What Corals Eat

MYTHS & REALITIES OF CORAL NUTRITION

By Ronald L. Shimek, Ph.D.
Excerpt from the January/February Issue of CORAL

The mid-oceanic waters surrounding Indo-Pacific coral atolls, with their spectacularly colored azures and transparent cobalt blues, are renowned for their clarity. Underwater visibility is often more than 100 feet (30 m). It is fair to say that these wonderful underwater coral vistas are one reason that the marine aquarium hobby has become so popular; people want to recreate miniature coral reefs in their living rooms. The average aquarist’s reef tank contains beautifully colored animals and waters of crystalline clarity.

Scientists are no less enamored of the beauty of a coral reef than are casual observers, and once they began studying reefs in detail it became apparent that there was a strong correlation between crystal-clear waters and healthy reefs. But this seemed like a paradox: Water clarity indicates a lack of suspended materials. Corals are animals, and by definition, all animals must feed. If there is nothing in the water for them to eat, what keeps coral reefs alive and thriving?

By the middle part of the last century, research began in earnest to solve this conundrum.

Plankton
One of the first things that had to be done was to verify that there actually was nothing for the corals to eat. By the late 1950s, the waters around coral reefs had been sampled in many parts of the world, and the plankton that researchers thought could have been coral food was nowhere to be found.

A calanoid copepod typical of those found in temperate plankton blooms. This rugged animal, about a centimeter (0.4 in.) long across the large antennae, can be collected in a plankton net without sustaining any damage. Photo by Ronald L. Shimek.

The sciences of oceanography and marine biology got their starts in the cold, rather murky waters of the north temperate regions—particularly in the North Sea, around New England, and in the dark waters of Puget Sound. These regions are characterized by very rich, seasonally abundant plankton populations called plankton “blooms.” To the average landlubber, the amount of life in the sea water during one of these temperate spring plankton blooms is unimaginable: a cubic meter (about 264 gallons) can easily contain over half a million animals that are visible to the unaided eye. The sheer abundance of these little creatures is, in part, what reduces the water clarity in these temperate areas, rendering it effectively opaque. 

When scientists who had taken samples from such dense temperate plankton populations sampled the water around coral reefs, they found nothing comparable. In fact, in many cases they found nothing at all. And, of course, they were not surprised—one needed only to look through the water to see that there was no plankton in it. Therefore, case closed—the oceans around reefs were “biological deserts.” As a corollary of this finding, it was assumed (but not proved) that corals must produce all, or at least the vast majority, of their own food. And the only way they could do this was to be mostly autotrophic. In other words, corals didn’t need to feed; they got all of their nourishment from their symbiotic zooxanthellae.

When Dinos Ruled The Reef…
It was during the early to mid-1970s that the physiology of the coral—including algal symbiosis—was first investigated in depth. The late Len Muscatine and his students and co-workers at the University of California, Los Angeles led this pioneering work. They showed quite conclusively that the dinoflagellates in most zooxanthellate corals produced a tremendous amount of photosynthate, and that, in many cases, the majority of this material leaked from the zooxanthellae into the coral host.

Photosynthesis makes only carbohydrates, basically sugars and sugar derivatives, made of carbon, hydrogen, and oxygen. The researchers found that as a part of the coral’s nutrition, the zooxanthellae could provide all of the coral animal’s daily carbon requirements. This was an amazing finding, and the knowledge that the zooxanthellae contributed greatly to the coral’s nutrition soon became widespread. This information was melded with the idea that there was little in the way of plankton in the surrounding waters, and then transmogrified into the notion that corals could live where they did because they didn’t need to feed at all.

Note the chain of events here: First, the plankton in the waters around reefs appeared to be insufficient to feed all the corals. Second, it was discovered that the zooxanthellae found in corals produced all the basic carbon compounds that their hosts needed. Third, somehow the statement “Zooxanthellae can produce 100 percent of the carbon, or sugar, required by corals” morphed into “Zooxanthellae can produce 100 percent of the nutritional needs of corals.” Finally, the conclusion was arrived at: “Corals do not need to feed.” Just shine bright sunlight on them, the conventional wisdom said, and their zooxanthellae would produce all the food the corals could use.

To the credit of the scientists working with corals or zooxanthellae, none of them proposed this ridiculous conclusion; however, it seemed to rise like so many bubbles of hydrogen sulfide out of the decaying sludge of poor scientific journalism—including, I must say, numerous aquarium references whose authors, in some cases, still seem to believe it. 

You Only Find What You Look For

A pelagic tunicate called a larvacean. Typical of mid-ocean gelatinous plankton, this small animal lives in a “house” made of mucus (arrows show the edges of the house). The animal pumps water through the house, filtering out food. When the filters are full, the animal eats them, detaches from and bails out of the house, and secretes a new one. The animal may construct 8 to 10 houses a day. Discarded floating larvacean houses become colonized by bacteria and when eaten by corals, provide a large amount of protein to a coral reef.  A larvacean house disintegrates on contact with a plankton net. Photo by Ronald L. Shimek.

Meanwhile, back on the reefs, a new group of scientists started putting two and two together and getting three. Things did not add up. If the waters around reefs were so plankton-poor, how come reefs were covered with plankton-feeding fish? Maybe the zooxanthellae did the trick for corals, but there are no zooxanthellate reef fish. And what about all the other plankton-feeding animals in the waters surrounding the reefs? Where did all their nutrition come from?

In several brilliant research projects from the late 1970s through the mid-1990s, researchers took a new look at the plankton puzzle. Remember, the early workers who had sampled the waters around reefs for plankton had done so using the same methodology and gear that they had been trained to use in the cold northern waters, where they had worked very well. The scientists had assumed those practices would work just as well in the tropics—and they do. They sampled the plankton in the tropics and, finding virtually nothing like the plankton found in the temperate regions, concluded that there was no plankton in these tropical waters. Unfortunately, what they should have concluded was that there was no plankton in the tropics that compared to that found in temperate habitats; the types of organisms from the two habitats were very different. 

Temperate plankton is composed largely of small crustaceans, fish larvae, some mollusks, and relatively large numbers of moderate-sized jellyfish and comb jellies. For their size, these are rugged animals. A large proportion of them can be collected simply by pulling a net through the water. Sure, some of them are battered and beaten up, but a good many of them survive the sampling process. A case in point: some of the most common planktonic animals in temperate regions are various calanoid copepods in the genus Calanus. It has been stated that some species of these calanoid copepods are the single most abundant large animal (large is relative) on the planet; Calanus finmarchicus has a body about the size of a grain of rice, and it is estimated that their annual productivity is in excess of 100 billion tons. That is a LOT of small planktonic animals. 

By the early 1990s, it was evident that tropical plankton was very different from its temperate counterpart. It comprises small gelatinous animals, such as larvacean tunicates; tiny jellyfishes; some huge, very complex, but exceptionally fragile jellyfishes; and medusoid animals, larvae, and huge amounts of bacterial particulate material. Instead of animals, most tropical plankton is made up of primarily bacterial aggregations. These sometimes form visible particles called “marine snow.” The huge shoals of planktonic calanoids are notable by their absence. Tropical plankton is simply too delicate to sample with the “standard” gear in use in the 1950s. A standard plankton net pulled through tropical waters doesn’t collect plankton; it shreds and squeezes it through the net, resulting in a slurry of small particulate material. The water clarity in the tropics is not due to a lack of plankton—it is a result of the plankton organisms being so small that they are like fog particles in the water.

Back To Corals…

A non-photosynthetic Dendronepthea coral in an aquarium, with polyps in feeding mode. Stony corals without zooxanthellae must eat to survive and reproduce. Photo by David Hammontree/www.Reef2Reef.com

Zooxanthellae provide a coral with all the sugars it can use; much of it is almost immediately excreted from the coral as mucus. Sugars are used for energy; consequently, zooxanthellae provide fuel, which the coral uses to feed on other things. What zooxanthellae cannot provide are the important nutrients such as proteins, phosphates, minerals, and a host of other materials. They do provide the energy to build and utilize the coral’s own plankton-sampling machinery: the tentacles, the nematocysts, the mouth, and the digestive tissues.  

One of the famous questions in biology is: “What is life?” It is amazingly difficult to define all life inclusively. The major property of life, however—its main defining characteristic—is that it evolves. And one of the major characteristics of natural selection is that it removes anything that is costly or unnecessary. All organisms are on a budget, and any organism that spends energy producing unnecessary body parts eventually loses the race to another organism that has eliminated superfluous items. British zoologist Sir Charles Maurice Yonge once famously noted that of all predators, corals devoted the largest proportion of their bodies to food capture and consumption. This property alone tells researchers, and should tell aquarists, that corals need to eat to obtain proteins for new tissue, phosphates for nucleic acids, and many other substances. On the other hand, those cnidarians that don’t need to eat—for example, a few species of Xenia or Mastigias, the jellyfish in landlocked marine lakes—don’t have mouths or functional guts. 

A reef is a wall of mouths; everything on the reef eats. By the late 1980s, good data on how much food is ingested by reef animals were starting to become available, and that amount is tremendous. One of the better papers on the topic was authored by W. M. Hamner, et al., in 1988. Their research showed that in a 24-hour period, over 2,000,000 food items, weighing about 750 grams, have an impact on and are eaten by the animals in a 1-meter-wide portion of a reef crest community with a volume equivalent to that of a 400-liter (slightly more than 100-gallon) aquarium. These food items are tiny, essentially invisible to the naked eye.
 

References
Alldredge, A.L. 1972. Abandoned Larvacean Houses, A Unique Source of Food in the Pelagic Environment. Science 177: 885–887.
Alldredge, A.L. and M.W. Silver. 1988. Characteristics, Dynamics and Significance of Marine Snow. Prog Oceanogr 20: 41–82.
Hamner, W.M., M.S. Jones, J.H. Carleton, I.R. Hauri, and D. McB. Williams. 1988. Zooplankton, Planktivorous Fish, And Water Currents On A Windward Reef Face, Great Barrier Reef, Australia. Bull Mar Sci 42: 459–479.
Muscatine, L. 1973. Nutrition of Coral. In: Jones, O.A. and R. Endean, eds. Biology and Geology of Coral Reefs, vol. 2, Biology 1, pp. 77–115. Academic Press, New York.
Yonge, C.M. 1968. Review Lecture: Living Corals. The Royal Society of London, Proceedings B, Biological Sciences, Vol. 169: 329–344.
 

Copyright © 2010 CORAL Magazine and Ronald L. Shimek, Ph.D.  All rights reserved.

Dr. Ronald L. Shimek is a marine zoologist and author of The PocketExpert Guide to Marine Invertebrates (Microcosm/TFH, 2004, and numerous scientific papers and magazine articles. He lives in Wilsell, Montana.