The gillfin replaced its ancestor the scraping beakworm as well as replacing diamiboard beakworm. It now has evolved a longer neck so it can scrape off the hydoglobes from the sides of diamiboards. Its beak is also strong enough to bite off part of the crystal itself.
It now has primitive gills on the dorsal fin. This allows it to stay active longer and able to move faster. This helped it to displace the other beakworms around the diamiboards. It is also immune to the hitchhiker hydroglobes' sticky clouds of drowsy liquid.
They also have a new method of reproduction: the female will lay her eggs in the water and stick them to her tail. Then the male will fertilize them on her tail. There they will grow on her tail until they hatch. Not all stay stuck however, and some will float off for scavengers to eat.