Thalassic Stercor
The Thalassic Stercor (Initiasterus stellamare) is descended from Stercors (I. fluminis) which were washed out to sea by river flow. Already very adaptable and capable of surviving in brackish conditions, they were able to adapt to their new saltwater environment and split off into their own species native to Zone 3.
The primary adaptation in the Thalassic Stercor is in how it handles salinity. As an organism with permeable skin, freshwater readily flows into its cells through osmosis and the excess has to be removed using active transport. However, in the ocean, the concentration of molecules in the surrounding water is instead higher than that in their cells. This meant that upon entering the saltwater ocean, rather than water, dissolved salt and other molecules would flow into its cells instead. The Thalassic Stercor resolves this by instead pumping out the excess sodium, chlorides, and other solutes.
Otherwise, the Thalassic Stercor differs very little from its ancestor. It is fairly adaptable and can live anywhere from the lush shallow coasts to the colder depths up to a kilometer deep as long as there is food available. It is most common in the sunlit shallows along coasts, as there is more food there. It reproduces asexually through fission every 5-15 days and sexually by broadcast spawning when conditions become unfavorable, changing sex if necessary should there be no individuals of the opposite sex nearby. Its eggs hatch in favorable, sheltered conditions after at least a day, and it can take from 7 days to a month depending on food availability to reach full size. It is translucent and has no color, though it can appear white under certain lighting conditions. It has an armspan of 500 micrometers.
The Thalassic Stercor is a generalist which eats anything it can fit in its many mouths. This includes microbes such as S. oceanensis, V. phagohemolaimus, L. multibranchus, and L. bryodomus; zoospores and fragments of organisms such as B. cirrufractulus, F. primus, and B. globusmile; eggs and juveniles of similarly-tiny fauna such as F. foliumicroeus and F. cenafolius; fragments of dead muscals and polyphs; the sperm or milt of organisms which spawn such as R. pupadarco and R. unguladontus; marine snow; and occasionally the gametes, eggs, and juveniles of their own species. Occasionally, it will be found not on the substrate but on floating detritus, such as driftwood from the mainland and the dead leaves of floating polyphs, where it survives just fine feeding off of the rotting polyph matter and its decomposers; because of this, the Thalassic Stercor can readily drift to new locations and colonize new coastlines.
The Thalassic Stercor is often incidentally consumed by detritivores such as F. catagraphahumilis, F. clamus, F. marineris, N. bonii, and S. toxicopoda, due to it commonly being found among detritus. It is more actively preyed on by C. duolineum and E. elasmus. Its primary defense against these is to just reproduce too fast for all individuals to be eaten.