Reef-building corals have a mutualistic relationship with zooxanthellae.   These unicellular algae are found in the outer tissues of the coral where they can photosynthesise.  They pass on some of the carbon they have fixed through photosynthesis to their hosts.  The cost to the zooxanthellae is that they lose some of their fixed carbon which they could otherwise have used to reproduce and increase the size of their population.  The zooxanthellae benefit by receiving nitrogen and phosphorus from the coral; these nutrients are scarce in seawater where corals grow.  In the dark, the zooxanthellae do not photosynthesise and are thus unable to provide carbon to the coral, which ejects them.  This suggests that there is a considerable cost to the coral in providing nutrients to the zooxanthellae.  It also implies that the coral is in control of the mutualistic relationship.