Screwcap shells are an incredibly diverse group of animals with hard outer shells. In most species, there are two shells with treads along their meeting edges to allow them to twist tightly shut. When open, the flesh of the animal stretches across the opening between the two shells and the animal extends its gills. The gills not only facilitate gas exchange, but gather food particles as well – although some species do not feed on plankton.
Many screwcap shells will form pearls of silicon carbide much like the calcium carbonate pearls of Earth oysters. These pearls are harvested by the reefbuilder crabs, which glue them together with their own secretions to build elaborate structures of all kinds. Over tens of thousands of years, the reefbuilder crabs have built thick walls right up to the water’s surface, dividing the reef into different zones and redirecting water currents. Even they do not entirely know why they began building, but it seems to have something to do with their religion. These walls serve as attachment points for organisms of all kinds. The reefbuilder crabs also build towers and structures that can only be described as abstract art.
The reefbuilder crabs defend the screwcap shell beds from predators that might try to eat them. In this way, they are sure to have a constant supply of pearls to harvest. To feed themselves, they also care for several varieties of weeds.