Most reef corals farm algae for food inside their cells
Corals are colonial anthozoan cnidarians whose polyps build shared limestone homes. Though tentacles can sting plankton and tiny fish, most shallow species draw the bulk of their energy from symbiotic zooxanthellae—unlike deep Lophelia that lives without those algae.
A coral colony is a clone army of genetically identical polyps, each a millimeter-scale sac with a mouth ringed by tentacles. Compact reefs arise when generations stack calcium carbonate. Classification puzzled thinkers for millennia because corals seem plantlike yet animal; Aristotle's pupil Theophrastus already wrestled with the ambiguity.
Those algal partners are single-celled dinoflagellates of the genus Symbiodinium, known as zooxanthellae, which also give corals their colour; most polyps pull in their tentacles by day and spread them at night to catch plankton as a supplement. Corals that do without the algae can live far deeper: the cold-water Lophelia survives down to about 3,300 metres and has been found at the Darwin Mounds north-west of Cape Wrath in Scotland and off the Aleutian Islands. Today corals sit in two groups, the six-fold Hexacorallia, including reef-building stony corals, and the eight-fold Octocorallia, including soft and blue corals.
Theophrastus called red coral a mineral in one book and a deep-sea plant in another. The Persian scholar Al-Biruni, who died in 1048, classed corals and sponges as animals because they react to touch, but most people thought them plants until William Herschel used a microscope in the eighteenth century to show animal cell membranes. Soft corals lack a solid skeleton and are stiffened instead by tiny calcium carbonate sclerites. Researchers favour Acropora, with over 120 species the most diverse hard-coral genus, as a model, and heat shock proteins help some species resist warming seas.
Source: Coral
