What is coral?


Corals are the foundation of one of the world’s most biodiverse and fascinating habitats, the coral reef, often considered as the tropical rainforests of the ocean. Corals can occur across the globe in shallow to deep waters but reef building (that is Scleractinic) corals are primarily found in tropical to subtropical habitats.

Image Source: Toby Hudson Wikipedia

Image Source: Toby Hudson Wikipedia

Reef building corals are invertebrates belonging to the phylum Cnidaria which also includes jellyfishes and sea anemones.

An individual coral polyp is comprised of the coelenteron, a radially symmetrical sac-like body cavity with a single mouth opening. This cavity is surrounded by tentacles covered in stinging cells also known as cnidocytes. The body wall consists of two cell layers- the epidermis and gastrodermis- which are separated by a jelly-like collagen rich layer, the mesoglea. Coral polyps build a skeleton from aragonite which is a crystal form of calcium carbonate. The function of the coral skeleton is primarily to strengthen and protect the organism.


coral polyp

Image Source: NOAA

Reef building corals are primarily colonial animals and an individual coral colony can consist of thousands of individual polyps (i.e. animals). However, solitary corals also exist, such as the mushroom coral Fungia. Corals are syngameons or reproductively isolated units, and can reproduce sexually or asexually. Asexual reproduction occurs through budding or fragmentation. Budding is when a polyp reaches a certain size and divides resulting in a genetically identical new polyp. Fragmentation happens when part of a colony breaks off and forms a new colony. Sexual reproduction can be internal or external and result in free-swimming larva.

This coral larvae can then settle on a hard substrate like rock or a recently dead coral, where it will grow into a coral polyp that deposits aragonite. The coral polyp then builds copies of itself and these clones then connect to each other to form the initial single-organism colony. Over time colonies grow and join with other colonies to form reefs; the oldest reefs began growing over 50 million years ago. The Great Barrier Reef off the coast of Australia is so big it can be seen from outer space!

Coral reefs obtain their colorful hues from zooxanthellae, a microalgae living in the gastrodermal tissues. Zooxanthellae are a type of unicellular dinoflagellate microalgae belonging to the genus Symbiodinium.

They form a symbiotic partnership with the coral host by performing photosynthesis which provides the coral animal with fixed carbon, in other words sugars. In turn, the coral animal provides a protective environment and essential nourishing compounds (e.g. nitrogen, phosphorus, and sulfur).


                                                                   Image Source: National Coral Reef Institute

Author: Patricia Fernandez-Waid (4/25/2017)                        

Links and Sources:

  1. Australian Institute of Marine Sciences
  2. coral.aims.gov.au
  3. Fernando Nosratpour, interview Birch Aquarium
  4. Jaffe Laboratory, SIO
  5. Houlbreque, Fanny, and Christine Ferrier-Pages. "Heterotrophy in Tropical Scleractinian Corals." Biological Reviews 84.1 (2009): 1-17. Web.
  6. NOAA
  7. http://oceanservice.noaa.gov/education/kits/corals/coral02_zooxanthellae.html
  8. http://oceanservice.noaa.gov/facts/coral_bleach.html