Horseshoe Clam

Description:

The same species of symbiotic algae that live inside corals, live inside clams. In fact they owe their existence to clams (perhaps all the creatures of the reef do), for when the catastrophic changes that caused the extinction of the dinosaurs killed off most coral, the algae survived inside the harder-shelled clams. Then, when the oceans became less acidic and corals were able to re-grow, the algae that they depended on were still there and modern reefs were able to reform.