The Coral Whisperer: Bleaching and Tissue Loss in Corals – What’s the Difference?

Many of the questions I am asked
have an “ailing” coral as the subject. Often, the
problem involves some amount of paling to the normal coloration
of a coral, or a visible white area on the coral. It is very
difficult to ascertain the nature of the problem under any
circumstances, but one of the most common mistakes is the
misidentification of coral bleaching. In the next few articles, I
intend to look at some of the causes, appearance, and effects of
bleaching and then, in subsequent articles, a group of problems
of various types characterized by actual tissue loss. These
events can be difficult to tell apart, may have similar or
different causes and effects, and may even be related to each
other.

Photo

Trachyphyllia geoffroyi is often
suscpetible to bleaching in aquariums. Part of the reason may
stem from the fact that many of the bright colored (red and
green) specimens are being collected in deep water. The coral
above has bleached, and is showing signs of tissue loss,
probably from starvation resulting from the loss of
zooxanthellae.

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Background

What is bleaching? Bleaching occurs in corals that maintain a
symbiosis with various types of dinoflagellates called
zooxanthellae. By one common definition, bleaching is the
release, rejection, or loss of zooxanthellae from coral
tissue.

Zooxanthellae are acquired by corals in two ways: first, they
may be given a “starter culture” by the parent if the
parent colony broods its planulae. Alternately, in corals that
release sperm and eggs into the water and where fertilization
takes place externally in the water column, planulae (lacking
zooxanthellae) can swallow the algae from the water column. Once
swallowed, the algae are not digested but are brought into the
cell and put into a small intracellular bag called a vacuole.
Once inside the vacuole, they are “trapped” and
somewhat at the mercy of the coral polyp. The golden brown algae
reproduce within the cell and form a dense, but very thin layer
within the polyp. The zooxanthellae are found mainly in the inner
tissue layer of corals called the gastroderm, although they can
occasionally be found in the outer layer (ectoderm) and are in
the tentacles of day-feeding corals. Night feeding corals have
transparent tentacles that normally lack zooxanthellae.

Photo

The Goniopora shown is bleached.
The tissue is transparent, indicating a dramatic loss in
zooxanthellae. This coral is probably not bleached from excess
radiation or temperature, as it was photographed in deep water
where light levels are low and temperature is relatively
constant.

Once inside the polyp, zooxanthellae are provided nutrients
that are controlled, and usually limited, by their host. In
return, the algae use sunlight to photosynthesize and provide the
energy rich products of photosynthesis (photosynthate) to the
coral polyp. The nutrients for the zooxanthellae are mainly the
products of coral metabolism; that is, carbon dioxide and
nitrogen.

One of the advantages to living within polyp tissue is that
zooxanthellae have constant access to nitrogen in the form of
coral metabolic waste products. In contrast, the usually nitrogen
deficient seawater may not be able to provide a plentiful source
of nitrogen for growth and reproduction. However, the coral can
and does control the amount of waste released intracellularly to
the zooxanthellae, excreting any excess back into the seawater.
Under normal conditions, the balance is very precise and there is
very little deficiency or excess, with virtually all of the
coral’s metabolic waste consumed by a precisely moderated
population of zooxanthellae.

Bleaching Variation

Conditions can occur, however, that change the balanced
symbiosis of algae and coral. Where there is chronic or acute
nutrient limitation, the coral may not produce enough waste to
sustain the zooxanthellae. Conversely, the zooxanthellae will not
be able to produce enough photosynthate to sustain the coral. If
the deficiency is great enough, the zooxanthellae density will be
reduced. This can happen in three ways: the zooxanthellae will
simply die and be ejected from the coral; the coral can digest
the zooxanthellae for its own energy needs (if it is a species
that can digest plant material, specifically cell wall
components); or the coral can release some of the zooxanthellae
from its tissues back into the water. This is bleaching.

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Photo

This Cynarina lacrymalis is
severely bleached. The tissue is clearly visible and inflated,
although without the pigmentation of zooxanthellae. The white
coloration comes from the skeleton visible under the tissue.
This coral will probably need to capture prey or be fed to
prevent starvation and recovery of a full complement of
zooxanthellae.

Similarly, although for different reasons, chronic or acute
excess of nutrients (especially nitrogen) can also cause
bleaching. Since corals can absorb dissolved nutrients directly
from seawater, they can benefit from energy obtained in this way.
However, when dissolved nitrogen is absorbed into the tissue and
cells, the zooxanthellae can also have access to the material. In
this case, there may be an excessive nutrient availability and
the zooxanthellae are less nutrient limited by the coral, and can
use the nitrogen to grow and reproduce. The growth to higher
densities of zooxanthallae is not necessarily good for the coral,
and the growth can become unbalanced and unchecked. If this
happens, bleaching may be required to maintain proper densities
of algae within the tissues.

Zooxanthellae also have finite life spans, and at any time
there are numbers of them that become senescent and are no longer
useful to the polyp. These zooxanthellae are ejected, and this is
also a form of bleaching.

The descriptions above sound like adaptive and productive
behavior involved in maintaining a balanced symbiosis, and they
are. So bleaching, by definition, is not necessarily a
detrimental occurrence as is widely held. However, there are
degrees of bleaching, and there are other factors that can cause
bleaching. These are cases where bleaching is not a normal
regulatory mechanism, but are caused by various factors that not
only jeopardize the symbiosis, but the health of each
partner.

Another Definition of Bleaching

Coral bleaching has another and more popularly conceived
definition. This definition states that a coral is considered
bleached when there is a visible lightening of the normal
coloration state, translating to an approximate loss of 50% of
the standing stock of zooxanthellae. Most people associate a
bleached coral with the images of stark white corals on a reef.
This is considered to be severe bleaching, with mass bleaching
defined as when an entire community of corals has become partly
or totally bleached.

When I say totally bleached, this is a bit of an
overstatement. There are, to my knowledge, no cases reported
where bleaching is total except in experimental conditions
(difficult to even achieve) and where some temperate corals can
exist naturally either with or without zooxanthellae. The
densities of zooxanthellae, at most, become extremely low so that
their brownish coloration is no longer visible and the coral
tissue becomes largely transparent, revealing the white skeleton
below.

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Mechanisms and Effects of Bleaching

The low numbers of zooxanthellae remaining in bleached coral
tissue are also the reason that bleached corals often recover. It
is unlikely that they recover to an appreciable extent by
acquisition from the water column, but rather from the
reproduction of those left in the tissue. However, if
zooxanthellae densities are extremely low, the coral cannot get
much energy from the products of their symbiotic algal’s
photosynthesis. This creates an energy deficit that must be
filled by either feeding or by direct uptake of nutrients from
seawater. While possible, severely bleached corals often do not
recover, and they die. Why?

Seawater is often nutrient poor, and so direct absorption may
not take place to a degree, or at a rate, that can provide enough
nutrition. Secondly, even if there are adequate prey items for
the coral to capture, the maintenance of capture mechanisms, such
as nematocysts, are energetically costly. The coral may not be
able to effectively maintain these structures and, therefore, be
unable to effectively feed. Furthermore, it costs energy to
swallow and digest prey items. For aquarists, this is readily
obvious in bleached corals that appear to have no interest or
ability to capture food items offered to them. In the end,
bleached corals operating at an energy deficiency must metabolize
their own tissues to survive, and this is seen as recession and
tissue death. It is also called starvation.

Photo

The Sinularia sp. pictured here is
bleached, although zooxanthellae are still visible in some of
the branches on the left.

The best solution to a severe bleaching event, beyond removing
the stressors that caused the bleaching in the first place, is to
provide enough nutrients to sustain the coral and to repopulate
zooxanthellae, as well as providing them in a form that requires
the least amount of energy to obtain and use. The best answer for
all of these requirements is to make sure that there is a good
supply of dissolved nitrogen in the water column. A high nitrogen
level will probably not be beneficial once a bleached coral
recovers, but it can be helpful in the recovery process. Bingman
correctly notes that many aquariums are already many times higher
than reefs in usable forms of nitrogen (pers. comm.). In such
cases, increasing the usable forms of nitrogen (nitrate and
ammonium) will probably not matter. However, many aquarists now
keep aquariums where nitrogen levels approach or are below
average reef water levels, and in such cases ammonium or nitrate
can be fuel for zooxanthellae reproduction. For further
information on the role of nitrogen in zooxanthellae
reproduction, see Marubini and Davies (1996), Hoegh-Guldberg
(1994), Hoegh-Guldberg and Smith (1989), and Mueller-Parker et
al. (1994).

Another problem that occurs with bleaching is the way in which
zooxanthellae are lost. Aquarists may be familiar with brown
mucous strands of zooxanthellae being released from the mouth of
a coral. Often, zooxanthellae removal or loss is a fairly
controlled process with the vacuoles containing the algal cells
moving towards the outer cell membrane, fusing with it, and then
releasing the contents into the coelenteron. However, rapid
bleaching or severe stress results in a much more detrimental
release, with the entire cell contents being blown out into the
coelenteron or, even more severely, the entire dermal cell being
detached and lost. It should be apparent that such traumatic
reactions have an even greater detrimental effect on a coral than
the loss of algal cells alone. In such cases, bleaching is often
severe enough and with enough concomitant injury that recovery
chances are slim.

Corals bleach for a number of reasons, some of which were
described above as regulatory processes. In more detail, the
quantity and quality of photosynthetic products is a major
factor. In particular, the production of oxygen by zooxanthellae
can be especially problematic. Excess oxygen, especially in forms
where singlet oxygen radicals are produced, or when coupled with
water to form hydrogen peroxide, can be damaging to coral tissue.
Corals do produce enzymes to detoxify these oxygen forms, but
under conditions that produce bleaching, they may not be able to
handle the amount of oxygen produced. Therefore, bleaching occurs
to prevent the oxygen poisoning of the coral cells.

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It is not well known yet if bleaching is ultimately under
coral or zooxanthellae control. There is evidence to support both
views, and perhaps various bleaching events depending on the
circumstances, may be under the control of both or either
partner(s). Further research is required to determine these and
other aspects of the bleaching response.

Other Aspects of Bleaching

The factors that can cause coral bleaching are numerous. In
the wild, the most established factor attributed to mass
bleaching events is a prolonged increase in temperature above
normal levels. Temperature as a bleaching cause may be
synergistic with other factors, including reduced water motion,
irradiance, and nutrients. A list of factors shown to cause
bleaching in various studies are:

  • Bacterial bleaching – Vibrio shiloi
  • Low light or darkness
  • Chemicals – pollutants, metals, pesticides,
    contaminants
  • Endolithic fungii
  • Ciliates – unknown type and role
  • Hyper salinity
  • Coccideans – unknown type and role
  • Hypo salinity
  • High temperature – sustained or short-term increase
  • Drugs
  • Low water motion (stagnant water, doldrums)
  • Competition
  • High irradiance – sustained or a rapid increase
  • Sedimentation
  • Ultraviolet radiation – sustained high levels or a rapid
    increase
  • Starvation
  • Rapid change in temperature – higher or lower
  • Physical injury or stress

“But my coral still has a light blue color,” queries
an aquarist, “it must not be bleached.” Untrue! Many of
the bright colors found in corals are due to fluorescing proteins
that are not apart of the zooxanthellae. These pigment complexes
lie in vesicles either above or below the zooxanthellae within
the animal tissue. They serve to modulate visible and ultraviolet
light in either enhancing or protecting roles. If zooxanthellae
are lost, these pigments can remain for quite some time. Because
they are no longer serving their function and they are
metabolically costly to produce, these pigments will eventually
be lost until they are required again. If there is recovery they
will be produced again by the recovered coral if required. But,
it takes some time for them to be metabolized (unless the
bleaching event resulted in the entire loss of cellular contents
or cell detachment), and so a coral may retain some colorful
pigmentation even when bleached of zooxanthellae almost
completely.

Conclusions and Notes for Aquarists:

In conclusion, bleaching is a common event in both wild and
aquarium corals. In many cases, minor bleaching may not even be
noticed, with zooxanthellae and coral pigments occurring in high
enough densities to prevent observation. When bleaching becomes
severe enough, a paling or transparency of coral tissue results
and the coral presents a resulting pale or white appearance.

When this occurs, it can be very difficult to assess whether
or not coral tissue remains. In some cases, tissue expansion can
be apparent and it is obvious that there is coral tissue
remaining, but that it is transparent. In other cases, and
especially when a stressor is still present, coral tissue may not
expand, or be reduced in mass, and remains tightly contracted. It
is then very difficult to determine if there is coral tissue
remaining, or if tissue loss has occurred. One of the fastest
ways to assess this is to watch for the rapid colonization of
diatoms and other algae. These algae will not settle on coral
tissue, but readily populate exposed skeleton and should be
visible to the eye within a day or so after skeletal exposure.
However, this too can be deceiving. Sometimes, bleached coral
tissue was present but then died as a result of the bleaching,
and the skeleton is now exposed. Also, recovery from bleaching
can sometimes occur quickly, and the recovering and reproducing
brown zooxanthellae within the tissue can be mistaken for diatoms
and other brown algae on exposed skeleton. Conversely, brown
diatoms are often mistaken for zooxanthellae recovery.
Furthermore, initial populations of diatoms are soon replaced by
other algae, many of which may be unicellular green types that
frequently give aquarists the false impression of recovery.
Aquarists often report that their coral is recovering because
they see a greenish color returning to the tissue, but it is
often just green algae growing on exposed skeleton.

Photo

Euphyllia parancora shows bleaching, but
retains flourescing protiens..

Whether or not a coral recovers from bleaching is mostly a
factor of the subsequent conditions following the bleaching event
and the severity of the bleaching itself. There are no hard and
fast rules to determine whether or not a coral will recover, and
time is often the only indication and cure. Because a coral
appears white, however, does not necessarily indicate that
bleaching has occurred. The same signs of a pale or white coral
can also be indicative of tissue recession, competition,
predation, environmental stress, and disease. Despite the
difficulty of always being able to recognize bleaching, it is the
still the easiest of these “white” coral problems to
identify. In the next article, I will discuss some of the other
causes of “white” corals and their recognition in
aquariums.

Websites with further information on coral bleaching:

Literature Used

(not exhaustive, but useful for anyone interested in aspects
of coral bleaching, and including excellent summary papers):

  1. Brown, B. (1997). “Coral bleaching: causes and
    consequences.” Proceedings of the 8th International
    Coral Reef Symposium
    , Panama.
  2. Brown, B. E. (1995). “Mechanisms of bleaching deduced
    from histological studies of reef corals sampled during a
    natural bleaching event.” Marine Biology 122:
    665-663.
  3. Brown, B. E. and L. S. Howard (1985). “Assessing the
    effects of ‘stress’ on reef corals.” Advances
    in Marine Biology
    . London, Academic Press, Inc. 22:
    1-63.
  4. Brown, B. E. and M. Le Tissier (1992). “Quantification
    of coral bleaching.” Proceedings of the Seventh
    International Coral Reef Symposium
    , Guam, University of
    Guam Press.
  5. Bunkley Williams, L. and E. H. J. Williams (1988).
    “Coral reef bleaching: current crisis, future
    warning.” Sea Frontiers(March-April): 81-87.
  6. Fagoonee, I., H. B. Wilson, et al. (1999). “The
    dynamics of zooxanthellae populations: a long-term study in the
    field.” Science 283(5 February 1999):
    843-845.
  7. Fitt, William K., et al. 2001. “Coral bleaching:
    interpretation of thermal tolerance limits and thermal
    thresholds in tropical corals.” Coral Reefs 20:
    51-65.
  8. Fitt, W. K., H. J. Spero, et al. (1993). “Recovery of
    the coral Montastrea annularis in the Florida Keys
    after the 1987 Caribbean “bleaching event”.”
    Coral Reefs 12: 57-64.
  9. Gates, R. D., G. Baghdasarian, et al. (1992).
    “Temperature stress causes host cell detachment in
    symbiotic cnidarians: implications for coral bleaching.”
    Biological Bulletin 182: 324-332.
  10. Glynn, P. W. and L. D’Croz (1990). “Experimental
    evidence for high temperature stress as the cause of El
    Nino-coincident coral mortality.” Coral Reefs 8:
    181-191.
  11. Harriott, V. J. (1985). “Mortality rates of
    scleractinian corals before and during a mass bleaching
    event.” Marine Ecology Progress Series 21:
    81-88.
  12. Hoegh-Guldberg, Ove. 1999. “Climate change, coral
    bleaching and the future of the world’s coral reefs.”
    Mar. Freshwater Res. 50: 839-866
  13. Hoegh-Guldberg, Ove. 1994. “Population dynamics of
    symbiotic zooxanthellae in the coral Pocillopora
    damicornis
    exposed to elevated ammonium
    {(NH4)2SO4}
    concentrations.” Pac Sci 48: 263-72.
  14. Hoegh-Guldberg, Ove, and G. Jason Smith. 1989.
    “Influence of the population density of zooxanthellae and
    supply of ammonium on the biomass and metabolic characteristics
    of the reef corals Seriatopora hystrix and
    Stylophora pistillata.” Mar Ecol Prog
    Ser 57: 173-86.
  15. Hoegh-Guldberg, O., L. R. McCloskey, et al. (1987).
    “Expulsion of zooxanthellae by symbiotic cnidarians from
    the Red Sea.” Coral Reefs 5: 201-204.
  16. Hoegh-Guldberg, O. and G. J. Smith (1989). “The effect
    of sudden changes in temperature, light and salinity on the
    population density and export of zooxanthellae from the reef
    corals Stylophora pistillata Esper and Seriatopora
    hystix
    Dana.” Journal of Experimental Marine
    Biology and Ecology
    129: 279-303.
  17. Kleppel, G.S., R.E. Dodge, and C.J. Reese. 1989.
    “Changes in pigmentation associated with the bleaching of
    stony corals.” Limnol Oceanogr 34: 1331-5.
  18. Kushmaro, A., Banin, E., Stackebrandt, E., and Rosenberg,
    E. (2001) “Vibrio shiloi sp. nov: the causative
    agent of bleaching of the coral Oculina patagonica.”
    Int J Sys Evol
    Microbiol 51: 1383-1388.
  19. Marubini, F., and P.S. Davies. 1996. “Nitrate
    increases zooxanthellae population density and reduces
    skeletogenesis in corals.” Mar Biol 127:
    319-28.
  20. Muller-Parker, G., et. al. 1994. “Effect of ammonium
    enrichment on animal and algal biomass of the coral Pocillopora
    damicornis.” Pac Sci 48: 273-83.
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 Eric Borneman

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