Sanguine o' Spheres

Over the course of millions of years, Gilltails and their kin have been one of the most common forms of aquatic life on Sagan IV. Found in nearly every body of water, from freshwater rivers to the inky black darkness of the deep ocean, they have been a success story, even if they've rarely gained access to the title of top predator wherever they've lived. Despite this success, very few, if any parasites have arisen to exploit them, unlike on a planet such as Earth where near every living species has a parasite. While this is not to suggest nothing parasitized them - plenty of polyfee and microbial species certainly did - for the most part nothing specialized in doing so. However, as with any food source that goes unexploited for so long, it was only a matter of time for something to evolve and take advantage of it.

Prior the ice age that engulfed Sagan IV, a curious evolution, which might have otherwise gone unnoticed amongst all the evolutionary oddities of this world, occurred. A lineage of hydroglobe, normally a photosynthesizing species of "ball flora" that survived through its use of resistant spores and their incredible capacity to bud and spread over nearly any solid object, had begun to grow exclusively off the Common Gilltails. While it was not uncommon for occurrence in nature - even to this day various species will grow off the sides of large marine organisms - for the most part it was an example of commensalism, as the flora benefited from the access. This was not the case in this species, though. This flora, which would come to be known as the Hemoglobe, was parasitic. Like some of the carnivorous ball flora of old, this species did not derive the majority of its energy from sunlight but from other organisms. Evolving a structure akin to a holdfast that pierced the flesh, they would use it to hold tight against the skin of a gilltail, even draining it of blood slowly through minute tubules that perforated them. Infestations of these could prove lethal over time, but for the most part the hosts' immune system would keep them at manageable levels until they died off.

Now, after millions of years, this is no longer the case. Several lineages of hemoglobe have evolved and split off into a new genus, one composed of individuals far more deadly than their ancestor ever was.

The sanguine o' spheres have taken its parasitic relationship with gilltails to the next level. No longer are they capable of performing photosynthesis, instead now taking all they need to survive from their "gracious" hosts. Once a spore - now bearing a much more mobile form and being more akin to zoospores - finds a gilltail by following its chemical trail in the water, it seeks out an opening, such as a small scratch or other such wound, and upon doing so it digs in. Once it has secured itself, it will begin to mature into its adult stage. As it grows, the typical sanguine o' sphere will secrete enzymes that slowly digest the surrounding flesh in order absorb the resulting nutrients, as well as for their own protection. This protection comes in the fact that they will take up the various chemical markers of their gilltail host, which helps to hide them the hosts' immune system. All all while this is going on, their holdfast-like structures are beginning to spread out and dig in to their fleshy surroundings. They will seek out blood vessels, and once they penetrate them, will begin to transfer the blood within into the main body. These structures also perform a secondary function, for while the sphere itself is capable of budding new individuals, so too can it. This can cause an infestation of sanguine o' spheres to quite rapidly grow out of control, for even if the initial infestation site is somehow removed, "roots" might remain that are intact enough to start it all over again.

As new spheres continue to form, spreading over the body of the poor gilltail, its health will begin to drastically decline. While some infestations kill their hosts slowly over the span of months, many do so instead in the span of weeks. The host, suffering from both anemia and its own body slowly being digested away, becomes weakened, sluggish, and overall fairly pallid in coloration. Eventually, they will have difficulty even swimming, if they are not picked off by predators, until they are incapable of moving enough water through their gill-systems, a fatal situation for them. Of course, this is assuming the infestation starts on a more open part of the body, but this is not always the case. Should spheres begin to mature around the base of an eye, they are quite easily capable of penetrating their way into it and render the gilltail blind in that eye, which in turn can drastically reduce its chances of both feeding and avoiding predation. Should the infestation begin near either end of the gill-system, they can quite easily - if unintentionally - clog it, resulting in a much more rapid death for their host. Fins can also be a point of infestation, which can cause interference with the gilltail's capacity to swim effectively.

While a host dying quickly is detrimental to most parasites that don't rely on complex, multi-host lifecycles, for the sanguine o' spheres it is of little concern. By the time the first spheres have reached maturity, they are already releasing their spores asexually into the surrounding water. While many will never find a host, all that's needed is one to be successful in order to start the process all over again, and with oceans rich in gilltails, there are many potential opportunities to find a host.

Oddballs
While the vast majority of the hundred or so species of sanguine o' spheres - typically at least one for every species of gilltail and their kin - follow a very similar lifecycle and bear almost indistinguishable appearances beyond size - many species can only distinguished with genome sequencing - there are still a few evolutionary oddballs that pop up from time to time. One lineage, for example, has developed a tolerance for brief ventures out of water, a necessity given their typical hosts are srugeings. Another lineage doesn't immediately release spores throughout its life, instead relying on the guts of large carpozoans to release them from their bodies, and to achieve this they subtly alter the chemical scent of their gilltail host to make them more pungent to these sorts of predators. Perhaps the oddest of all is a lineage that is particularly deadly to its host, primarily because once the gilltail has died, the spheres will extend outwards from the corpse on thin stalks in order to better infect small scavengers with their spores which are in turn consumed by un-infested gilltails. Of course, all these examples are merely outliers on the evolutionary tree of the sanguine o' spheres. Time will tell whether or not they will lead to vibrant new limbs, or if they are but mere branches to be pruned all too soon.