By: Nelson Vayada and Cristina López Miranda
Figure 1. Coral heterotrophy. Credits: Cnidarians, FlexBooks 2.0 Corals generally obtain food from the symbiotic relationship they share with algae that live within their tissue. While shallow water corals can usually obtain all of the food they need from this symbiotic relationship, they still possess the ability to capture food with their tentacles to feed (heterotrophy). Studies have shown that corals may actively feed to make up for times when their algae are not producing enough food or to obtain key nutrients for growth that their algae cannot provide for them.
"Coral can meet up to 100% of their daily metabolic requirements completely from heterotrophy"(Franny Houlbre`que et al, 2008)
This research is incredibly important because as corals bleach, which is them expelling their symbiotic algae partner, it is possible for them to recover if they can survive through the bleaching event and regain algae. Corals that are more adapted to active feeding are more likely to survive a bleaching event and continue to support life on the reef. While it is not ideal to have corals bleach as it will likely cause permanent damage to the animal, if they do bleach, it is better for them to bleach and survive than to bleach and die.
Figure 2. Coral anatomy. Credits: Cnidarians, FlexBooks 2.0 
Figure 3. This figure is a summary of many studies on active feeding in corals. It shows how fed corals showed twice as high protein concentrations, photosynthetic rates, and calcification. They also showed a 60% increase in synthesis of their organic matrix during the day and 100% increase at night. Credits: Cnidarians, FlexBooks 2.0 A study on active feeding of tropical corals found that up to 66% of the carbon found in the coral skeleton could be traced back to active feeding. A group of studies showed that fed corals had double the photosynthetic production and protein synthesis. The fed corals also calcified at a higher rate than unfed corals showing that feeding increases the rate at which corals grow.
Figure 4. This figure shows the lipid concentration of the Acropora corals after 93 days of being fed with different treatments. Showing which treatment was more successful depending on the coral species. Credits: Cnidarians, FlexBooks 2.0 Another study focused on the different heterotrophic diets that guarantees the most survival of the corals. The study centered on the nutritional requirements, specializing on lipids, the corals need in order to grow bigger, this way having more chances to survive. By using four different treatments that the lipid concentration varies on the diet and the diet that showed better results on survival rates was also the one that did not affect the lipid content unlike the others. Still the study proves that a nutritionally adequate feeding regime can guarantee coral survival.
Learn More (Citations)
Houlbrèque F, & Ferrier-Pagès C. (2009). Heterotrophy in tropical scleractinian corals. Biol. Rev. 84: 1-17