Crystalgrazer
The crystalgrazer split off from its ancestor and developed characteristics that allowed it to feed on crystals—which were otherwise an untapped food resource in its habitat. Its mouth is always sucked inward relative to its ancestor, and it has developed four beaked jaws formed by enlarged and fused mouth bristles. These are opened and closed using an existing ring of muscles around its mouth, previously used by its ancestor to hold the mouth closed while processing food. Its bite is very strong, more than capable of breaking through the outer shells of crystals to give it access to the fungal core. The jaws mean it no longer has the ability to suck in prey by inverting its mouth, but this is fine because its feeding method was never dependant on that anyway.
Internally, the crystalgrazer’s digestive system has been transformed to be far better at breaking down plant matter. A portion of its “intestine”, previously only a waste funnel, has been converted into a primitive gut connected directly to the mouth. It has a large, tough stomach filled to the brim with melter detritis it obtains from the environment; these break down the chunks of flora it swallows, and in turn it digests the excess melter detritis produced in its gut, much like a terran ruminant—albeit by absorbing them directly into its cells, rather than secreting digestive enzymes. As a result, it is much chunkier than its ancestor to account for all the extra space needed for such a digestive system. The part of its “intestine” organ which ran through its limbs has been repurposed for distributing nutrients and oxygen throughout its body, like a very strange circulatory system. It has no blood pigment, instead simply dissolving the oxygen and nutrients in water and using that.
With the presence of predators lurking around, the crystalgrazer’s walking limbs have become flat and flipper-like. It still walks around on the seafloor like its ancestor did, but it is also capable of swimming in short bursts to escape predators. As it could not do the same to its anal arm without harming its ability to mate, the anal arm is much shorter to reduce drag created by it while swimming. As its gut anatomy came with increased density, it has a shape far better suited for walking in adulthood and it has developed a bundle of cartilaginous support structures in its limbs.
The crystalgrazer has developed a pair of gills. Like the gills of fish, water passes through them via the mouth. This helps support its larger size by allowing it to get more oxygen.
The crystalgrazer is otherwise much like its ancestor. It mates by linking its anal arms together using its anal fingers, and its offspring are released in eggs with shells made of sheets of cells and a yolk-like substance to feed them until they hatch. Its larvae are radially symmetric, have fully developed jaws at hatching, and become bilateral as they mature.