There are 2 main classifications for corals: 1. Hard coral (scleractinian and stony coral) [13] which form reefs by a calcium carbonate base, with polyps with 6 stiff tentacles, [14] and 2. Soft coral (Alcyonacea and ahermatypic coral) [13] which are pliable and formed by a colony of polyps with 8 feather-like tentacles. [14]These two classifications arose from differentiation in gene expressions in their branch tips [12] and bases that arose through developmental signaling pathways such as Hox, Hedgehog, Wnt, BMP etc.

Many corals, as well as other cnidarian groups such as sea anemones form a symbiotic relationship with a class of dinoflagellate algae, zooxanthellae of the genus Symbiodinium, which can form as much as 30% of the tissue of a polyp. [31] :23–24 Typically, each polyp harbors one species of alga, and coral species show a preference for Symbiodinium. [32] Young corals are not born with zooxanthellae, but acquire the algae from the surrounding environment, including the water column and local sediment. [33] The main benefit of the zooxanthellae is their ability to photosynthesize which supplies corals with the products of photosynthesis, including glucose, glycerol, also amino acids, which the corals can use for energy. [34] Zooxanthellae also benefit corals by aiding in calcification, for the coral skeleton, and waste removal. [35] [36] In addition to the soft tissue, microbiomes are also found in the coral's mucus and (in stony corals) the skeleton, with the latter showing the greatest microbial richness. [37] a b c d e f g h i j k l m Hemond, Elizabeth M; Kaluziak, Stefan T; Vollmer, Steven V (2014-12-17). "The genetics of colony form and function in Caribbean Acropora corals". BMC Genomics. 15: 1133. doi: 10.1186/1471-2164-15-1133. ISSN 1471-2164. PMC 4320547. PMID 25519925.Budding involves splitting a smaller polyp from an adult. [49] As the new polyp grows, it forms its body parts. The distance between the new and adult polyps grows, and with it, the coenosarc (the common body of the colony). Budding can be intratentacular, from its oral discs, producing same-sized polyps within the ring of tentacles, or extratentacular, from its base, producing a smaller polyp. National Oceanic and Atmospheric Administration – New Deep-Sea Coral Discovered on NOAA-Supported Mission". www.noaanews.noaa.gov . Retrieved 2009-05-11.

Neave, M.J., Apprill, A., Ferrier-Pagès, C. and Voolstra, C.R. (2016) "Diversity and function of prevalent symbiotic marine bacteria in the genus Endozoicomonas". Applied Microbiology and Biotechnology, 100(19): 8315–8324. doi: 10.1007/s00253-016-7777-0. Water temperature changes of more than 1–2°C (33.8–35.6°F) or salinity changes can kill some species of coral. Under such environmental stresses, corals expel their Symbiodinium; without them, coral tissues reveal the white of their skeletons, an event known as coral bleaching. [105]hompson, J.R., Rivera, H.E., Closek, C.J. and Medina, M. (2015) "Microbes in the coral holobiont: partners through evolution, development, and ecological interactions". Frontiers in cellular and infection microbiology, 4: 176. doi: 10.3389/fcimb.2014.00176. Anthony, K.R., Kline, D.I., Diaz-Pulido, G., Dove, S. and Hoegh-Guldberg, O.(2008) "Ocean acidification causes bleaching and productivity loss in coral reef builders". Proceedings of the National Academy of Sciences, 105(45): 17442–17446. doi: 10.1073/pnas.0804478105. Dauphin, Y.; Cuif, J.P.; Williams, C. T. (2008). "Soluble organic matrices of aragonitic skeletons of Merulinidae (Cnidaria, Anthozoa)". Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology. 150 (1): 10–22. doi: 10.1016/j.cbpb.2008.01.002. ISSN 1096-4959. PMID 18325807. In medicine, chemical compounds from corals can potentially be used to treat cancer, neurological diseases, inflammation including arthritis, pain, bone loss, high blood pressure and for other therapeutic uses. [133] [134] Coral skeletons, e.g. Isididae are being researched for their potential near-future use for bone grafting in humans. [135]