Coral

Tiny jellyfish-like polyps that build a hard limestone structure. On this world, coral began as a sponge which the polyps dwelled inside for protection. Over time, many different symbiotic strategies evolved — from hardened limestone reefs (like in Earth's corals) with no spongelike traits remaining, to "sporal" hybrids where the soft sponge remains inside a hard shell.

Climate control — captures carbon from the atmosphere, returning it to the deep ocean when the coral dies.

Photosynthesizer — many corals convert light to biomass with symbiotic bacteria.

Predator — coral polyps capture prey from the surrounding seawater.

Earth analog: reefbuilding corals.