Ilze K. Berzins
Coral reefs are the most diverse marine ecosystem with more than 25% of all marine species depending on its resources while covering less than 1% of the earth's surface. Definitely a valuable resource. But what are corals? Animal, vegetable or mineral? Actually, sort of all three! Corals are marine invertebrates in the phylum Cnidaria. The group is complex, and the various species which comprise reefs are scleractinians, commonly known as hard or stony corals. Most are colonial and comprised of numerous, genetically identical, interconnected polyps. Polyps secrete a calcium carbonate "exoskeleton" at their base, forming boulders and branches contributing to the overall reef structure. The polyps can catch small fish and plankton using cnidocytes (aka nematocysts, stinging cells) in their tentacles, but tropical and subtropical reef corals obtain the majority of their nutrients from photosynthetic unicellular dinoflagellates (aka zooxanthellae), genus Symbiodinuim, located in the gastrodermal tissue. So corals can be thought of being made of all three - animal (polyp), vegetable (zooxanthellae) and mineral (calcium carbonate "exoskeleton"). Understanding the dynamics of this "coral trifecta" is essential to defining coral health and addressing coral disease in attempts to prevent coral demise.