slowly down into the deep cold water and to the bottom, taking nutrients with them. While currents called upwelling bring nutrients from the bottom back up to the surface fueling blooms of plankton in temperate and polar waters, upwelling is rare in tropical waters. The result is low nutrient levels in warm tropical surface waters, low densities of plankton, and hence clear water.
ZOOXANTHELLAE
The low nutrients in warm, clear, shallow tropical waters pose a paradox for coral reefs. Coral reefs have abundant organisms, and high rates of photosynthesis and growth. How can this occur in waters that are low in nutrients? How can this oasis flourish in such a biological desert? A variety of mechanisms probably contribute, but perhaps the most important is the algae living in corals. Corals are animals related to sea anemones and jellyfish. Corals have small polyps that are nearly identical to sea anemones. However, they contain within them the seeds of their success. These are the tiny single-celled algae known as zooxanthellae. The algae are members of a group called dinoflagellates. The algae and the coral animals live in a mutualistic symbiosis (i.e., mutually beneficial coexistence). The waste products of the animal contain the nitrogen and phosphorus that the algae need. The algae perform photosynthesis in the sunlight, and leak much of what they produce into the surrounding animal cells (around 80%). So the algae benefit from the nutrients the coral animal produces, and the coral benefits from the food that the algae produce. In effect, this is a tight recycling arrangement, with nutrients passed from one partner to the other, and then back to the other partner. As a result, the combination of these two partners needs a smaller input of nutrients from the outside, and can survive in nutrient-poor, warm, clear, tropical waters.
Plants on Coral Reefs
On most coral reefs, animals are obvious and appear to be common, while plants are less obvious and appear to be less common. But only plants can produce food through photosynthesis. When animals eat plants, most of the food is used to produce energy to run the animal’s bodily functions, while only a small part is added to the material of the growing animal. As a rule of thumb, only about 10% of what an animal eats is incorporated into its body in growth. As a result, there must be about ten times as much biomass of plants as herbivores that eat them. And there must be about ten times as much herbivore mass as carnivore mass that eats them, and so on up the food chain. Thus there must be about 100 times as much plant biomass as carnivore biomass for carnivores that eat herbivores; 1,000 times as much to support carnivores that eat other carnivores. And yet on coral reefs animals are obvious and appear abundant, while plants are usually less obvious and appear less abundant. How can this be? First, some plants are hidden. Zooxanthellae are found in hard corals, soft corals, giant clams, and a few other animals on coral reefs. They provide a large part of the food production on a coral reef, and yet are not obvious. They actually provide much of the color in corals and giant clams, yet we normally don’t recognize them as plants. Second, the algae which can be seen on coral reefs include some species which are small and hard to see but are highly productive. Large fleshy algae grow slowly and put most of their growth into defenses such as woody cellulose that is hard to digest, calcium, and chemicals that are bad tasting or toxic. Defenses are necessary for a plant to grow large on a coral reef, since there are many hungry mouths of herbivores, such as fish, sea urchins, and snails. Fish alone bite algae about 40,000 to 156,000 times per square meter of reef per day! So herbivory is intense on coral reefs. A second group of algae is the filamentous algae. These algae are made of tiny strings of cells with little or no defense. Their main defense is their ability to grow very rapidly. Herbivores bite most of their growth off daily or hourly, but the base of each filament attached to rock rapidly grows the string back. So filamentous algae are highly productive fast growing algae, but have a very low standing biomass and are hard to see.
Species Diversity
Coral reefs are not only geological structures, but also biological communities. Coral reefs are amazingly diverse and complex ecosystems. They are the most diverse marine ecosystem known (i.e., they are the most species-rich). Sometimes they are said to be the most complex ecosystem on the planet, but they actually have fewer species than tropical rainforests. Rainforests have large numbers of insect species, and insects are by far the most species-rich group of organisms on the planet. There are more insect species known than all other organisms combined, and there are more insects in tropical rainforests than anywhere else on earth. There are very few marine insects, and none known on coral reefs. The total number of species is not known for either coral reefs or rainforests. Around 1.8 million species have been described on earth, with a majority of those being insects. Estimates for the total number of species on earth range from about 3 to 120 million, with about 5–10 million being most likely. About 85% of all species are arthropods, and a majority of those are insects. On coral reefs, one estimate is that 93,000 species may have been described, but the total may be ten times higher (Reaka-Kudla 1995a,b). However, at the level of the largest groups of animals, known as phyla (such as Mollusca, Arthropoda, and Chordata) there are more phyla of multicellular animals (metazoa) in the oceans (about 29 out of 32) than on land (about 13) and in freshwater (about 16) (Rupert and Barnes 1994), and coral reefs probably have more phyla than any other ecosystem (about 26).
Relationships among Organisms
Coral reefs have very large numbers of species living together and interacting in a very complex web of relationships. We have already spoken of the mutualistic relationship between corals and the zooxanthellae they host. There are many other examples of coral reef species that have mutualistic relationships with other species, such as the Anemonefish (Amphiprion) that live among the tentacles of sea anemones; cleaner fish, such as Labroides dimidiatus, and shrimp, such as Periclimenes, that clean parasites off of fish; snapping shrimp (Alpheids) that excavate burrows which they and guardian prawn gobies (Gobiidae), occupy, and the Guard Crabs (Trapezia) that live in the branches of corals (Pocillopora) and defend them.
In another type of symbiosis, commensalism, one organism lives on another and benefits, while the host organism is neither helped nor hurt by the relationship. An example is the shrimp Periclimenes that live on sea anemones, starfish, nudibranchs (sea slugs) and other animals. A third type of symbiosis is parasitism, where one partner benefits at the expense of the other. Many small crustaceans and flatworms live on the skin of fishes, eating mucus and tissue off of the fish. Some crustaceans called isopods live attached on the outsides of fish, sucking tissue and blood. There are so many species of flatworms that are parasitic on snails in at least one stage of their life cycle that it is said that nearly every species of snail is parasitized by a species-specific parasitic flatworm. One parasitic flatworm, Plagioporus, lives in corals, Porites, at one stage in its life, causing the host polyp to expand and turn pink. The larger pink polyp stands out and is often eaten by butterflyfish (Chaetodontidae). This transfers the flatworm to its next host (Aeby 1991).
New relationships like these are being discovered all the time. For example, the snail Dendropoma maxima produces an uncoiled shell in a coral. The snail, called a vermatid because its shell resembles a worm tube, secretes mucus, which it drapes over the coral surface. The snail pulls the mucus in, dragging along with it additional mucus produced by coral, and eats it. The snail is thus parasitic on the coral, and stunts the growth of the surrounding coral (Fenner, under review a). Infectious diseases are similar to parasites, except that the agents that cause the diseases are generally microorganisms such as protozoa, bacteria, and viruses. Coral diseases have increased in number and severity in recent years, both on Caribbean and Indo-Pacific reefs. Diseases were detected at 10 of 45 (22%) sites during a recent survey at the Raja Ampat Islands (McKenna, Boli, and Allen 2002).
Nearly all animals on coral reefs are either predators or herbivores, the exceptions being those hosting endosymbiont algae such as corals. Predation may shape some communities, with apex predators dominating fish communities in pristine coral reefs such as the northwest Hawai’ian Islands. Predators may increase diversity by preying on the most abundant species. Two important predators of corals are the Crown-of-Thorns (COTS) starfish (Acanthaster planci) and snails of the genus Drupella. Crown-of-Thorns starfish came to the attention of scientists and the public in the mid-1960s when there
