In today’s changing global environment, the need to understand global ecosystems has become more necessary than ever. One vital part of the global environment is the marine ecosystems around the world, especially those with large coral components. These coral ecosystems are astonishingly diverse, possibly containing hundreds of thousands of species. The diversity of these ecosystems makes them incredibly fragile, and therefore all the more important [5]. Many of these coral ecosystems, however, are in poor condition. One study by the National Oceanic and Atmospheric Administration in their Coral Reef Conservation Program found that about half of US held coral ecosystems are in fair or poor condition [1]. The need to mitigate the harmful trends in coral ecosystems and better understand the effects of changes in these coral ecosystems has led to increased research into the ways corals grow and respond to changes their environment. Both to understand the ways corals grow best in the wild and to better propagate them in research and recreational settings, it is necessary to understand how corals gain the energy that fuels their growthSome corals use a mix of processes to obtain their energy, and if it is known what processes they use, then corals can be more readily understood in the wild and/or propagated in a research setting. If the way a coral gains energy in the wild is known, then it can be known how changes in the environment, such as reduction in sunlight via decreased water clarity, or change in the kinds of organisms in the environment, will affect the growth and survival of certain coral species. Also, if corals can be propagated more efficiently by knowing their preferred method of obtaining energy, then research on other important questions regarding coral can be performed more easily. Finally, and possibly most importantly, easier propagation of coral in captivity will lead to a decrease in pressure put on wild coral populations by the growing marine aquarium trade. According to a 2004 report, approximately $200-330 million worth of marine ornamental species went through this trade into the US alone [2]. This harvesting of coral from the wild greatly damages the fragile coral ecosystems. If corals can be more easily grown in captivity, then less coral will need to be harvested from the wild leading to a more sustainable and healthier coral reef ecosystem

Two Competing Models

Corals are animals that fall somewhere between the heterotrophic and autotrophic feeding models traditionally used to classify organisms [4]. The corals themselves are heterotrophic organisms, related to jellyfish, using specialized nematocysts to capture prey, but many corals also harbor a symbiotic zooxanthellae (an algae) within their tissues which provides the coral with a chemical energy source via photosynthesis. It is known that corals range from completely autotrophic to completely heterotrophic, and anywhere in between, depending on the coral species and environmental conditions. Based on this, two competing models of coral energy acquisition can be proposed:

Plant Model

Sunlight is used by the symbiotic zooxanthellae within the corals to create glucose from carbon. This glucose can then utilized by the corals as a chemical energy source. The plant model suggests that this is the primary method of energy acquisition for the coral, with little, if any, input from the feeding of the coral polyp itself. Nutrients would be obtained primarily through direct absorption from the surrounding water. The implications of this model would be that little if any supplementary feeding would be needed for a coral to thrive and grow in the wild or in captivity.

Animal Model

Some outside organic substance, eg. another organism, is captured and used by the coral to create a form of chemical energy to power the organism. The nutrient acquisition in this model would come from the digestion of the captured prey item. Little, if any, energy would be supplied to the coral by the photosynthetic zooxanthellae. The implications of this model would be a coral that would benefit from extensive plankton populations in the wild, or by supplemental feeding in captivity.

The Coral Studied

To test the effects of supplemental feeding, the corals from the genus Anthelia were used. Anthelia sp. is a soft coral that grows in a mat shaped colony with the polyps growing up from the mat. Each polyp is a stock growing from the mat, as seen in figure 5. Anthelia polyps in a colony reproduce asexually and grow to form a single mat, through which nutrients and energy are shared. Anthelia sp. tends to be a rapidly multiplying coral, growing well in most conditions. It is generally thought to only require sunlight to produce energy and grow [3]. This trait makes it an optimal coral to use when studying the effects of one variable on coral growth.

Data Collection

In order to quantify the effects of supplementary feeding on Anthelia growth, the number of coral polyps in each group were counted each week. A single coral polyp is noted by arrow A in figure 5. The number of polyps were tracked by "plug", enabling growth to also be tracked by colony.

Habitat Description

This experiment was conducted in a large tank environment under natural light. The water was an average of 78ºF and had a specific gravity of 1.023 g/ml.


This experiment was designed to determine whether or not the supplementary feeding of Anthelia sp. coral greatly effected the growth rate of that coral, thereby testing the two models of coral energy acquisition for Anthelia. After the initial data set was collected, it appeared that the supplemental feeding benefited the coral colonies (Figure 1) yielding a 85% increase in polyp number versus a 54% increase in the control group at the end of the 6 week data collection period. Upon reshuffling the corals and running a second, although shorter, trial, the results suggest another explanation for the increased growth. The second trial yielded a 22% increase in polyp number over a three week period in the control group versus 13% increase in the fed (experimental) coral colonies (Figure 2). However, when both the data from figures 1 and 2 are looked at in conjunction, it seems that feeding does not have an effect on the growth of Anthelia, but rather some other variable is effecting the growth. Figure 3 shows the two trials in conjunction over a period of 3 weeks. Here, the two experimental groups have vastly different growth rates, while the control groups also have a substantial difference in growth rates. The comparison of the percentage growth rates between the groups of the two trials shows that the results of the two trials effectively cancel the effect of feeding out of the equation for Anthelia growth, since in the first trial feeding was correlated with high growth, while in the second trial it was correlated with low growth.

The answer to that problem seems to be that the growth rate was not effected by supplementary feeding, suggesting that the Anthelia coral fits more of a plant model for energy acquisition than the animal model. The experimental results suggest that there is indeed a variable affecting growth of the Anthelia coral in these conditions but that variable is not supplementary feeding. One possible cause of the shift is the placement in the tank, as that is the only thing that changed between the two trials, suggesting a variable such as water flow as the cause.

Possible Error

The fact that this experiment was conducted in a large tank with many other types of coral and organisms lends itself to a possibly substantial margin of error. While the overall environment remained fairly stable throughout the experiment, the Anthelia had to compete with other organisms for nutrients, resulting in possible error. Also, the Anthelia coral were often displaced by fish, sea urchins, and other animals, which may have had an effect on the growth rate of the coral. The length of the trials could have also had an impact on the results of the experiment. While trial 2 showed seemingly conclusive results, it was only run over 3 weeks, a relatively short period of time. It is possible, albeit unlikely, that the results of the 2nd trial could have been altered significantly with a longer period of time. Finally, subtle differences in water flow patterns around the tank may have effected growth. The rate of water flow in the tank could fluctuate over a short period of space, making the environment for corals different even if they were placed close together. This problem of water flow could have greatly affected the experiment conducted, since placement in the tank was changed between trials and deemed to be possibly important.

Further work

This experiment has shown that even corals grown in the same environment will exhibit different growth rates based on a difference in placement by only a few inches. It is possible that flow rates have effected the growth rates of the two sets of corals, but there are endless possible causes for the difference in growth rate, all of which lend themselves to more experimentation and investigation. The findings to these investigations and the answers that are produced from them will all have important implications for coral growth, both in captivity and in the wild, just as this experiment gave important insight into the way Anthelia sp. reacts to supplementary feeding and to how it gains its energy.

This experiment has also shown that supplementary feeding likely does not effect the growth rate of Anthelia sp. coral, and that the coral tends to use the Plant model of energy acquisition rather that the Animal model. Anthelia, however, is only one species out of thousands of coral species around the world. Each of these species acquires its energy differently and will use a different combination of plant and animal models. This opens thousands of possible new experiments to determine how certain corals gain energy and how best to propagate them. As more and more corals are studied, the extremely diverse coral ecosystems will be better understood. This understanding will hopefully lead to an improvement in the coral ecosystems around the world, improving the world as a whole and preserving the natural beauty of coral reefs.