3. Coral Diseases (pathologic syndromes of reef corals)
In order to understand the growing appearance of coral diseases and to stabilize at least the current status quo,
coral pathology has to step out of its neglected existence to become a growing side-branch of coral reef science.
Reefs are very sensitive to environmental conditions. They are adapted to extreme oligotrophic conditions.
Under normal conditions, diseased or even dead corals never exceeds 5% of the total undisturbed reefs.
But changes in coral health and vitality (disease, algal overgrowth, bleaching, etc.) may be more sensitive indicators
of changing environmental conditions. All the man-made stresses (chemical and thermal pollution, sedimentation, dredging,
blasting, boat anchoring, recreational activities, etc.) not only exert considerable pressure on these organisms,
but also enforce the frequencies of coral pathogens. Thus, pathologic syndromes of reef corals are commonly grouped
into those acting without and those mediated by a pathogen.
3.a Disease working without a pathogen: Although no pathogen is involved in the progress of
the disease, the pathogenic reaction is caused by external (i.e. abiotic) influences as in the cases of Tissue Bleaching (TBL),
Shut-Down-Reaction (SDR), and White Band Disease (WBD).
- Tissue Bleaching (TBL): Coral bleaching or tissue bleaching refers to the whitening
of coral colonies brought about by a reduction in the number of zooxanthellae from the tissues of polyps, by a loss of
photosynthetic pigment, or by a combination of both. This loss exposes the white calcium carbonate skeletons of the
coral colony (fig.7). Corals naturally loose less than 0.1% of their zooxanthellae during processes of regulation and replacement.
However, adverse changes in a coral's environment can cause an increase in the number of zooxanthellae lost.
There are a number of stresses or environmental changes that may cause bleaching including excess shade, increased
levels of ultraviolet radiation, sedimentation (necrosis in soft corals), pollution, salinity changes, elevated atmospheric
CO2 levels, industrial seawater pollution, or excess freshwater runoffs. But the strongest evidence was found to
be an increased surface water temperature that is quite often linked to the phenomenon known as
ENSO (El Nino and Southern Oscillation) - see
also the NOAA web site about Sea Surface Temperatures.
(TBL in deeper water, without perceptible temperature changes, remain largely unexplained).
TBL is not necessarily lethal, and corals are usually able to recover their symbionts once environmental conditions
return to normal. However, for the duration of the bleaching event, tissue biomass, growth rate, and reproduction
rate are all negatively affected, and, if stresses last long enough, the condition may eventually lead to the death of
the specimen or even to the demise of the entire section of a reef tract (fig.8).
Corals bleach in response to prolonged temperature change (due to the combined affect of clear skies, calm sea and
maximum summer solar irradiation) and not due to rapidly fluctuating temperatures. Lab experiments show that corals bleach
when water reaches or exceeds a constant 32°C and an increase of UV exposure. The amount of mycosporine-like amino
acids in a coral's tissues helps to determine how much UV it can withstand without bleaching.
Fig.7 TBL (120kB)
Fig.8 Montastrea faveolata
before and after (190kB)
- Shut-Down-Reaction (SDR) - often referred to as Rapid Wasting (RW), Rapid- or Stress-
Related Tissue Necrosis (RTN/SrTN), White Plague (WP), or White Death (WD) - (fig.9). Observations in
laboratory experiment and field observations of corals under sublethal (abiotic) stress such as elevated temperature,
sedimentation, chemical pollution, have revealed that specimens can die from a simple scratch.
Such sudden disintegration of the coral tissue, that starts at the margins of the injury, is characterized by sloughing off the tissue
in thick strands of blobs from the coenosarc, leaving behind a completely denuded coral skeleton. From the initial interface, the
phenomenon proceeds in an enlarging circle on massive corals, or moves along the branches in ramose forms, spreading to all
side-branches upon reaching a junction. It is still unclear if SDR represents a disease on its own, as the thriggers match those
in WBD or WS (see below), although there seem to be significant differences regarding the speed this disease effects a colony.
Thus, SDR is especially dangerous as it can spread with an average speed of 10cm/hour - fast enough to be visually
observed! Being contagious, SDR can be transmitted by a floating strand of dissolved, contaminated tissue to produce an onset
on a neighboring stressed colony. Thus, triggering a catastrophic chain reaction, which may occur several times during the
course of a season. It usually affects species of the Caribbean, such as small star corals Dichocoenia stokesii, pillar
corals Dendrogyra cylindrus, and boulder corals Montastrea annularis (fig. 10).
Fig.9 Mycetophyllia ferox (70kB)
Fig.10 Necrosis on Montastrea annularis (70kB)
Fig.10 Favia pallida (150kB)
Fig.11 Diploria strigosa (left);
Agaricia agaricites (right); even interspecific competition may triger WBD (70kB)
Fig.12 WBD on Acropora palmata (70kB)