Changing Planet: Our Fading Corals

Grace Kim

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Features of Corals

  • Coral polyps are small, soft bodied organisms that are related to sea anemone and jellyfish. In their center, they have a limestone skeleton called a calicle.
  • Corals can reproduce sexually through budding, as well as asexually by releasing their fertilized gametes into the water. Eventually, these gametes turn into polyps and attach to a rock, where they will eventually divide into many other corals.
  • Corals prey on zooplankton and small fish. They use their barbed, venomous tentacles to grab and kill their prey.
  • Zooxanthellae is a type of algae that live inside many coral polyps. These small plant cells are what give corals their brilliant colors. Corals and zooxanthellae share a mutual relationship in which the coral provides shelter for the algae, which then supply the coral with glucose, glycerol, and amino acids, which are the products of photosynthesis.

Abiotic Factors

  • Sunlight is extremely important for the survival of coral, because zooxanthellae algae need it to produce necessary nutrients in the process of photosynthesis. Because of this need for sunlight, many corals are located in shallow water where they have access to sunlight.
  • The algae requires a certain temperature to function. A sudden increase in water temperature will cause the algae to leave. As a result, many corals die from lack of nutrients, and many other become bleached.
  • Calcification of coral's skeletons depends on the salinity of pH. The the pH increases, the skeletons will be weaker and more prone to breaking. Most coral's salinities are anywhere between 32% - 40%.
  • Corals need clear, moving water to survive. Murky water contains sediments that block out sunlight from reaching the coral and the zooxanthellae. Water movement pushes sediment and debris out of the way, and it brings nutrients and warm water to the coral.
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Characteristics of Coral Ecosystems

  • Corals make up for a vast majority of the reef ecosystem. It provides life and a habitat for many other organisms. It also acts as a predator in the cycle of life, promoting healthy balance and biodiversity
  • The trophic levels of coral ecosystems are as follows:
  1. Primary producers: Phytoplankton, Seaweed, Zooxanthellae
  2. Primary Consumers: Zooplankton, corals, sea anemone, herbivorous fish
  3. Secondary Consumers: coral-eating fish, plankton feeders
  4. Tertiary consumers: dolphins, sharks, large fish

  • Corals and zooxanthellae are a prime example of a symbiotic relationship in coral ecosystems. Another example of a symbiotic relationship in the coral reef is between crabs and sponges or seaweed. The crabs place the sponges and seaweed on their back to camouflage themselves from predators, and in return the sponges and seaweed get a more diverse selection of nutrients.


  • Coral reefs increase biodiversity. They provide a safe habitat for approximately 1/4 of the fish species in the ocean, and they act as a barrier between potential storms and the coastline.
  • Coral reefs offer the practical benefit of providing a breeding ground for many fish, which increases the activity of the fishing industry. They are also extremely popular tourist attractions due to the bright colors of the coral along with the multiple varieties of fish and other organisms.


  • Increasing water temperatures are driving zooxanthellae algae out of the corals, resulting in coral bleaching and a lack of a major food source for the corals. Water acidity is also increasing, causing the limestone skeletons to become weaker and more prone to breakage. Any changes in water movement or clarity could cause sediments to build up and block sunlight to the algae.
  • Diseases are a very real threat that can greatly damage the tightly knit coral reef community. In the past, unknown and unexpected diseases have killed off almost all of the herbivorous sea urchins in the ocean, causing an imbalance in algae population, and subsequently throwing off the entire ecosystem.
  • Storms have been known to cause damage to coral reefs, which act as a barrier between said storms and the land nearby. However, it is not entirely bad. These storms often allow new reefs to flourish in place of the older reefs.
  • Humans pose one of the largest threats to coral reefs. boating and fishing causes physical and structural damage to the ecosystems. Carbon output from human life has created a gradual increase in ocean acidity, and pollution and liter poses a threat to all ocean organisms.
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CORAL REEF SYMBIOSIS - Coral Reefs - Ocean World. (n.d.). Retrieved November 9, 2015, from

Coral Reef Food Web. (2012, April 4). Retrieved November 9, 2015, from

Corals. (n.d.). Retrieved November 9, 2015, from

Corals, Coral Pictures, Coral Facts - National Geographic. (n.d.). Retrieved November 9, 2015, from