Types
of Coral Disease and Their Identification
Bleaching |
Bleaching in corals is a response to stress that causes the coral
to lose its symbiotic algae (zooxanthellae), or the photosynthetic
pigments of the zooxanthellae that give the coral its normal yellow,
brown or green color. In either case, the coral appears lighter in
color (D. labyrinthiformis
- right), mottled (M. faveolata,
below), or white (Agaricia lamarcki
- below) as the white skeleton is seen through the now translucent
coral tissue. |
A partially
bleached brain coral, D. labyrinthiformis |
A bleached colony of lettuce coral, Agaricia lamarcki |
Localized
bleaching affecting a few corals or a few reefs may occur in response
to a sudden or prolonged stress such as an increase in sedimentation
associated with heavy rainfall and run-off, a change in salinity,
increased temperature or elevated visible light or ultraviolet radiation.
Over the last two decades regional or global bleaching events have
been reported with increasing frequency. These have been associated
with warmer than normal sea water temperatures, extended periods of
doldrum conditions where penetration of visible light and UV radiation
may increase, and are especially common during El niño (ENSO)
years. Scientists have also discovered that bacteria and protozoan
parasites can cause bleaching. |
| Colonies
may remain bleached for six months or more. In some cases, all
or part of bleached tissues may die, especially if water temperatures
remain above the normal maxima for several weeks to months. |
Mountainous
star coral (M. faveolata) affected by mottled bleaching.
Some of the bleached tissue has died and is evident in this photo
in the lower right lobe as patches of white, exposed coral skeleton
next to fully pigmented tissue.
|
Mountainous star coral (M. faveolata) that bleached
during the 1995 bleaching event.
|
Same
colony six months after it bleached. The colony has regained
most of its pigmentation and did not experience any partial
mortality. |
Often, once
environmental conditions return to normal, colonies regain their
symbiotic algae and dark pigmentation. |
