking is optional, and many male seastriders grow a thick growth of green algae over their light-colored backs from spending so much time grazing in these shallow sunlit waters.
The largest living dolfinch is now the porpedo, a small dolphin-like omnivore with substantial sexual dimorphism, which lives in sunlit coastal waters along both the northern and southern continents. These porplets, once amphibious, have gone the opposite way of the little platyporps and are again completely aquatic - males are too large and ungainly to come ashore and while females can do so, they have little reason as here are few predators around which can harm their young and so usually they give birth at sea. Porpedos are very streamlined and lack the thick blubber of ocean age dolfinches as the water in their coastal environment is warm, lending them a torpedo-like shape from which their name is derived.
These dolfinches are extremely gregarious and live in large pods, which help protect one another from threats, but mostly seem to live their lives to have fun. Adults have few significant enemies, and so with their numbers protecting their young, they can spend their days frolicking. Individuals frequently synchronize leaps from the water and engage in innovative games, using their intelligence as much for amusement as for actually seeking out food.
Male porpedos are larger and much more colorful than females, up to eight feet long and 325 pounds, and have evolved bright display markings to impress potential partners. They serve no direct role in finding food for the young, but do protect them. and as adults tend to live in their own loose social hierarchies nearby but not directly among more stable, matriarchal female pods. Females, which weigh just 150 pounds and measure five to six feet in length, are devoted parents and even unrelated adults may cooperate to feed and protect the juveniles, while males spend most of their time goofing off on the perimeter of their herds, competing - not usually aggressively - with one another to win female attention. Yet if enemies approach the females and the young, it is the males which will be the first to confront it, using their larger size and intimidatingly heavy beaks to attack and drive away the threat.
Females and males, especially when young, interact freely but do not compete significantly for food resources, which may be why the social pecking order of each sex is largely separate. Nearly all of their diet of the female is vegetable, mainly comprised of sea-puffgrass grazed in shallow coastal waters, while the males are significantly more omnivorous. Being up to twice as large, they also use their sturdy chewing jaws and well-developed tooth battery (situated upon the tongue and upper jaw, in typical viva fashion) to crush shelled molluscs found by diving to deeper depths off the coastal shelves into more turbulent, rocky waters below. Though females also consume smaller molluscs and soft-bodied worms, they are generally unable to break open the largest bivalves or snails, nor dive deep enough to reach them, and so will often rely on males to provide them these nutrient-rich supplemental meals. Males which provide the most nutritious snacks to potential partners score the best chance of being allowed to mate with her. Like earth dolphins, male porpedos sometimes may try to force themselves on females uninterested in their advances, but the female's social groups are strong and will often come together to protect their own from such harassers, limiting the behavior and leaving mate choice largely up to the female.