FEEDING CORALS

Rhizotrochus typus, also known as the Rhizo Coral, is a rare stony coral with a solitary polyp, often stunningly beautiful and seen in a rainbow of colors. It lacks zoxanthellae and demands regular feeding. Some sources suggest that this species should not be purchased, as it may have been illegally shipped from Japan, where it is legal to collect but not to export. (Update: Clarification here.) Image by the author.

A fresh look at the diversity of dietary needs of corals in the wild and in our aquariums

By Tim Wijgerde, M.Sc.

When the earliest reef explorers peered into tropical waters and saw colorful things growing on the substrate, they assumed that they were seeing underwater flowers and shrubs. Further study revealed that many corals had stony skeletons, and there ensued years of debate about whether these curious objects were plant or mineral.

In the early days of marine biology, the great unexplained paradox was that corals seemed to grow in gin-clear waters devoid of nutrients. Only with the discovery of zooxanthellae in 1881 did the mystery begin to unravel. Students of corals gradually came to understand the complex symbiotic relationship between a stony coral—with its mineralized skeleton, layer of tissue, and fleshy polyps—and the millions of microscopic zooxanthellae cells thriving in its flesh. The sun fed the zooxanthellae and the zooxanthellae fed the coral, which was able to build carbonate skeletons and imposing reefs.

However, the story is not as simple as that, and just how corals manage to nourish themselves has been a point of debate and much controversy for years. For those of us who endeavor to keep corals in the aquarium, knowing how best to meet their energy needs is a vital question with many different answers.

Some still argue that corals with symbiotic zooxanthellae need no more than appropriate lighting to do very well in captive conditions. For years, the drive to find lights that would maximize coral coloration and health has dominated the reef aquarium hobby, and today we have lighting options barely dreamed of a decade ago.

Although hobbyists may still quarrel about the various ways in which corals derive their daily energy, marine scientists have pretty much reached a consensus about what they actually consume. A coral's diet can be roughly divided into three main categories; how big a role each of them plays depends on the species concerned.

These categories are (1) photosynthetic products which are synthesized by symbiotic algae; (2) food items such as plankton and drifting detritus; and (3) dissolved nutrients.

Let's take a closer look at these nutritive sources and the different coral groups that depend on them. For anyone who aspires to keep and grow healthy corals, a basic knowledge of coral nutrition is essential and also enriches our appreciation of these amazing animals.

PLANT OR ANIMAL?

Asteroides calycularis, an azooxanthellate stony coral found in the Mediterranean, feeding on live brine shrimp nauplii. For these corals, collecting plankton is a crucial means of survival. Photograph courtesy Jean-Louis Teyssié, IAEA Monaco.

For centuries, people unfamiliar with the strange creatures called corals have tended to consider them plants. They, at least the soft corals, do indeed resemble a flowering shrub or tree. When told that these organisms can actually harness the sun's energy, these people often consider their theory confirmed. However, further investigations eventually lead them to an undeniable truth: corals are animals. They have a primitive nervous system, digestive system, and muscles. Their cells lack chloroplasts, cellular organelles that form the machinery used for harnessing the sun's light energy, as well cell walls, the protective layers that surround the cells of plants and fungi. So corals are animals—and animals need to feed.

Stony corals like Montipora spp. feed on photosynthetic products, plankton, detritus, and dissolved molecules. Photograph: Tim Wijgerde

Autotrophy and Heterotrophy

Feeding is a very broad term, which must be further clarified. Both plants and animals naturally require feeding for growth, metabolism, and regeneration. The main difference is that plants produce their own organic molecules to feed on, while corals need ready-to-use organic molecules because they cannot synthesize these themselves. Plants use the sun's energy to convert inorganic CO2 into organic compounds such as carbohydrates, fatty acids, glycerol, and amino acids.

This ability categorizes plants as being autotrophic, which literally means self-feeding (noun: autotrophy). Animals, on the other hand are not blessed with this unique ability, and they have to consume plants or other animals in order to obtain their own organic molecules. This is what makes all animals heterotrophic, which means unable to synthesize their own food and dependent on feeding on complex organic substances such as carbon and nitrogen compounds (noun: heterotrophy). More simply put, heterotrophy equates to consuming different sources of energy.

Corals are quite special when it comes to these matters, as many species actually harbor plants inside their tissues. This is the basis for a lot of confusion. The plants in question are actually symbiotic algae of the genus Symbiodinium, commonly referred to as zooxanthellae. Algae are very adept when it comes to providing solar power for themselves and their coral hosts. They produce so many photosynthates, organic compounds arising from photosynthesis, that they actually translocate a lot of it to the coral's tissue. The coral uses this energy, which is provided mainly in the form of carbohydrates and glycerol, to build its skeleton.

Pocillopora damicornis is one of the most-studied corals on the planet and is very adept at using a variety of energy sources: light, plankton, and dissolved nutrients are all utilized for rapid growth.

The zooxanthellae are contained in the endodermal cells of the coral. Corals have two cellular layers, referred to as the ectoderm (epidermis, or outer layer) and the endoderm (gastrodermis, or inner layer). Each layer consists of two cells, which are glued together by a jelly-like substance called mesoglea, which makes up a large part of any Cnidarian. This substance is also found between the ectoderm and endoderm. The endoderm, or gastroderm, lines the digestive cavity of a coral polyp and has specialized cells that produce digestive enzymes. Other cells are able to take up nutrients, as human intestines do.

Corals also feed on plankton and detritus, which are organic particles. They also take up dissolved molecules from the water; these may be either organic (such as amino acids) or inorganic (such as ammonia, urea, nitrate, and phosphate). The latter category is used by the zooxanthellae—they require these inorganic building blocks to survive and grow. In the end, corals have both autotrophy and heterotrophy at their disposal. Therefore, they are best regarded as polytrophic, basically meaning multiple feeding (noun: polytrophy).

One can see, then, that light alone is not sufficient to keep healthy corals; other nutritive sources, like plankton, are important as well.

Plankton: the basis for life in the ocean

The term “plankton” is a common name for an astoundingly large group of organisms that can be categorized in various subgroups according to size. The term stems from the Greek word planktos, meaning drifter. Planktonic organisms come in all shapes and sizes, which are measured in micrometers (µm). One micrometer is equal to a mere 0.00004 inch! The figure below shows a commonly accepted division into pico-, nano-, micro-, and mesoplanktons.

Major plankton size classes, ranging from picoplankton, nanoplankton, microplankton and mesozooplankton, as part of the diet of marine (in)vertebrates. (A, B) Scanning electron micrographs of (A) Prochloroccocus sp., 0.6 μm and (B) Synechococcus sp., 1 μm. (C) Fluorescence microscope image showing one nanoflagellate cell indicated by a yellow arrow. Image of (D) ciliates, mean total length is 100 – 200 μm, taken under a phase contrast microscope and (E) crab zoea, mean total length is 1000 μm. 1 μm equals 0.00004 inches. Copyright Houlbrèque F. & C. Ferrier-Pagès, Biological Reviews, 2009, reprinted with permission.

These groups consist of cyanobacteria, small protozoa, and the smallest of algae (picoplankton), algae and medium-sized protozoa (nanoplankton), microscopic crustaceans such as rotifers and large protozoa (microplankton), and countless species of crustaceans like copepods (mesoplankton). Fish and invertebrate larvae can be further categorized into micro- and mesoplankton, depending on the species. Between 2 and 20 mm (0.08–0.8 inches), larger crustaceans such as Northern Krill (Meganyctiphanes norvegica) are categorized under macroplankton.

Finally, every living organism larger than 20 mm is classified as megaplankton. This is of relevance to LPS corals, which can devour larger shrimp or small fish entirely. Phytoplankton forms the basis of most marine ecosystems. These organisms are the so-called primary producers, which convert CO2 into organic compounds using sunlight. Zooplankton subsequently consumes the phytoplankton, which is, in turn, devoured by other, larger animals.

This is basically how the marine ecosystem works. Cyanobacteria, seaweeds, and zooxanthellae are also capable of photosynthesis and help support ecosystems such as seagrass beds and coral reefs. Without zooxanthellae, for example, stony corals wouldn't be able to build the vast coral reefs that exist today, which serve as a living space and food source for thousands of species. 

(Continued in the January/February Issue of CORAL, Number 7:1)

Images by the author, pictured above at Tenerife.
 

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