Russian Journal of Marine Biology, Vol. 28, Suppl. 1, 2002, pp. S32–S40.
Original Russian Text Copyright © 2002 by Biologiya Morya, Leletkin.
1063-0740/02/2801-S $27.00 © 2002
The sharp change in the physiological state of corals
and some hydroids harboring symbiotic zooxanthellae
in response to elevated water temperatures has
undoubtedly been among the topical issues in the phys-
iology of reef-building corals over the last decade. This
phenomenon is deﬁned as coral “bleaching” since it is
accompanied by mass liberation of zooxanthellae from
the animal hosts, thus depriving them of their charac-
teristic color. Coral bleaching is frequently concomi-
tant with the mortality of colonies and even entire coral
reefs. In fact, that is why this natural phenomenon has
been of concern in the world (and not only scientiﬁc)
In the last decade, coral bleaching events have been,
literally, of global extent. In different years, the world’s
coral reefs were affected by bleaching to various
degrees, which has left “green wounds” on the reefs.
Below are only a few reports on natural disasters on
coral reefs over the last six years. The year 1998 was
particularly unfavorable; bleaching involved 654 coral
reefs of the Great Barrier Reef in Australia, where 60%
of the corals lost their zooxanthellae [9, 42]. At the
same time, some reefs in the Maldives lost 98% of the
corals of the genera
coral cover in lagoons decreased from 42 to 2% .
On the Bahama reefs, between 1994 and 1998, the
dominant coral community was replaced by macro-
phytes as a result of the coral mortality following
repeated bleaching events. On some reefs of the Anda-
man Islands, 85% of the corals partly lost zooxanthel-
lae and 10% lost them completely . On some reefs
along the western coast of India, 90% of the corals
bleached over two months, and 26% of these subse-
quently degraded . Corals on the southern atolls of
the Seychelle Islands and on reefs of the Ryukyu
Islands bleached, slowed down their growth, and were
partly forced out of the biotope by macrophytes [60,
 also reported, in a review
paper, on high coral mortalities on the reefs of Sri
Lanka, Kenya, and Tanzania, where the shallow-water
areas lost up to 90% of their coral colonies and, at
depths to 20 m, about 50% of the corals perished as a
result of the mass bleaching event in 1998.
Earlier, bleaching has also affected coral reefs and,
although the consequences were not as dramatic as
after the year 1998, attained a fairly signiﬁcant magni-
tude. Thus, in 1996, coral bleaching occurred in 54% of
the coral area of New Guinea . In 1994–1995, on
the reefs of Thailand, Tahiti, Belize, and Puerto Rico,
50% of the coral reef areas was affected by bleaching
and coral mortalities reached 10% [11, 15, 16, 43, 44,
46, 71]. A large-scale bleaching event occurred in the
Gulf of California and off the Florida coast in 1997 [47,
68]. This process repeated with striking constancy in
the Galapagos reefs: the years 1983, 1987, and 1993
were particularly remarkable in this respect .
The processes associated with the bleaching of reef
corals and the plausible reasons and mechanisms of this
phenomenon have been the subject of much research.
Apart from the remarkable publication of Wilkenson
, mention should be made of the works of
other notable investigators [23, 24, 41, 45, 52, 53, 59].
Based on the results of these investigations, the fol-
lowing conclusions can be made. First, coral bleaching
is a recurrent phenomenon whose magnitude (spatial
extension or the number of coral colonies or reefs
involved) and extent of disturbance to the reef organ-
isms (loss of zooxanthellae and photosynthetic func-
Bleaching of Hermatypic Corals
V. A. Leletkin
Institute of Marine Biology, Far East Division, Russian Academy of Sciences, Vladivostok, 690041 Russia
e-mail: email@example.com; firstname.lastname@example.org
Received September 26, 2001
—In this paper, I review data on the magnitude and extent of reef coral bleaching events and consider
modern hypotheses on the mechanisms of this natural phenomenon and experimental data lying at their basis.
Four possible mechanisms of color loss by hermatypic corals have been conﬁrmed experimentally: bacterial
infection, change of zooxanthellae type in the polyps to improve the heat resistance of the photosynthetic func-
tion of coral to elevated seawater temperature, intoxication of zooxanthellae by animal metabolic wastes at high
temperature and light levels, and thermal and photodestruction of the animal and algal cells. The heating effect
of photosynthetic active radiation on the zooxanthellar cells in coral polyps was veriﬁed theoretically. The cal-
culations showed that in the natural environment, the additional light-induced heating of zooxanthellae is not
C and that it cannot cause disruption of the animal and zooxanthellae symbiosis.
Adaptation, hermatypic coral, zooxanthellae, bleaching, heat resistance.