Coral Diseases

Corals are subject to many stressors, including parasites, infectious diseases, and extreme environmental changes. They often respond with loss of zooxanthellae, tissue necrosis, abnormal growths, and diebacks. Around the world, large-scale loss of scleractinian corals is associated with many biological and physical causes.

Variations in weather may damage corals. Heat, excessive ultraviolet radiation exposure during calm winds or at low spring tides, or unusual tidal events that expose near-surface corals are all stressors capable of killing corals at some level of exposure. When combined with other stressors (salinity changes, pollutants, turbidity, sedimentation), the lethal threshold is reduced and coral populations decline. Hurricanes primarily destroy macroalgae, and to a lesser extent the hard corals. In many deteriorating reef systems, storm-damaged corals do not appear to be recovering.

Changes in predator-prey relationships can dramatically reduce coral populations. Population explosions of the crown-of-thorns starfish devastated coral colonies widely in the Pacific. In the Atlantic, die-offs (from unknown causes) of Diadema and other algal-grazing sea urchins resulted in loss of algal control. Overfishing of tangs and parrotfish removes algal control on the reef.

With fewer herbivores cropping macroalgae, settling sites for larval corals become rarer and recruitment of new coral colonies declines. The very young coral colonies that do get a foolhoW are subjected to abrasion and shading from adjacent macroalgae. This slows their growth, exposes bare calcareous rock that the algae invade, and confines the young cnidarians so that they have no basal space in which to expand. The reef becomes macroalgal-dominated rather than coral-dominated.

A few coral diseases are of special interest.

Coral Reef Bleaching

Bleaching, the loss of color in corals and other invertebrates, is dramatic in branching or massive stony corals that lose their brown or pastel hues and become milky white as the corallum skeleton shows through the translucent tissue layer. Bleaching also affects brightly colored soft corals, Milepora tire corals, sea anemones, sponges, and other invertebrates.

Bleaching is the result of expulsion of symbiotic zooxanthellae from host corals and/or the loss of accessory pigments (chlorophyll c. peridinin, diadinoxanthin) in symbionts not expelled.

Bleaching is neither contagious nor infectious, but can affect all the corals in a largo area. Occurring worldwide, bleaching is often a response to prolonged, elevated seawater temperatures such as occur off the Pacific coast of South America during midsummer in El Nino years. The combination of calm seas that allow increased exposure to UV radiation and of prolonged midsummer water temperatures only 2°C above normal may trigger the response.

Corals adjust to the normal high temperatures of their region. Corals from the Florida Keys can tolerate maximum temperatures much lower than the maximum temperatures tolerated by corals from more tropical locales. The interaction -- of temperature and time for a specific region sufficient to induce bleaching -- can be calculated. A formula in degrees heating weeks (DHW) developed for Belize and Jamaica predicted that 26 DHW would induce mass bleaching.

Cold upwellings and reduced salinities can also induce localized bleaching, indicating that it is a general stress response. Bleaching develops in a few days but may go on for weeks or months. The longer the event, the less likely the colony will recover and avoid succumbing to infectious disease or other stressors. Mortality rates of 95 percent of coral colonies have been recorded following mass bleaching, with whole reefs lost in the eastern Pacific.

Montastrea and other corals frequently recover from a few days or weeks of bleaching, but long-term bleaching (over a month) can cause death of corals that cannot survive without their symbiotic zooxanthellae. Some partial, short-term bleaching is not unusual; the increasing frequency and severity of bleaching has been attributed by some to global climate change.

Bleaching in non-coral hosts results from the loss of symbiotic algae or cyanobacteria or the explosive multiplication of symbiotic fungi. It is not uncommon throughout the animal kingdom to eliminate symbionts during periods of stress, but some symbionts are more important to the host than others.

Corals often fade or change color in reef aquariums, sometimes as a response to a different light regime rather than temperature; that is not bleaching. In other cases, corals seem to undergo bleaching identical with what occurs in nature during long hot spells. If fading occurs, the temperature should be measured; if over 80°F, corrective action should be taken to gradually reduce it to about 72°F. Presumably, high temperature-induced rapid fading of color in home aquariums is no different from coral bleaching in nature. However, because it is a stress response, other stressors may also induce fading, including those (such as a strong chemical excretion from one of the invertebrates) that do not cause bleaching in nature.

White-Band Disease

White-band disease may follow a recent hurricane or occur for no discernible reason. The polyp tissues die and slough off from the base to the very tips of the branches in acroporids (elkhorn and staghorn corals), leaving the bleached white skeleton of the corallum exposed. Primarily affecting acroporids, the disease has also been seen in other corals where it has been termed white plague or white death. White-band diseased corals contain bacterial colonies in the calicoblastic epithelium, the layer of tissue that secretes the skeleton, but it is not known if the bacterial colonies are a cause or a result of the disease.

Stress-Related Necrosis

A similar sloughing of tissue has been observed in acroporids, faviids, and poritid corals, where no bacteria are apparent. This condition has been termed stress-related necrosis (SRN). White-band disease and SRN may be manifestations of the identical disease, the intratissue bacteria only opportunistic invaders of tissue already dying from some other cause. Tissue sloughing in captive hard corals is sometimes reversible by moving the coral to another aquarium.

Black-Band Disease

Black-band disease is an infection of faviid scleractinian corals and certain gorgonians; the cause is the pathogenic cyanobacterium Phormidium corallyticum and other bacteria. Often beginning at the site of an injury, the tissue is destroyed and disappears from the coral skeleton at a few millimeters a day. the advancing margin marked by a thin black line or band of Phormidium. The role of accessory microbes, including fungi, is not clear. Black-band disease causes severe damage in the summer or when the water is warmer than usual, but stops with the onset of cold.

Platygyra and Goniastrea are most often affected, but the disease is also known in Monlastrea, Sideraslrea, Diploria, and Colpophyllia. Diseased faviids are often widely separated on the same reel, with adjacent faviids uninfected. However, a recent outbreak in the Florida Keys showed many simultaneous infections of adjacent corals Work at the University of Puerto Rico indicates that the disease may be transferred by infected pieces of coral carried elsewhere by currents The gorgonians Pseudopterogorgia in Florida and Gorgonia in Costa Rica have also been found infected with Phormidium corallyticum and other bacteria.

Phormidium is susceptible to antibiotics and intolerant of cold Outbreaks in a minireef aquarium are rare. Try removing the infected coral to a separate 70°F aquarium and treating the water with 10 mg/L of oxyteuacycline hydrochloride, with a new dose in new water every two days until the disease clears.

Bacteria

Corals injured by rocks, crabs, fish bites, boring snails, or rough handling often die if the polyps are interconnected, but sometimes survive if polyps are separate. Tissue tears can become deadly when invaded by opportunistic pathogenic bacteria. Under the microscope, these bacteria are usually Gram-negative, short rods that may be susceptible to 10 mg/L of nitrofurazone. Treatment should always be in a separate aquarium, because many antimicrobial drugs, especially neomycin (250 mg/gallon), Chloromycetin (80 mg/L for 25 hours), and erythromycin (any dose) may kill non-target nitrogen cycle bacteria, red algae, and cyanobacteria.

The bacteria identified so far in coral diseases are not short Gram-negative rods, but members of different groups that use sulfur products. Beggialoa is a member of the gliders, sulfide-oxidizing filamentous bacteria that live in sediments at the interface of anoxic and aerobic zones Desulfovibrfo is an anaerobic sulfate reducer shaped as a curved rod, sometimes a spiral Beggiatoa and Desuifovibrio have been found in corals, but whether they are primary or secondary invaders is not always clear.

Growths and Tumors

True tumors or neoplasms are known from acroporid corals, and can be seen as raised, white calcarous nodules. The tissues producing the calcareous deposits overgrow the polyps. As the tumors enlarge, the covering tissue loses its mucus secretory cells and the tumor eventually becomes ulcerated. The skeleton becomes exposed and can be invaded by filamentous algae or other organisms. This neoplasm or true cancerous condition is called calicoblastic epithelioma The cause of calicoblastic epitheliomas is unknown Other skeletal anomalies in corals are the result of overgrowth of a foreign invader.

Just as parasites or sand grams can induce nodule formation such as pearls in oysters, some endoparasitic algae can irritate corals sufficiently to wall them off. A parasitic trematode, Plagioporus or a relative, whose host is the coral-eating butterfly fish, Chaetodon multicinctus, has an encysting larval form that penetrates Pontes corals and induces nodule formation These nodules are then selectively eaten by the host fish.

Gorgonians parasitized by algae also develop tumorlike nodules. A filamentous green alga, Entocladia endozoica, is an endoparasite of the gorgonians Pseudoplexaura and Pseudoptorogorgia. Its algal filaments irritate coral tissues, which subsequently wall them off. Another endosymbiont of (usually) stony corals (probably the alga Ostreobium), induces hyperplasia (cell multiplication), followed by walling off with gorgonin when the alga infects the sea fan, Gorgonia