Corals have a large number of ways in which they can obtain their nutrition, for the most part the corals we keep in aquaria are photosynthetic and as such are able to obtain much of their nutritional demands through their symbiotic relationship with photosynthetic algae known as Zooxanthellae. However corals also have the ability to prey on other live organisms or absorb nutrients from the surrounding water. In this article I will cover some of the basic feeding techniques employed by various Cnidarians.
Although corals gain much of their nutritional requirements through photosynthesis they also rely partly on their ability to capture and feed on other organisms. The amount of nutrition that corals gain through plankton capture varies from species to species, studies by Muscatine and Porter (1977) found that in colonies of Monastrea cavernosa no more than 20% of the corals energy requirements are met by capturing plankton. Johanes and Tepley (1974) found that in the coral Porites lobata around 10% of the energy requirements are met by plankton capture, other studies have shown that around 5% of corals energy requirements are met in this manner. The overall morphology of a coral can give us an idea as to how much a coral relies on plankton capture as a means of gaining it’s nutritional requirements; corals with numerous large polyps will be more effective at plankton capture than species with few or very small polyps. One of the main mechanisms by which a coral captures plankton is by the use of stinging cells known as nematocysts. The nematocyst is a sack-like structure with an inverted thread that houses a barb at the end. The nematocyst is filled with fluid which often contains toxins used to paralyse prey. The thread of the nematocyst is hollow and upon firing pressure in the fluid sack causes the thread and barb to be extended out of the sack and into the prey item, fluid and toxins are then injected from the sack, through the thread and barb and into the prey. The pressure contained in the nematocyst cell is very high with estimates of up to 2,200 lb per sq inch, this high pressure causes the nematocyst barb to be fired at a very high velocity into the prey. The number and design of nematocysts varies from species to species, there are over 25 different types that have been described in the group Cnidarians and these vary in abundance and distribution over the surface of the animal. In some sea anemones the number of nematocysts can be as high as 10,000 per square millimeter and many thousands of nematocysts can by used to capture a single tiny prey item, once the nematocysts have been fired they cannot be reused and they are discarded and replaced by new ones. The stimulus to cause a nematocyst to fire can vary, often it is physical contact by a prey item on the trigger of a nematocyst that causes it to fire, however Rupert and Barnes (1994) have also shown that chemical stimulus and combinations of other stimuli can also be responsible. Corals also feed on bacteria and unicellular plankton by means of mucus excretion. Many corals excrete a mucus that forms a thin layer over the surface of the coral, this mucus contains high levels of carbohydrates which is the primary food source for bacteria. Bacteria from the surrounding water settle on and colonise the mucus layer, this mucus and bacteria is then sorted by cilia and fed on by the coral. It is also possible that the high concentrations of bacteria on the mucus layer attracts unicellular zooplankton which feed on the bacteria and are in turn, fed on by the coral. Although corals are able to gain much of their nutritional requirements through photosynthesis we should not ignore their need to capture plankton and bacteria. As our understanding of coral biology and nutrition increases there has been a move in the reef keeping hobby to provide higher levels of plankton for our corals to feed on and this can only be a good thing. The type of plankton that corals feed on will vary enormously according to the species in question and the type of plankton that is available. Many species of coral, particularly soft corals will filter fine dead organic material out of the water column, other species require live zooplankton of the right type to elicit a feeding response. Overall live zooplankton will feed a much greater diversity of coral species than dead particulate matter. On coral reefs the majority of zooplankton captured by corals are various types of copepods. Copepods vary greatly in morphology and size, often there are many species of copepod already present in a well maintained reef system and populations of these can be increased by adding phytoplankton to your reef system which the copepods will feed on. Rotifers and newly hatched Artemia also make a good source of food for many species of coral and these along with copepods can easily be cultured to regularly feed to a reef system. The capture of plankton provides corals with essential nutrients such as phosphates, amino acids and vitamins and as such is an aspect of coral nutrition that we should not ignore.
The zooxanthellae that are found in the tissues of corals require organic nutrients such as phosphates, nitrates and carbon for photosynthesis. How the coral obtains these nutrients has long been studied by biologists, some of the nutrients come from plankton captured by the coral which are digested and passed on to the zooxanthellae for use in photosynthesis. Amino acids produced by the zooxanthellae are passed on to the coral and used in respiration, the waste products of which are passed back to the zooxanthellae and hence nutrients are recycled. However the waste products produced from plankton capture and digestion by the coral which are passed on to the zooxanthellae are not enough to meet the requirements of the algae for photosynthesis. It has long been known that corals are able to absorb dissolved nutrients through the body wall, studies have shown that corals are able to absorb glucose from solution, Stephens (1962) showed that Fungia scutaria is able to absorb 1-2mg of glucose per hour from solution. As coral reefs waters are so low in organic molecules the ability of a coral to absorb nutrients through the body wall and to capture plankton is essential as without these processes the coral would not be able to pass enough nutrients onto the zooxanthellae to enable photosynthesis to continue.
Photosynthetic corals cannot gain all of their nutritional requirements from plankton capture or nutrient absorption so they must rely largely on photosynthesis. Organic molecules produced by the zooxanthellae during photosynthesis are passed on to the coral host tissues and provide the coral with much of it’s daily nutritional requirements. Zooxanthellae are unicellular algae that have a symbiotic relationship with the coral and photosynthesise in the same way as other algae and plants. By using waste products from the coral as well as nutrients absorbed from the water the zooxanthellae photosynthesise and provide the coral with glycerol, glucose, organic acids and organic phosphates.
The zooxanthellae are located in the upper tissues of the coral known as the endodermis, the diagram in figure 2 shows the basic structure of the coral tissue with zooxanthellae present. Light is obviously one of the key ingredients for photosynthesis to occur, however corals live at a great range of depths and light intensities on a coral reef and so must be able to adapt to make the most of the environment that they are living in. Fortunately this ability to adapt works well for us and our endeavours to keep corals in aquaria. By adjusting the concentration of the zooxanthellae present in it’s tissues, changing the growth shape of the colony, altering the absorption spectrum of the chlorophyll pigments and increasing or decreasing the number of polyps present the coral is able to adapt to the light intensity that it is receiving and optimise it’s rates of photosynthesis for a given environment.
The amount of energy that a coral obtains via photosynthesis varies in different species of coral, in some studies the lower values reported are in the region of 50% of the daily nutritional requirements and the upper values are in the region of 98% of the daily nutritional requirements being met via photosynthesis. Zooxanthellae like all plants require nutrients for photosynthesis, particularly nitrogen and phosphorus, however readings on coral reefs have shown that the concentration of these nutrients is too low to support the level of photosynthesis by the zooxanthellae. To overcome this problem the coral recycles nutrients to aid in the process of photosynthesis. Nutrients are first absorbed through the body wall of the coral and passed on to the algae cells and used in photosynthesis. The products of photosynthesis are then passed from the zooxanthellae back to the coral hosts tissues and are used in respiration. The metabolic waste products from the coral are in turn, passed back to the zooxanthallae and used again for photosynthesis. In this way the coral and zooxanthellae are able to co-exist in an environment where nutrient levels and food sources can be extremely low.
The situation in aquaria is slightly different as there are generally higher nutrient levels than are found on coral reefs – particularly when concerning nitrates and phosphates. However well maintained aquaria can also have very low levels of nutrients such as phosphates and nitrates and it should not be thought that these nutrients are always in abundance in comparison to coral reefs. Aquariums highly stocked with coral that are growing healthily can become devoid of nutrients needed for photosynthesis, in fact some coral and clam farmers deliberately add these nutrients to the water so that they do not become the limiting factors for photosynthesis.
Careful monitoring of nutrients such as nitrates and phosphates is necessary to maintain good photosynthetic and growth rates in corals as too much of these substances can be as damaging as too little. Feeding your coral is also recommended to help maintain a healthy reef system. The rule of thumb is to add as much food to your system as you can without deteriorating water quality. Live zooplankton such as copepods, rotifers and Artemia as well as phytoplankton make an excellent food source for your corals. However you should pay particular attention to the target species of coral that you are feeding, some species of coral such as the LPS corals naturally feed on larger prey items such as shrimp or small fish whereas filter feeders such as gorgonians will feed on smaller particular matter.
Whatever species of coral you keep they all have developed particular feeding strategies usually incorporating all three techniques to one extent or another; plankton capture, nutrient absorption and photosynthesis. By providing the corals with the correct levels of light intensity, nutrient abundance and enough plankton to capture will you be able to obtain a flourishing and vibrant reef aquarium