Wormreef
Wormreefs first split from wormstems through a simple change in behavior, which turned into something more profound. In both, a mobile juvenile stage crawls across the ocean floor and, in adulthood, settles into an immobile lifestyle. Wormreefs, however, settle by preference on other adults of their own species.
Each individual, then, is a polyp in a larger reef, which grows into a heaping mound. Living polyps have much the same form as their ancestors: a series of cylindrical segments, arranged in a J-shape, resting at the base and growing upward at either end. Thorn-leaves grow out of each segment. At the top, one end is the head, with a filter-feeding proboscis over its mouth, while the other end contains the anus. Each segment has a green outer shell, capable of photosynthesis, which now secretes an inner protective layer of calcite. Within the shell is the soft core, which digests the intake from filter-feeding. These shells and cores are technically separate organisms.
As new polyps settle over the old ones, they block out the sunlight, and the outer shells fade and lose their ability to photosynthesize. Meanwhile, the inner cores continue to filter-feed organic material that passes into the reef. They secrete a nutritious pool of digested material, thus sharing it throughout the reef and providing sustenance for the shells and juveniles within the reef. Eventually, the polyps die entirely, but the calcite skeletons remain, producing a framework that the reef grows on.
The reefs don't grow into much of an organized shape, mostly just a heap, and a fairly unstable one at that, with individual polyps liable to shifting. They do, however, form holes in the overall mound shape, providing channels for water to flow in and out. These reefs are able to extend above the surface of the water, generally as a result of tides. The hard shells provide a resistant barrier, preventing the inner cores from drying out. A typical reef may grow to several meters in height, at which point they're liable to collapse. However, on particularly level and stable surfaces, in deeper areas, they may grow to a few tens of meters, though this is exceptional. It is also not unknown for wormreefs to grow on top of the remains of older reefs. There are always many scattered smaller reefs of under a meter tall as well. The heap shape means that these reefs always settle into a shape much wider horizontally than they are tall.
Reproduction now occurs largely inside the reefs. Polyps spawn egg and sperm cells, which come in pairs: one from the shell and one from the core. Typically, these will unite within the reef and grow into a crawling juvenile, similar to the ancestral thornworm. Bathed in the nutrient-rich pool, they easily grow to adulthood; often, the strongest juveniles eat the rest. They will then crawl out of the reef, where they might find a spot to settle on their home reef, or go out in search of another reef, or even find a spot to found a new one. Occasionally, eggs and sperm will find each other outside a reef and grow into a juvenile out in the open, which will have to fend for themselves, but are just as capable of founding a new reef or settling onto an existing one.