Before we can talk about coral reefs, we need to start with a basic question—what is a coral? Is it animal, vegetable or mineral? The correct answer is that it can be all three. Corals are animals that may have a special relationship with a microscopic plant and that can make an external limestone (or mineral) skeleton.
What we see as a large head (or colony) of coral is actually made up of thousands of tiny individual animals called polyps. These polyps range in diameter from about 1 mm to 1 cm or more. Each polyp looks somewhat like a small sea anemone, which is not surprising, as corals and sea anemones are cousins. Another member of the coral’s family is the jellyfish. All three of these groups contain stinging cells called nematocysts. Most corals’ nematocysts are not strong enough to penetrate human skin, so we cannot feel them sting, but they are able to use their nematocysts to defend against some predators and to catch food.
Coral reefs form in tropical areas. As we’ve mentioned briefly, reef-building corals have a partnership with a type of microscopic plant or algae. Since plants need sunlight in order to make food, reef-building corals can only survive in fairly shallow areas where sunlight can penetrate. Most corals cannot tolerate cold water temperatures; those that can handle the cold do not grow very quickly and do not form reefs.
This relationship between corals and algae is a type of symbiosis. Both the coral and the algae (known as zooxanthellae) benefit from the partnership, so this is an example of a mutualistic symbiosis. The zooxanthellae are safe from predators inside the coral tissue (remember, corals have stinging cells) and the coral provides them with nutrients in the form of excreted nitrogen and phosphorus. In return, the zooxanthellae provide the coral with sugar compounds that they make when they photosynthesize. Most corals appear brown in color. This is a result of the millions of brown zooxanthellae that are found in their cells. Occasionally, when corals are stresses by high water temperature or other factors, the zooxanthellae will be lost, leaving the corals appearing almost pure white. This phenomenon is known as coral bleaching. The coral may die following bleaching, but often it will recover and become repopulated with zooxanthellae.
Coral bleaching can be the result of many different types of stressors. We most commonly think of it in relation to increased water temperatures, but UV light, high or low oxygen tension and prolonged darkness can also cause bleaching. If corals become covered with sediment, both oxygen levels and light levels will decrease and bleaching can also result. We do not know whether it is the coral animal that “kicks out” the zooxanthellae, or the zooxanthellae that “choose” to leave. We do know that bleaching involves loss of the zooxanthellae, and not just loss of the photosynthetic pigments (chlorophyll). When the coral is bleached, the animal is still there, just without the zooxanthellae. In many cases, corals can and will recover from bleaching events. We do not know whether a few zooxanthellae that are left in the coral simply multiply and re-populate the coral, or if zooxanthellae from the water or sediment somehow get into the coral and repopulate it.
Since bleaching is most often associated with increased water temperatures, it should be no surprise that most bleaching is reported in summer months. In the Atlantic, corals usually bleach first in the Caribbean, then bleaching events are reported later in the summer for Florida and Bermuda. One interesting observation is that in many cases, the entire coral head does not bleach. Bleached patches on large coral heads, usually on the sides of the colony, often occur. There may be two colonies side by side, one of which is bleached and one which is not.
One climatological phenomenon that has an impact on coral bleaching is the El Nino Southern Oscillation, often called simply “El Nino” or ENSO. This phenomenon occurs in the Pacific Ocean, but it impacts climate globally. Under normal conditions, winds blow ocean currents near the equator in the Pacific from east to west. As surface water flows away from the Pacific coast of South and North America, cool bottom water is pulled up to the surface, causing upwelling. This cool, nutrient-rich water provides food for plankton and for a large anchovy fishery off Peru. For some reason, about every 3-4 years, the winds in the Pacific equatorial region reverse direction, and warm water from the western Pacific flows towards the Americas. This influx of warm water results in large-scale bleaching events in Hawaii, Panama, Colombia and other coastal areas. This figure shows data from reported bleaching events in Australia. The arrows indicate El Nino years. You can see that in most El Nino years, there are increased numbers of bleaching events reported. Meteorologists do not understand exactly what causes El Nino years to occur, although there have been suggestions that El Nino events may be related to global warming.
Scientists were puzzled by the patchiness of bleaching, especially when parts of a single coral colony would bleach and parts would not. In the mid-1990’s. researchers in Guam and Panama discovered that there were some genetic differences between zooxanthellae found in single coral heads. These differences are not great enough to be able to call the zooxanthellae different species, but the scientists were able to group the zooxanthellae into three groups that they called “clades”. Each clade has different temperature and light tolerances and may be found in a different location on the coral head. For example, zooxanthellae from the clade that has low light tolerance are found on the sides of the coral colony, while those with high light tolerance are found on the upper surfaces. When temperatures increase, the zooxanthellae with the lowest temperature tolerance bleach first, resulting in the patchy bleaching.
Dinflagellates are common components of phytoplankton. In fact, red tides are caused by blooms of certain types of dinoflagellates. However, zooxanthellae are almost never found in plankton samples. When zooxanthellae are inside the coral animal, they are simply round balls, with no flagellae or hairs. When zooxanthellae are cultured, or grown in lab containers, they develop flagellae and can swim. In the wild, this does not seem to happen. It is more likely that “wild” zooxanthellae live in or on the sediments, in a non-flagellated form. It is almost a certainty that “wild” zooxanthellae exist, as corals have to get their zooxanthellae from somewhere, but they apparently are very good at hiding…
Corals are capable of both sexual and asexual reproduction. In addition to the splitting of individual polyps, coral colonies can reproduce asexually by fragmentation. This usually occurs as a result of storm damage, when coral colonies can be broken into pieces and the pieces may be scattered. If the coral pieces do not become covered with sand, they may be able to resume growth and form a new colony. This is most common in the branching corals like elkhorn and staghorn corals. All corals are capable of sexual reproduction, although the reproductive strategy varies with species. Some corals are hermaphroditic, where individual polyps contain both male and female reproductive organs. Others are gonochoric, where polyps are either male or female. Genetic studies have shown that hermaphroditic corals are capable of self-fertilization as well as cross fertilization (where sperm would come from another coral colony). Some corals release eggs and sperm into the water column and the eggs are fertilized externally—this is called broadcast spawning. Other corals keep the eggs inside the polyps and they are fertilized internally and develop into larvae inside the mother polyp. This strategy is referred to as brooding.
Broadcast spawning usually only occurs once or twice a year, in late summer. It is often referred to as mass spawning, as all of the same type of coral colonies on a reef will release their gametes within about a one-hour window. This typically occurs at night and the dates can be predicted based on the lunar cycle. Spawning may occur over 3 or 4 nights. In the Florida Keys, the branching Acropora corals spawn 3-4 nights after the full moon in August and/or September, while the Montastraea corals spawn 4-5 nights later. Different species will spawn within different time windows in the same night.
Individual polyps release bundles that contain both eggs (which are very buoyant because they contain a large amount of lipids or fats) and sperm. When the bundles reach the water’s surface, the water tension causes them to break apart and the eggs are then fertilized. They develop into larvae in a matter of hours.
Brooders, like the mustard hill coral shown in this picture, retain the eggs, then larvae, for several weeks. The release of brooded larvae is also fairly predictable, based on the lunar cycle. My dissertation research showed that in the Florida Keys, the mustard hill coral releases its larvae over several nights, with peak release right around the new moon. Unlike mass spawners which have only 1 or 2 reproductive cycles per year, brooding corals may have up to 12 reproductive cycles per year.
Coral larvae are called planulae. They range in size from less than 1 mm to several mm in length. They are usually oval in shape with the mouth located at one end. They are able to swim using tiny hairs or cilia, which cover the surface of the planula. Larvae that are brooded often contain zooxanthellae when they are released from the mother polyp. Larvae will swim near the water’s surface for varying periods of time before they dive down to the bottom where they begin crawling over the surface, apparently in search of the perfect place to attach. Once they find this spot, they attach to it and transform into a polyp shape. The new polyp will then begin to secrete its limestone skeleton. The picture here shows skeletons from two individual baby coral polyps. As we learn more about coral settlement, we are discovering that many larvae require a specific chemical cue before they will settle and metamorphose. Very often, this is a chemical produced by a group of red calcareous algae. Without the cue, the larvae will swim around until they die. Some coral larvae will settle in response to other corals of the same species, so several larvae may settle right next to each other. This is referred to as gregarious settlement. Within a matter of days, the baby coral polyps may start dividing and building their new colony. Some of the large coral heads that we see today are hundreds of years old.
For several decades, reef researchers and managers have been concerned about the status of coral reefs globally. Reefs that are close to major metropolitan areas are particularly threatened, as are those that are near popular tourist destinations.
At a meeting of reef researchers and managers in 1994, the participants were asked to rank the threats that they felt were facing coral reefs worldwide. Overwhelmingly, they identified three factors that they felt had the biggest negative impacts on global coral reefs. These were overfishing, sedimentation and nutrient enrichment. Other factors listed included chemical pollution and physical damage. Other factors that are probably affecting coral reef health, but that cannot be directly linked to human populations are global warming, global sea level rise and increased ultraviolet light.
If you talk to anyone who has been diving or fishing in any tropical region for any length of time, they will tell you that they have seen changes in the abundance and types of fish on the reefs. Most of the reef fish that we like to eat are top predators—examples are grouper and snappers. Poor management of these fisheries has resulted in overfishing of many species. As a result of the overfishing of grouper and snappers, in many Caribbean islands fishermen are collecting and selling fish that were formerly considered “trash” fish—like parrotfish and surgeonfish. These fish are grazers, which means that they eat the algae that grows on the reefs. When the populations of grazing fish are reduced, this allows algae to overgrow and kill corals.
Nutrient enrichment refers to the addition of elements to the environment that are required for an organism to grow. If a nutrient is not readily available in the environment, but organisms need it in order to grow, it is said to be a limiting nutrient. Fertilizers usually contain limiting nutrients—when we add the fertilizer to our plants, the plants are able to grow. The most common limiting nutrients in the marine environment are nitrogen and phosphorus.
In the water, nitrogen is available in many forms and bacteria can convert it from one form to another. Plants in the marine environment prefer to take up ammonium, although some can also take up nitrate and nitrite. Some animals can take up dissolved organic nitrogen; most animals obtain their nitrogen by eating plants or other animals—particulate organic nitrogen. Dissolved inorganic phosphorus is referred to as orthophosphate; organic phosphorus is available in dissolved form or again as plant or animal tissue.
There are two main sources of nutrient enrichment in the oceans. One which is a major concern in urban or heavily populated coastal areas is sewage, from leaky septic tanks, sewage pipe breaks, or deliberate pumping of sewage into the ocean. In the US, sewage treatment plants are now required to treat sewage (removing all solids and some of the nutrients) before they can dispose of the effluent in natural waters. However, this was not always the case. Until the late 1970’s, sewage treatment plants were allowed to pump raw, untreated sewage into the oceans. In Hawaii, this resulted in the overgrowth of coral reefs in Kaneohe Bay by a brown “bubble algae” which killed all the corals in the bay. Thanks to efforts of local coral researchers, the reefs are starting to recover in Kaneohe Bay. Many of our major cities have infrastructure, including sewage pipes, that is old and decaying. For example, Miami has major pipe breaks each year, causing closure of beaches and waters near the city. Sewage is a human health risk because of the likelihood of the presence of human pathogens, but it poses a risk to reefs by causing nutrient enrichment as well as an increased sediment load. Fertilizer runoff is more difficult to control than sewage pipes. This is because fertilizer runoff is an example of non-point-source pollution. In the case of sewage pipes, we know the source of the liquid in the pipes, and can hold one particular source responsible for violations of limits. Fertilizer tends to enter the ocean through stormwater drainage pipes, which can hold liquids that have come from large areas of watershed. If a toxin or high levels of pollutant are found in stormwater, it is nearly impossible to determine the source of the pollution.
As on land, nutrient enrichment in the ocean will encourage plant growth. The plants that can impact coral reefs fall into two categories—macroalgae and microalgae. These are not actually true plants, as they do not have roots and flowers, but are types of photosynthetic algae. Macroalgae are the large, easily-visible algae like the sea lettuce shown here. These plants pose a risk to corals as they can literally grow over the top of the coral, reducing the amount of light that can reach the coral and increasing the amount of sediment falling on the coral. Microalgae, or phytoplankton will also grow in response to nutrient enrichment. These algae are able to rapidly take up nutrients, then go into a growth phase where their numbers increase exponentially, causing an algal bloom. One type of phytoplankton bloom is that which causes red tides.
Red tides, along with blooms of other toxic algae, are a problem to humans and marine life. Many of the algae that cause “harmful algal blooms” release toxins into the air and water. When people inhale or swallow these toxins, the effects can be severe. The dinoflagellate called “Pfisteria piscicida” (which literally means fish killer) has been profiled in a fascinating book titled “And the waters turned to blood”. Pfisteria is responsible for killing thousands of fish in estuaries, and can cause nerve-related damage to people. Whenever fish with lesions are reported in the St. Johns River, Pfisteria is the first suspect. Fortunately we do not seem to have this particular type of pest here…yet. Red tides have been linked to manatee deaths and some people feel that they may be responsible for some marine mammal stranding events.
While increased algal growth can have a negative impact on corals, increased nutrients also have a direct physiological impact on corals. While we are not sure of the physiological processes involved, scientific research has shown that nutrient enrichment under laboratory conditions causes decreased calcification by corals. Corals lay down skeleton in much the same way as trees—in layers which show up as bands in cross-section. In the winter, coral growth is slow and the bands form very close together, making the dark stripes you see here. In the summer, growth is more rapid, and the bands are further apart, appearing lighter. Under enriched conditions, corals lay down less skeleton than their unenriched counterparts. It is known that phosphorus can inhibit the binding of calcium carbonate molecules, but it is less clear how nitrogen enrichment causes this decrease in calcification.
If zooxanthellae help corals to lay down skeleton, and nutrients cause zooxanthellae populations to increase, then it seems that coral calcification should also increase. However, the opposite is true. In 1989, researchers proposed that there was a fairly simple explanation for how nitrogen enrichment could reduce coral calcification. The theory states that under “normal” conditions, zooxanthellae are nitrogen limited and cannot use all of the photosynthetic carbon that they produce. The excess carbon is translocated, or given to the coral host. If nitrogen is provided, the zooxanthellae can start to reproduce and grow, and therefore can use more of that photosynthetic carbon themselves, giving less to the coral. The coral then cannot produce as much skeleton.
This theory seemed reasonable and was generally accepted by the coral reef research community. However, in the early 1990’s, I decided to test the theory, and my findings became the main part of my doctoral dissertation. I used radioactive carbon to trace the amount of carbon taken up by zooxanthellae in enriched and control corals, and was able to also measure the amount of radioactive carbon in the animal tissues. What I found was that it was true that each enriched zooxanthella gave less carbon to the coral compared to control zooxanthellae. However, because there were more enriched zooxanthellae per cm² of coral, there was just as much radioactive carbon in the animal tissues of enriched corals compared to controls. How then can we explain the decreased calcification of enriched corals? One theory of mine, which I did not have the time or equipment to completely explore, is that the specific type of carbon compounds that are translocated by enriched and control corals may be different. If the coral requires a sugar compound to help with calcification and it is instead getting a lipid compound, it is still receiving carbon, but it is the “wrong” type of carbon for its needs.
We tend to hear about oil pollution when there are ship groundings or large spills, however, these types of events, while catastrophic, only account for a very small amount of the oil that ends up in the ocean. When we talked about nutrients, we talked about non point source pollution and how it is difficult to control. Most of the oil and chemical pollutants that end up in the oceans come from runoff, like stormwater drainage. We do not have a good feel for how these types of pollutants are affecting coral reefs, although we do know that you can find relatively high concentrations of some pesticides in coral tissues.
The great majority of coral reefs occur in parts of the world where people are relatively poor and uneducated. Most of these reefs are in the Pacific and Indian Oceans, where fish are more colorful and exotic than Atlantic fish. There is a demand among aquarium hobbyists in the western world for these exotic fish, and the locals are trying to supply that demand in any way possible. Unfortunately, the easiest way for them to collect fish is by poisoning the reefs with clorox or cyanide (so the fish float to the surface where they can hopefully be revived) or by destroying the reef to drive the fish out of crevices and into nets. Smaller-scale, but potentially just as devastating is the damage caused by boats and ships, and even curious, careless scuba divers.
While I am blaming humans for all the coral reefs’ problems, it is important to point out that there are natural phenomena that can also cause great physical damage to reefs. Hurricanes have destroyed outcroppings of elkhorn and staghorn corals in Jamaica and Puerto Rico in recent years. Hurricanes can also overturn large boulder corals like the brain corals, killing most of the living tissue. However, one major difference between natural disasters and human influences is that events such as hurricanes are acute—the effects occur over a short time period, then the reef has potentially some period of time to recover. Most human actions are chronic—the impacts may be less severe than that of a hurricane, but they are continuous, so there is never any recovery time for the reefs.
Most of what I have talked about today has dealt with things that directly impact corals. However, it is important to remember that corals are part of a food web which includes and relies on many other types of animals and plants. If something happens to change their populations, the health of the corals may be affected, as we have seen with overfishing. Marine debris is a topic which is of increasing concern as our population grows and we become more and more reliant on plastics. Plastics last for hundreds of years in the environment and clear plastics are often difficult to see underwater. Balloons and plastic bags are often mistaken for jellyfish by leatherback sea turtles. When the turtles eat these items, they cannot digest them, so their intestines become blocked and the turtles starve to death. In the past 20 years, discarded monofilament fishing line has been the reason for 1 in every 5 manatee rescues. In the past 5 years, 163 entangled sea turtles were recorded in Florida alone. In the same time period, at least 35 dolphins in the southeastern US have died from monofilament-related injuries. Each year hundreds of birds die from monofilament entanglement. This problem has become so severe, that there is now a statewide entanglement workgroup looking into ways to reduce these risks.
There are some simple things that you can do to help protect the marine environment. If you do your own oil change, take the used oil to an auto parts store to be recycled. It doesn’t cost you anything, except a little time, and that’s 3-5 quarts of oil that won’t end up in the landfill where it can contribute to runoff. You can also recycle fishing line—currently you have to take it to specific tackle shops, but soon you will start seeing recycling stations like this one at local beaches, boat ramps, fishing piers, etc. The monofilament can be melted down and turned into artificial fish habitats and tackle boxes. Whenever you are working with chemicals, including fire ant poison and fertilizer, be sure to read and follow the instructions. Doubling the application dose will not double the effectiveness, it will only contribute to contaminating the watershed. If your car is leaking oil or antifreeze or other fluids, fix the leaks. Antifreeze is not only a hazard to aquatic life—it tastes sweet and animals, especially dogs, love to lick up puddles of it, but it will kill them.
We live in a challenging time. As human populations grow, our impact on the natural environment around us is constantly increasing. It is vital that we all do our part to help protect the natural ecosystems around us—we rely on it not only for food but for air quality, for potentially life-saving drugs and probably many other reasons that we don’t yet understand. We think that coral reefs play a role in decreasing global warming, by removing carbon dioxide from the air. A chemical compound found in soft corals appears to help fight cancer. Hopefully you now have a new understanding of how human actions affect the marine environment in general, and coral reefs in particular. I hope you will do your part to help minimize these impacts.
Coral Reefs—health and hazards Dr. Maia McGuire University of Florida/Sea Grant Photo by Mike White, FKNMS