Skeletal eroding band
First coral disease caused by a protozoan, forming a dark band.
Skeletal eroding band (SEB) is a coral disease that shows up as a black or dark gray band creeping slowly across coral, leaving behind a patchy, spotted area of dead skeleton. It is the most widespread coral disease in the Indian and Pacific Oceans, and also occurs in the Red Sea. SEB is especially common in warmer or more polluted waters.
The disease was first noticed in 1988 near Papua New Guinea and later near Lizard Island on Australia’s Great Barrier Reef, but was initially mistaken for a gray version of black band disease. It was recognized as a distinct condition after surveys in the Red Sea in 1994. The band advances at about 2 millimeters per day in the Red Sea and 2 to 3 millimeters per day on the Great Barrier Reef. Corals in the families Acroporidae and Pocilloporidae are most vulnerable. One study found that the infection spread at roughly 2 millimeters per day in colonies of *Acropora muricata*, eventually killing 95% of affected corals. However, experiments showed the disease spread easily onto already dead or dying areas of coral but did not attack healthy, undamaged tissue.
SEB is the first coral disease confirmed to be caused by a protozoan—a single-celled eukaryote—rather than by bacteria. The culprit is the ciliate *Halofolliculina corallasia*. This sessile protozoan secretes a bottle-shaped protective shell called a lorica, which it anchors to a surface. When a mature cell divides, it produces two worm-like larvae that settle on undamaged coral just ahead of the dark band. Each daughter cell then spins to form its own flask-shaped lorica, and the spinning combined with hardening chemicals crumbles the coral skeleton and kills the polyps. The discarded loricae of the parent cells remain, creating the spotted dead zone behind the advancing band.
A similar disease was found in the Caribbean Sea during a 2004 survey, affecting 25 coral species across six families. Although initially thought to be caused by *H. corallasia*, closer examination revealed a different, unnamed species of *Halofolliculina*. This Caribbean version is most common in oceanic waters, while the Indo-Pacific form is more frequent in coastal waters. Because of these differences, the Caribbean disease was named “Caribbean ciliate infection.” Coral disease research is still relatively new, and standardized terminology has not yet been settled.
- First noticed
- 1988 near Papua New Guinea
- Spread rate red sea
- 2 millimeters per day
- Spread rate great barrier reef
- 2 to 3 millimeters per day
- Most vulnerable families
- Acroporidae and Pocilloporidae
- Mortality rate in study
- 95% of infected Acropora muricata colonies
- Causative agent
- Halofolliculina corallasia
Lore & Background
Skeletal eroding band was first noticed in 1988 near Papua New Guinea and then near Lizard Island in Australia's Great Barrier Reef, but was regarded as a gray variant of black band disease, as were instances off Mauritius in 1990. Surveys in 1994 in and around the Red Sea first identified the condition as a unique disease. It is now considered the commonest disease of corals in the Indian and Pacific Oceans, especially in warmer or more polluted waters. The spread of the disease across an infected coral has been measured at 2 millimeters in the Red Sea and 2 to 3 millimeters around the Great Barrier Reef. Corals of the families Acroporidae and Pocilloporidae are the most vulnerable to infection. A study in 2008 found that the infection spread at about 2 millimeters per day in colonies of Acropora muricata, eventually wiping out 95% of its victims. However, experiments showed that the disease easily spread to already dead and dying areas of corals but did not attack undamaged corals.
Reader's Guide
Skeletal eroding band is significant as the first recorded coral disease caused by a protozoan, and thus the first known to be caused by an eukaryote, whereas most coral diseases are caused by prokaryotic bacteria. The identified agent, Halofolliculina corallasia, is a sessile ciliate that secretes a bottle-like lorica. When a mature cell divides, daughter cells settle on undamaged coral just ahead of the black band, secrete their loricae, and spin, which crumbles the coral skeleton and kills polyps. The discarded loricae of parent cells leave the distinctive spotted region. A disease with very similar symptoms found in the Caribbean Sea was given a different name, 'Caribbean ciliate infection,' because it is caused by a different, unnamed Halofolliculina species and occurs in a different environment—oceanic waters versus the coastal waters of the Indian and Pacific Oceans. This highlights the complexity of coral disease research and the need for standardized terminology.
Did You Know?
- Skeletal eroding band is the first coral disease known to be caused by a protozoan.
- The disease leaves a spotted region of dead coral, distinguishing it from black band disease which leaves a completely white area.
- The ciliate Halofolliculina corallasia drills into the coral's limestone skeleton to produce its lorica, killing polyps in the process.
- A similar disease in the Caribbean is caused by a different, unnamed Halofolliculina species and is called 'Caribbean ciliate infection'.
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