Serina: Urchin-Bellied Maceback

With the evolution of giant, macropredatory calacarnas, and other such predators capable of hunting equally giant shimmershiners, like sea serpents and snippers, there has been a proliferation of more extreme defences amongst these huge marine snarks to counter them. Simply getting larger was infeasible as they risked constant starvation at large sizes and their predators grew larger still and able to hunt them in packs, so they became faster, smarter, more numerous, armed to the teeth, or coated themselves in thick armour.  The macebacks are a subgroup of shimmershiners which have fortified themselves with batteries of bony spines, calluses, and plates on and underneath their skin, growing massive and bulky to present themselves as nigh-impregnable living fortresses. To reduce weight, they have developed air bladders to help counterbalance the mass of this armour when swimming, although most species are bottom-dwelling animals. Unlike the primarily filter-feeding sabrefins, macebacks are largely incapable of great migrations between fertile feeding grounds due to their bulk making long distance swimming strenuous, so they have become comparatively sedentary grazing animals, feeding primarily on aquatic vegetations. Compounding the mass from the armour is the large gut they have developed to digest this, resulting in a much longer and more rotund frame than the sabrefins, as well as a very enlarged proboscis filled with rows of plate-like grinding teeth for macerating plant matter.

Most macebacks are founds in shallow waters, as only these rich sunlit waters provide the lush meadows of marine grasses, seaweed forests, mangrove canals, and alga-encrusted reefs that can support them, and they are able to easily rest on the seabed, but there is one species which survives far out in the middle of the Unbroken Ocean. Here is an oasis in the midst of a functional desert many thousands of miles from anything else habitable to sustained animal life, the unique floating forest biome. This collected mass of sargassum-like algae several hundred miles wide supports a similarly unique ecosystem of ocean life isolated on this shimmering emerald jewel. The largest native denizen of this habitat is the urchin-bellied maceback; although it is one of the smaller maceback species, growing between seven and ten feet in length, and potentially up to six-hundred kilograms, as the floating forest is not large enough to support populations of animals much bigger than that. Aside from some rare vagrant predator species like sea serpents or kraviathans that pass through the area, no carnivore of the region can threaten an adult urchin-belly, and they can reach exceptional ages, possibly over two-centuries, and breed very seldom.

This species likely became isolated in this environment when a maceback shoal followed a temporary current far from their fertile homelands and became stranded, but were able to adapt to the floating forest environment. The urchin-belly is unusual compared to shallow-dwelling maceback species in numerous respects to facilitate a permanently pelagic existence. Their armour is reduced to save weight, as there is no predator large enough to threaten them once full-grown. Their body is slimmer and more flexible to facilitate navigation through the tangled strands of kelp-like seaweed, which they often rest in, as it grows heavily enough in spots that it can support their underwater mass. They lack countershading, as they often spend much of their time swimming upside-down or horizontally browsing the underside of the massive seaweed barges. The adult urchin-bellies are much less socially inclined than normal for macebacks and do not naturally congregate, as their size alone is sufficient to keep most predators at bay and associating in high population densities can easily result in overgrazing, but they retain strong dark-and-light banding for visual recognition from a distance. Although they play a minimal defensive function as adults, the spikes help the urchin-bellies cut through thicker tangles of algae and are occasionally used for intraspecific conflicts, as the animals will ram themselves into one another over mating or foraging disputes, although this is uncommon.

The urchin-bellies can take up to thirty years to reach sexual maturity and often do not mate until they reach their forties, due to their very low mortality rates and very lengthy lifespan. The macebacks are born in litters of two to six, and become independent only within a few months of birth and are radically different in appearance to the adults, being far slimmer, vibrantly green, a thicker coat of spines, and with much larger proportionate fins. Due to their size, adults rarely venture into the thickest tangles of the larger rafts of the floating forest, since it is all too easy to become trapped, but the more slender youngsters can far more easily slip in-between the kelp-like strands. They often shuffle and push their way through the seaweed, rather than swim, congruently similar to the movements of early tetrapods or the distantly related gups. Unable to keep themselves safe from predators through size, juveniles rely much more heavily on camouflage and armour; when threatened, they can quickly ingest water and inflate themselves like a pufferfish, attempting to wedge themselves in the thickets of seaweed, hooking into the vegetation with their spines, while eyespots on their tail fins keep the predator's attention away from their head.

The greatest threat to adult urchin-bellies is actually starvation; during leaner years of sustained unfavourable weather conditions, the size of the floating forest can drastically shrink as the seaweed dies off. This is obviously disastrous for native animal life, but for the large macebacks that require the most food, this can be catastrophic, the entire species' adult population can drop by a third within six months in the worst famines. In typical conditions, the seaweed easily grows back quick enough to deal with their grazing, but in poor years, their grazing can have a detrimental cascading effect, worsening the shrinkage. To help cope with this, macebacks can actually shrink in size by more than a foot in length to reduce resource consumption during years of poor grazing (they grow back to normal once ample food supplies return), and they consume a greater degree of animal matter (usually via scavenging), which normally makes up an inconsequential amount of their diet due to being specialized for digesting vegetation. Macebacks become less socially tolerant during this period, aggressively staking out their feeding grounds to remain as well-fed as they can until their green buoyant rejuvenate themselves.

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(To reiterate, this was a sponsored commission for a species of floating forest maceback, a group which had not been seen up to this point)