Less well known are the algal relationships with animals. Reef-building corals are basically social Cnidarian polyps. Corals are dependent on light, because the algae are important partners, and they require light. Corals have evolved structures, often tree-like, which offer the algae maximum access to light. The coral weakens the algal cell walls, and digests about 80% of the food synthesised by the algae. The corals' waste-products provide nutrients for the algae so, as with lichen, both partners gain from the association. The algae are golden-brown flagellate algae, often of the genus "Symbiodinium". A curious feature of the partnership is that the coral may eject the algae in hard times, and regain them later. The ejection of the algal partner is called "bleaching", because the coral loses its colour.