Blog Authors: Jenna Weisman and Liza Wolf
Research Team: Ellen Malone and Christian Burke
Figure 1. Star Coral Eating photograph by Nancy Sefton.
In order to survive and reproduce, all organisms must have sustainable means of getting food. This falls into two primary categories: autotrophy and heterotrophy.
Autotrophs are organisms that make their own food, most commonly from sunlight through the process of photosynthesis.
Heterotrophs, on the other hand, are those that get their food from outside sources, meaning they often eat other organisms, such as plants and animals.
Corals are very diverse animals that require the ability to produce their own food and consume other organisms.
There are some special cases in which organisms have the ability to feed through both autotrophy and heterotrophy, commonly known as mixotrophic or polytropic. One such example of a polytropic organism is coral. Corals are very diverse animals that sometimes require the ability to produce their own food and consume other organisms.
Figure 2. Colonies of Stylophora pistillata (A) Starved (B) Fed.
When focusing on autotrophy, corals rely on photosynthesis. They do so by means of the algae that lives within the corals’ tissue, called zooxanthellae. This symbiotic relationship between the coral and the zooxanthellae is mutually beneficial: the coral provides the zooxanthellae with a place to live and reproduce, while the zooxanthellae helps the coral grow by providing nutrients from photosynthesis. This process of photosynthesis begins as the ocean absorbs carbon dioxide from the atmosphere. The carbon dioxide is then taken by the zooxanthellae, and with the help of sunlight, photosynthesis occurs, producing nutrients that are shared with the coral.
Figure 3. Polyps of coral Stylophora pistillata, feeding on crustacean Artemia salina (Photo by É. Tambutté, CSM, Monaco).
Corals are also heterotrophs, having the ability to consume small organisms. They can do this with small tentacles that extend from their exoskeletons, reaching out to sting their prey. Their target prey is plankton in close proximity to them.
Figure 4. Acropora cervicornis,
extending it's small tentacles.
Corals did not show preference between autotrophic and heterotrophic feeding habits. It seems as if they participate in whichever is more convenient at a given time.
One study in particular aimed to determine whether, when faced with long periods of higher concentrations of carbon dioxide, corals would reduce their heterotrophic feeding habits. Authors, Smith J. et al., confirmed their hypothesis, that heterotrophy would lessen. However, the data collected determined that, in neutral conditions (without artificial levels of carbon dioxide), corals did not show preference between autotrophic and heterotrophic feeding habits. It seems as if they participate in whichever is more convenient at a given time. For example, a coral could be autotrophic during the day, and heterotrophic at night. Through studies such as this one, it can be determined that corals survive through both autotrophic and heterotrophic feeding habits, making it easier for them to adapt and thrive.
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