Flop Ribbon
As reef ribbons developed their taste for microbial films, some left their reefs to feed on further films in the outside world, where they became the flop ribbon. Retaining their laterally compressed body, they lie on the seafloor like Earth's flounder. They're still born as radially symmetrical larvae, but as they grow older, they flatten, and one eye moves to the other side of the body. Like the reef ribbon, two of the arms develop into frilly gills, and one arm develops into three gripping tentacles. However, the tentacles now form out of a side arm, and are used to hold it in place when resting or feeding. They scoot across the ocean floor by pushing themselves with these tentacles and their anal fingers. While still capable of swimming, it's quite cumbersome.
To blend in while feeding on mats on the ocean floor, they've developed new mottled coloration: a mix of purples and greens, like Sagan 4's most common photosynthesizers, and some sandy and silty colors. The proportions of these colors vary widely from individual to individual, as genes that express different pigments unlock randomly during gestation, so each individual's patches are unique. Their gills are always fairly pink, however; lacking pigmentation, they are colored by hemoglobin.