Reefstar

The reefstar diverged from the shellstar when it developed the ability to grow shells together with other individuals (polyps). The ancestral shellstar secreted a shell of calcium carbonate in a ring around its base, not part of the organism's body, but attached by modified baits. In the reefstar, adjacent polyps join their secretions together to form a continuous lattice: a much larger shell with holes for each polyp throughout. Furthermore, they will build on to the shells of deceased polyps, further extending the structure in layer upon layer, building a reef.

The polyps themselves are little changed from their ancestor: a simple star-shaped body of four arms, in which the center clings to the shell, while the arms reach outward. Each arm is covered in many small bristles for filter-feeding, but these now also extract oxygen from the water, though none have yet specialized as gills. Each arm has an eye at the end.

Internally, their bodies are mostly undifferentiated tissue, but they have the beginnings of organ systems. A network of nerves runs throughout the body, taking in sensory input from the eyes, chemicals, and vibrations in the surrounding water, and coordinating the ability to release its own chemicals into the water. This way they can detect predators passing by and also warn their neighbors, allowing them all to retract their arms into the shell.

As before, they can reproduce through both sexual spawning and asexual fragmentation. However, this fragmentation now proceeds only one arm at a time, which will detach and grow a new full body. This is one of the main ways that their reefs expand, as the new polyp will settle somewhere nearby and start secreting its own bit of the shell. Whole sections of a reef will consist of such clones. When spawning, they emit spore-like gametes into the water, which allows them to spread further away, though the survival rate is much lower. If the larvae make it through their free-swimming shell-less stage, they secrete their own shell and drop to the ocean floor, where they may found a new reef or join an existing one.

Reefs try to maximize their surface area, the better to have more material for filter-feeding. There's no coordinated plan, of course, but when polyps settle into new locations, they instinctively prefer areas where they will have a small number of neighbors but not many. A large reef base ends up dividing into smaller branches as it reaches upward, subdividing into narrow towers and spires. For the most part, these just grow upward, and the gaps may gradually fill in as the reef ages, but occasional branches may continue off diagonally as well. Reefs that get too filled in may become stagnant and their death rate overtake their birth rate, to eventually be replaced by a new generation of reefs.

These reefs typically grow about a meter tall, and may grow to several meters, but the most active living zone will be quite a bit smaller. They cannot grow past the surface of the water, as they have no ability to tolerate desiccation.