Siluros
Splitting from their ancestor, the siluros have diversified and spread throughout the oceans of Sagan IV. Inhabiting the dark depths, they are innumerable in both species and numbers, having flourished over the past several thousand years due to the relative lack of competition for their niche. With time, they have also been incorporated into the diets of various larger predators, as well as the various parasites that also inhabit their environments. Most species live either in the Midnight or Twilight zones, with more specialized lineages inhabiting the Abyssal zones, as well as the various sea caves, cold seeps, and deep sea vents. Because of this great diversification, they have become an incredibly common sight in the waters of Sagan IV.
While anatomically they are very similar to the common siluro, a fairly new evolutionary feature has become incredibly widespread in this genus. The production of oil is performed within an organ that evolved very early on in this genus, now as an oil bladder. This oil helps the siluro to remain buoyant in the water column, without the threat of being damaged by the extreme pressure of the ocean depths. Oil production varies depending on the species, with those that live closer to the surface producing much less of it compared to those that dwell within the abyss.
Like their kin, siluros must molt periodically in order to grow. Their boney plates are covered in a layer of keratin, separated from each other by a thin layer of tissue and blood vessels that supplies oxygen and nutrients. As the siluros mature and grow, the keratin portion of the plates will routinely be shed, one at a time, exposing the underparts for a brief period of time before the new, larger keratin plate grows in. Discarded plates litter the seafloor, and are broken down by various scavengers and decomposers. Other times, these plates make their way onto beaches, where they are then bleached under the blinding glare of the midday sun.
The number of plates present on adult siluros tend to be stable at around five of them, just as it was with their ancestors like the common siluro. Some lineages, however, deviate from that number, based on a combination environmental pressures and exposure to predation. Some have had one or more plates fuse together, creating a sturdier, thicker exoskeleton, but at the cost of their overall flexibility. Others lineages, meanwhile, have had their plates split even further, increasing their flexibility while swimming and turning, but leaving larger gaps in their bony exoskeletons that parasites and such can exploit. Overall, however, such lineages as these are considered the exceptions to the rules, and are not particularly common, despite evolving independently of one another multiple times.
Just as with the common siluro, broadcast spawning is the most common form of reproduction in this genus. Parents show no care to their young whose eggs are dispersed into the ocean currents. When they hatch after a few days, the larvae within will float amongst the zooplankton for several months. As they grow and mature, they will eventually begin to settle into the depths and take on their adult lifestyles.
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Example species, clockwise
Ruborosarmis communialudo: A species that is representative of the most common basal form of this genus, most open water siluro resemble this. They are passive filter-feeders, sucking in water that is then expelled through the gills, while bits of organic matter and microorganisms are ingested. Species such as this will often head towards the surface during the night, so as to exploit the more plentiful food sources that are present there.
Ruborosarmis paulopiscis: A small, primarily scavenging species, those like this lineage often dwell around Cold Seeps. Feeding on corpses that either fall down there or which were poisoned by the seeps themselves, siluro like these have very slow metabolisms, as food is not always common. More numerous spikes adorn their boney exoskeletons, which help to protect them from potential predators that might otherwise swallow them whole.
Ruborosarmis caudateneris: One of the larger, bulkier species native to the waters off Fermi, this and similar species have evolved a unique symbiotic relationship with the glow detritis, which grants them bioluminescence. Whereas these microbes would often merely be passed though the bodies of scavengers, in this case they instead are able to take up residence within special patches of flesh along the tail (and bodies, in some cases). They are fed on nutrients from the bodies of their host, and in return they are used as a form of communication between siluros of the same species. However, these microbes are not passed on from parent to offspring, so newborn siluro of these lineages must consume glow detritis if they are to develop these bioluminescent patterns as well.