What do corals eat in the wild?
All living organisms need food to live. It is no different with corals. However, the food of aquarium corals depends on what they eat in the wild. From the previous post Do I need to feed my aquarium corals?, you know that feeding corals is necessary, even those that live in symbiosis with zookstanthellae. Now you will learn what their diet is like in the wild.
Coral diet
What do corals eat in the wild? It turns out that most corals can derive food from a symbiotic relationship with autotrophic zooxanthellae. It would seem that these corals have won the lottery of evolution and have had it easy for millions of years when it comes to acquiring food. Unfortunately, global warming is causing many of these corals to be endangered due to a process known as coral reef bleaching. Corals that lose zooxanthellae can still live for several months as long as there is no shortage of food for heterotrophic nutrition. If the water temperature drops below critical during this time, there is some chance of restoring zooxanthellae in the tissues.
Corals that lose zooxanthellae can still live for several months as long as there is no shortage of food for heterotrophic nutrition.
In this scenario corals, which evolutionarily did not develop the ability to symbiosize with zooxanthellae, are in a much better position, as they feed by catching food from the water. These corals, called AZOOX or NPS by aquarists, are completely independent of light. In the wild, they often inhabit underwater caves and gorges, where a strong current provides them with water rich in organic particles. In the aquarium, AZOOX corals are rather less popular due to the need for target feeding, which often requires adjusting the entire system for such feeding. Unfortunately, unless we live on the shores of one of the islands of Indonesia or Oceania, we do not have the opportunity to use natural and live food all the time. Therefore, we need substitutes.
Aquarium as a complex ecosystem
Marine aquaristics is one of the few hobbies where you need to take care of the entire reef ecosystem. It is not self-sufficient and complete. On the other hand, it is probably the most complex ecosystem that we can keep in an aquarium and even obtain simple, more or less stable cycles of matter and food chains. That is why we should rather talk about feeding the entire system rather than a specific group of organisms.
After all, fed fish will excrete ammonia, which will be converted by bacteria into nitrates. Nitrates, on the other hand, will be used by unicellular autothropic organisms. These, in turn, will be food for rotifers, which will be eaten by corals or crustaceans, which will serve as food for fish. Fish excrement will feed the corals again and the circle is closed. Fish food, will not be ignored by corals or polychaetes. Coral food will be eagerly eaten by smaller fish.
Corals kept in an aquarium together with fish have greater growth rate and better condition than corals kept in tanks without fish.
Unfortunately, it is not a self-sustaining system, and the aquarist must take care of an adequate food supply so as not to disrupt the biology of the entire aquarium. Too much food will cause the increase in the supply of inorganic nutrients such as NO3 or PO4, which, if the situation lasts long enough, will be utilized by rapidly proliferating autotrophs such as algae. When there is not enough food, the condition and population size of certain groups of organisms, such as gammarus and snails, will be affected.
Natural foods eaten by the corals
When we consider feeding corals, we must remember that they are able to use different types of food. They include dissolved organic matter (DOM), dissolved inorganic matter (DIM), particulate organic matter (POM) and plankton.
Dissolved organic matter (DOM)
As early as 1960, scientists discovered that corals of the genus Fungia were capable of absorbing radiolabeled glucose directly from water. Researchers often distinguish between several important types of DOM:
- DON – dissolved organic nitrogen
- DOC – dissolved organic carbon
- DFAA – dissolved free amino acids
- UREA – a less toxic form of ammonia excreted by many marine animals.
Corals feed on these substances, but absorb them in very low concentrations of nanomoles/1L of water. For nitrate, 1nM translates into 1 ug NO3/1L of water. Stylophora pistillata takes up to 21% of the nitrogen budget from DFAA. A similar volume of absorbed DOM can be observed for other calcifying corals.
Dissolved Inorganic Matter (DIM)
We must not forget that the minerals dissolved in the brine are also part of the necessary food. Especially in the case of corals, which form limestone skeletons. DIM is mainly a mixture of macro and micronutrients and gases. In an aquarium, we can dose DIM using various methods – calcium reactor, Balling/kalk method and regular brine changes.
Particulate organic matter (POM)
It is mainly detritus (undecomposed remains of organisms) suspended in water. Many scientists also include bacterioplankton in this category. In the aquarium, we also have the excess food not decomposed by the bacteria. Both on the reef and in the aquarium, detritus accumulates as sediment in areas with slower water flow. The process of decomposing detritus is called mineralization. Sediment contains bacteria, protozoa, their excretions, microscopic invertebrates, algae and non-living organic matter. Such sediment is a food source for many corals, especially in turbid waters where light penetration is difficult. Numerous experiments have been conducted, showing that Fungia horrida and Acropora millepora readily absorbed sediment. It was also noted that the more detritus there was in the water the more was absorbed by the corals. It was calculated that in Montastrea franksi, Diploria strigosa and Madracis mirabilis 50-80% of the absorbed sediment was converted into coral biomass.
Plankton
For many years, plankton was not considered an important part of the corals’ diet. Scientists believed that the concentration of plankton on the reef was too low to be of any significance. In the meantime, more thorough studies using much more modern techniques have been done. It turned out that the amount of available plankton varied not only during the period of the year, but also during the day. Zooplankton includes actively swimming organisms that can move between the reef and the water column.
Immediately after sunset, the concentration of zooplankton rises sharply. This is when these animals migrate into the water column from their hiding places on the reef. As a result, at night, for example, the amount of Copepod (500-700 μm) is five times greater than during the day. The concentration of other small invertebrates increases fourfold overnight. Then the larvae (>700μm) of other organisms such as tunicates and polychaetes also appear.
You can observe a similar situation at night in an aquarium when you illuminate it with a strong stream of light. Unfortunately, this effect is largely destroyed by the presence of mechanical filters that cannot distinguish what is useful and what is not.
What do aquarium corals eat?
Studies have shown that corals are excellent at assimilating fish feces. This is because they still contain a lot of nutritional value. Moreover, fish feces contain probiotic bacteria. These bacteria can be additional food for corals. The aforementioned study showed that corals kept in an aquarium together with fish have greater growth rate and better condition than corals kept in tanks without fish. Another conclusion that emerges from this research is that it is very important to feed fish with good quality food containing balanced nutrients. Unfortunately, many aquarists feed fish with frozen artemia, which is not of marine origin and have low nutritional value. Frozen Mysidacea is much better.
However, in terms of nutritional value, the most beneficial are dry foods in the form of granules or flakes, the composition of which is selected specifically for marine aquarium fish. Such foods are rich not only in protein, but also in amino acids, vitamins and fatty acids. And let’s not forget about the natural food produced by the reef itself in the aquarium. Many organisms in the aquarium reproduce or excrete feces, and since nothing dies in nature, everything can have some nutritional value for corals and filter feeders. That’s why biodiversity in any ecosystem is so important, and an aquarium is no exception.
Literature
Renaud Grover et al. (2008), Uptake of dissolved free amino acids by the scleractinian coral Stylophora pistillata, Journal of Experimental biology, volume 211, issue 6.
