Branching Coral

"This is a typical Protean coral. It works differently than Earth coral. The gross morphology is a branching tree, like some species of Earth's extinct Acropora. We all call it coral because we know the word. But the difference matters.

"On Earth coral are basically tiny jellyfish living in stone houses. The jellyfish sting prey. They also let zooxanthellae live with them, houseguests who make food from the sun.

"On Proteus...I think what we're seeing here is actually a sponge acting as a landlord. The sponge makes a house. It rents space to these tiny coral polyps, which sting prey and capture sunlight as energy for the sponge.

"I think the sponge used to be in charge. But eventually the coral polyps got so good at their job that the sponge stopped pumping water and just lived off rent from the coral polyps. And to keep its tenants alive, it evolved a rocky shell — like Earth's sclerosponges.

"If I'm right, we should see other coral-sponge pairs that have worked out different deals. Each deal suited to a particular ecological niche."