Yes, corals are animals and most are colonial and are in symbiosis with an algae (Phyla: Chlorophyta, Rhodophyta, or Phaeophyta) or a dinoflagellate (Phylum: Dinophyta).  In other words, they exchange nutrients or other services with one another in order for both to survive better. Corals are made up of several nearly identical units called polyps that are bound together at their base by a skeleton made of calcium carbonate that they secrete. Corals eat by filtering tiny particles of food out of the water.

Coral is made up of many tiny polyps that are attached to each other by a calcium carbonate skeleton that they secrete. Alone, all coral would be white, the color of the calcium carbonate skeleton. However, each polyp contains several unicellular yellow-brown dinoflagellate algae called zooxanthellae. These zooxanthellae have a symbiotic relationship with the coral. The coral provides protection and access to light for the zooxanthellae and the zooxanthellae provide nutrients for the coral via photosynthesis. Since the zooxanthellae must accomplish photosynthesis, they are filled with various pigments to absorb sunlight. It is these pigments that give coral its color.

Coral bleaching occurs when the corals lose their symbiotic zooxanthellae  or dinoflagellate algae that live in the coral polyps, which is what give the coral its beautiful color. The calcium carbonate skeletons of the coral colony are exposed with the loss of their symbiotic counterpart, causing the coral to become completely white. This process is called bleaching. Scientists are not quite sure why this happens, but there is some evidence to suggest that a gradual rise in water temperature may be involved.  A relatively small change in temperature can cause an organism stress and even death. A possible cause of this rise in temperature is the heavily debated global warming situation. Bleaching may also be caused by other stressors like disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, or salinity changes.

Yes, they have a central mouth which is surrounded by tentacles with nematocysts, stinging cells that paralyze and entangle small marine animals. Sea anemones primarily reproduce sexually and produce a free-swimming larva that finds a place to attach to, usually a rock or coral.

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