Coral disease

About 30 diseases of corals have been recognised since they were first discovered more than 30 years ago. There is still little known about the causes and effects of coral disease although diseases of coral can be caused by bacteria, fungi, algae and worms.

White Syndrome has taken hold of this Tabulate Acropora coral

Tabulate Acropora with White syndrome.
Photo: AIMS

Coral disease has a major impact on Caribbean reefs, where 80% of coral has been lost to disease in the last 20 years. On the Great Barrier Reef, the Long-term Monitoring Program (run by the Australian Institute of Marine Science and supported by CRC Reef) has found an increase in coral disease on surveyed reefs over the last five years. However, only about seven diseases have been recorded from corals on the Great Barrier Reef and disease is not considered a major threat to the Reef.

Scientists believe that the increase in disease abundance in some areas may be linked to higher sea temperatures because more diseases are seen in summer. Some scientists also believe that run-off of nutrients and sediment from the land may allow coral diseases to thrive.

Coral bleaching

High sea temperatures can cause corals to eject the symbiotic algae or ‘zooxanthellae' that normally produce food for the coral. This makes corals appear bleached, and can ultimately kill the coral. While coral bleaching is not a disease, some scientists have linked it to a virus in zooxanthellae which is triggered by heat. Some corals can recover from bleaching if the sea cools. However, corals that have been bleached are weaker and more prone to attack from disease.

BLACK BAND DISEASE
Appearance on coral	A black band or line 5-40mm wide. The coral in front of the progressing band is healthy, while behind it the coral is dead and looks white. The band can move across the coral colony at a rate of 4mm a day.
Coral Families affected	Widespread in the families Pocilloporidae, Acroporidae, Faviidae, Poritidae, Pectiniidae, Mussidae, Dendrophylliidae, Siderastreidae and the orders Alcyonacea and Hydrocorallina.
Cause	There have been no studies of black band disease on the Great Barrier Reef. However, research in other parts of the world suggests that the disease is associated with cyanobacteria.
Distribution and abundance	Widespread throughout the Great Barrier Reef on inner, midshelf and outer reefs. However, the number of infected coral colonies has remained low and stable over the last five years.
Comments	Black band disease was the first coral disease observed by scientists, with reports of the disease in the Caribbean dating back to 1973. It was not found on the Great Barrier Reef until 1994.
WHITE SYNDROME
Appearance on coral	White bands, spots or patches.
Coral Families affected	Pocilloporidae, Acroporidae, Faviidae and Poritidae.
Cause	‘White syndrome' is a name given to a number of diseases exhibiting similar symptoms. The causes of white syndrome are as yet unknown.
Distribution and abundance	White syndrome has increased in abundance 20-fold in the last five years, with increases on inner, mid-shelf and outer-shelf reefs along the length of the Great Barrier Reef.
Comments	White syndrome diseases such as white pox, white band and white plague have had a major impact on Caribbean reefs, and are considered a potential threat to the coral communities of the Great Barrier Reef. In areas of the Great Barrier Reef surveyed, white syndrome, along with skeletal eroding band, was the most common disease.
SKELETAL ERODING BAND
Appearance on coral	A black band similar to that of black band disease. However skeletal eroding band produces a white skeleton speckled with the empty black shells of the ciliate (see below) that causes the disease.
Coral families affected	Pocilloporidae, Acroporidae, Faviidae, Poritidae, Fungiidae and Merulinidae.
Cause	Clusters of the ciliate (a single-celled organism covered with cilia which look like hairs) Halofolliculina corallasia penetrate healthy coral tissue, disrupting it in the process of secreting their protective shells or loricae.
Distribution and abundance	Thought to be common and widespread across the Great Barrier Reef. There is little information about whether the disease is stable or increasing on the Reef.
Comments	This disease has been found on the Great Barrier Reef and Indo-pacific, but not in the Caribbean or Atlantic reefs.
BROWN BAND
Appearance on coral	A brown band of varying width, dividing healthy coral tissue in front of the band from the exposed white skeleton of coral that the disease has already killed. Sometimes there is also a white zone between the brown band and healthy tissue.
Coral families affected	At least 18 species of Acroporidae, and a few species of Pocilloporidae and Favidae.
Cause	The brown band is caused by dense populations of single-celled organisms called ciliates that are covered with ‘cilia'. The ciliates eat the symbiotic algae or ‘zooxanthellae' within coral polyps. The ciliate Helicostoma nonatum may cause the disease which appears as a brown jelly-like condition on aquarium corals.The cause is yet to be confirmed.
Distribution and abundance	Little is known about the spread or number of Brown Band cases. It has been recorded in low numbers at intensively monitored sites around Lizard Island (northern Great Barrier Reef, mid-shelf), No Name Reef (northern Great Barrier Reef, outer-shelf), and Heron Island (southern Great Barrier Reef, outer shelf).
Comments	Brown band was recorded for the first time on the Great Barrier Reef in 2003.
BLACK NECROSING SYNDROME
Appearance on coral	Dead, black patches eating away tissue and leaving a white skeleton.
Coral Families affected	Gorgonacea
Cause	A fungus that has not yet been identified.
Distribution and abundance	Found on many gorgonian corals on mid-shelf and outer-shelf reefs in the northern Great Barrier Reef.
Comments	Gorgonians, or sea fans, are soft corals with a semi-rigid calcareous skeleton. In the Caribbean, they are very susceptible to disease, but little is known about gorgonian disease on the Great Barrier Reef.
PINK SPOT
Appearance on coral	Swollen pink polyps.
Coral species affected	Porites compressa
Cause	Pink spot is caused by the larval stage of the parasitic flatworm Podocotyloides stenometra. The flatworm has three life stages, the first is parasitic on a mollusc, while the second affects tissues of the coral, causing polyps to appear swollen and pink. This makes the polyp more vulnerable to predation by butterfly fish, which are the final host for the parasite.
Distribution and abundance	Pink spot has only recently been recorded on the Great Barrier Reef, in low numbers at intensively monitored sites around Lizard Island (northern Great Barrier Reef, mid-shelf) and No Name Reef (northern Great Barrier Reef, outer-shelf).
Comments	Once the infected polyp has been eaten by the butterfly fish, healthy polyps regenerate from the coral.
CORAL TUMOURS
Appearance on coral	Spherical lumps raised about 4.5cm from the surface of the coral.
Coral species affected	Tumours have been found on two types of Platygyra or ‘brain worm' coral, as well as on tabular, bushy and isoporan forms of Acropora, and on Montipora on the Great Barrier Reef.
Cause	Tumours are formed by groups of polyps with increased growth rates. The cause is unknown.
Distribution and abundance	Little is known about the spread or number of coral tumours, but it has been recorded in very low numbers at intensively monitored sites around Lizard Island (northern Great Barrier Reef, mid-shelf), No Name Reef (northern Great Barrier Reef, outer-shelf) and Heron Island (southern Great Barrier Reef, outer shelf).
Comments	In high densities, tumours can reduce the ability of the coral to function and grow.

Coral Disease links

The Coral Disease Page (focus on Caribbean reefs)
World Conservation Monitoring Centre Global Coral Disease Database

References

Bruno JF, Petes LE, Harvell CD, A Hettinger. 2003. Nutrient enrichment can increase the severity of coral diseases. Ecology Letters 6:1056-1061.

Willis B, Page CA, Dinsdale EA. (In preparation) Coral Disease on the Great Barrier Reef.

Wilson WH, Francis I, Ryan K, Davy SK. 2001. Temperature induction of viruses in symbiotic dinoflagellates. Aquatic Microbial Ecology. 25:99-102.