The spiny castrum split off from it's ancestor, the suction crastrum. When a symbioship diamiboard went through Soma Coast, some suction crastrums stuck onto them, where they then carried them off. However, the small crastrums were easy to eat for any that ate them. So, it grew some large spines in the same vein that the thorny crastrum grew thorns. The spiny crastrum has five large spines, one at each indent, and one in the center. Two of the ones on the sides point forward, catching any small prey that might be beneficial for the diamiboard and its inhabitants.
The crastrum's benefited when the whole symbioship moves, causing currents to form around the crastrum, aiding in photosynthesis by carrying away waste gasses. Another way that the spiny carstrum aids the diamiboard is that they grow all over the edges of the ship, which helps to keep larger predators away, and to that end their shape has changed to better tessellate with each other. By this logic, some symbioship hydroglobes got crowded out at the edges, but the hydroballs benefit from the crastrums protection. The crastrums are not harmed by the hydroballs because they do not sleep.