White Corals,
Part II: Bleaching and Disease "Look-Alikes" All
photos by Eric Borneman except as noted.
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In the last article, I
discussed coral bleaching and its appearance in corals. In particular, I emphasized the
variations in bleaching, and alluded to the fact that bleaching may, in fact, resemble
other events that are distinctly not bleaching. In the next article, I will cover the
final part of the series, white syndromes as they relate to coral diseases. In this
article, I limit myself to those corals having white areas that are not caused by
bleaching or disease.
I also
mentioned that coral bleaching might be very hard
to distinguish from tissue loss. Bleached corals
have transparent tissue so that the skeleton is
visible beneath it, or in some cases the tissue
is very hard to see because it is greatly withdrawn.
Here, various causes may evoke bleaching, tissue
loss, or both.
Mechanical Stress and
Injury
Corals have very thin tissue that is easily
damaged. It does not take much effort to cause injury or abrasion. Because coral tissue
lies immediately superficial to skeleton, anything that breaks through two tissue layers
(epidermis and gastroderm), themselves often composed of a single cell layer, will expose
the white skeleton below. The simple act of moving a coral from place to place within an
aquarium can result in finger pressure piercing tissue as the sharp skeletal elements
break through the layers. Having just placed a few new corals in my own tank, I know that
pressing the coral down onto an epoxy mount caused the septal adornments to pierce through
the tissue of an Echinophyllia species. Similarly, corals that are overturned or
broken in the wild, and corals tumbling from their placement in the aquarium, can sustain
tissue injury that causes the skeleton to become exposed.
Figure
1. The sharp adornments on the upper edge of coral septa can act like blades, cutting or
piercing thin coral tissue. This Cynarina lacrymalis skeleton is very
"toothed" and can easily puncture the thin inflated tissues in the living coral.
Figure
2. This Siderastrea siderea was overturned so that the white area was upside down
on the sand. The discolored area adjacent to the white area is a common sign in stressed
tissue.
Tearing or piercing is not, however, the only
way injury can result in white areas on a coral skeleton. Corals are very susceptible to
abrasion by sand, and to local bleaching or tissue loss by sedimentation. On the reef,
shallow waters are subject to disturbances that can abrade tissue during periods of heavy
wave action, especially when the sediments consist of sands. In the aquarium, this happens
more frequently when a powerhead falls from its mounts onto the bottom of the tank.
Powerheads and pumps can pose another hazard if dislodged since the force of water itself
can blast tissue off corals. As an aside, some research utilizes a variation of this water
blasting when coral tissue must be collected for study: Water Piks and airbrushes are used
to remove coral tissue from the skeleton.
Figure 3. This Acropora palmata
fragment shows three reasons for white areas. First, the growing margins are white, and
this is normal as they are growing rapdily and do not yet have zooxanthellae. Second, a
wire is visible that was used to attache these fragments in a reconstruction effort. The
wire has caused whitened areas where the coral is growing over it. Finally, patchy white
areas are seen on the front fragment, and these were caused by a broken wire abrading the
surface.
Another deleterious result of sediments is
smothering. Fine particulate and organic flocculent material (detritus, silts, etc.) is
easily borne into the water column where it can settle on coral surfaces. The mucus
covering of corals, while generally helpful in removing such debris, is also a sort of
sticky trap for such materials. While corals feed on organic particulates, excess can be
problematic for several reasons. First, accumulations of particulate matter prevent light
from reaching the coral surface in the local area of settlement. This can result in
bleaching. Second, particulate material can smother the tissue, resulting in local hypoxia
(a deficiency of oxygen reaching the tissues) or anoxia (hypoxia that causes damage). A
great deal of partial mortality, and even total mortality, in corals both on the reef and
in aquariums occurs from sedimentation damage of this sort. When this happens, both
bleaching and tissue loss can be the result. A final problem with smothering is that the
often organically rich material is enriched with microorganisms, including bacteria,
ciliates, cyanobacteria, fungi, and other flora and fauna that can directly or indirectly
cause bleaching and/or tissue loss. Some of these components may even be primary
pathogens, although the aspect of pathogenicity will be covered in the next article.
Figure
4. The spots in the center of the coral are areas caused by sediment deposits. These are
older, but sediments deposited on the coral surface will first appear white until they are
grown over by other flora or fauna
Figure
32. This coral, along with the entire reef, is being
covered by sediments from coastal development. Such
deposits endanger the tissue of living corals.
Predation and Competition
Probably the most common source of white
corals in aquariums and in the wild results from the action of other coral reef plants and
animals. All too often, corals show signs that are easily confused with bleaching or
disease. Mistaken identifications are equally as common among aquarists as they are among
trained scientists. At least two cases of predation have been officially mistaken as a
"new" coral disease, with many more descriptions that are mislabeled as existing
coral disease. When I mentioned that it was often difficult to tell the difference between
conditions when corals have white areas, I was not exaggerating!
Figure 28. I was perplexed at this spotty
white area on a Montastraea faveolata. It was not until examining the photo that I
caught the perpetrator. Look closely nearby for a small face. Note how the upper surface
tissue is gone, but the polyps beneath "mouth’s reach" remain in their
corallite unscathed.
Rapid Wasting Disease was first mentioned on
the mailing list serve for NOAA’s "Coral Health and Monitoring Program."
Soon thereafter, an article appeared in the newsletter for the International Society for
Reef Studies, Reef Encounter (Cervino et al. 1997). The description of Rapid
Wasting Disease was primarily for massive corals in the Caribbean showing signs of tissue
loss, a sharp white band delineating healthy tissue from the disease band line, and the
erosion and dissolution of the coral skeleton. Preliminary results indicated the presence
of fungal hyphae and these were suspected of being the causative agent pending further
study. Rapid Wasting Disease was discussed at length and reported from areas throughout
the Caribbean, and even into the Pacific. It was on the verge of being deemed an
epizootic. However, some researchers were suspicious that this was not a disease at all,
but the grazing behavior of parrotfish. Andy and Robin Bruckner conclusively put Rapid
Wasting Disease into the "false alarm category" with several papers that showed
this condition to result from unique biting behaviors of parrotfish (Bruckner and
Bruckner, 1998a, 1998b, 1998c, in press). Spot, focused, and repeat biting behavior (where
the fish repeatedly bites the same spot on a coral, even after swimming away to return to
the same spot later), is an interesting behavior with few explanations - worthy of its own
study. It is not, however, a coral disease (Bruckner et al., 2000). The fungal hyphae are
believed to be normal endolithic fungi or perhaps even those growing on parrotfish beaks.
Figure 5.
Figure 6.
Figure 7.
Figures
5,6,7. The stoplight parrotfish, Sparisoma viride,
and others, participates in various biting behaviors
that produce signs on a coral that were incorrectly
thought to be a new disease called Rapid Wasting Disease.
This is not the only group
of fish that bite corals, either. Many, including, damselfish,
butterflyfish, triggerfish, pufferfish, angelfish, and even
some gobies, nip or graze coral tissue or individual polyps.
In such cases, spotty areas that may show partial or complete
tissue loss are seen. Rarely are these bites as deep as they
are with parrotfish, and small nips may not even break the
tissue. However, local stress can result in bleaching of the
area. Furthermore, there can be a continuum of pale or white
areas as new bites are made and old bites heal. Therefore,
the nature of these areas may not be easily recognizable.
Figure 8.
Figure 9.
Figure 10.
Figure 11. Photo: Mike Kirda
Figure 12.
Figures 8-12. Many fish may leave
bite marks on coral that can be deceptive. These corals
from both the Caribbean and the Pacific have characteristic
patchy white marks that result from bite marks in
various stages of healing.
Ridge Mortality Disease is another
"infamous" false alarm (Abbot 1979, Bruckner, pers. comm.). Massive Caribbean
corals such as Diploria spp. and Colpophyllianatans were reported to
be losing tissue with a distinct pattern along their upper ridges. As with Rapid Wasting
Disease, the initial description was soon followed by a slew of reports - another
impending epizootic was at hand. I suppose the coral disease community has a right to be a
bit paranoid considering the extent of real "new" diseases. It was noticed that
the tissue loss along the ridges of affected corals shared two things: a proliferation of
filamentous algae over time, and the presence of damselfish around the colony. Debbie
Santavy, a coral disease researcher in Pensacola, Florida, admits that many cases re
certainly attributable to damselfish activity, but she also feels that there is legitimate
ridge mortality disease exclusive of damsels nipping and killing coral tissue to grow
algal turfs as a food source (Santavy, pers. comm.).
Figure 13.
Figure 14.
Figure 36.
Figures 13, 14, 36. These patchy white blotches are caused
by damselfish. However, it may closely resemble a beginning white band disease or fireworm
predation. Note the green algae on some of the branches; these farmed filamentous algae
patches are why the fish nip the coral tissue.
Figure 15.
Figure 39.
Figure 15,
39. While no damselfish are visible, the algae on
the older nipped skeleton almost assures one that
they are nearby. This pattern of nipping the upper
margins of the meanders was called Ridge Mortality
Disease, although this is not a disease at all.
Many other organisms
prey on corals or can induce signs of disease or bleaching
by producing whitened areas on corals (Table 1). Among the
more common denizen diners of coral are various corallivorous
gastropods. The Caribbean fireworm, Hermadicecarunculata,
grazes many stony corals, and may even eat gorgonians and
sponges. Large fireworms can swallow an entire branch of Acroporacervicornis. Grazing trails are frequently present,
but not always. Its important to remember that Hermadice
is the only polychaete known to eat coral (though many bore
into skeleton), and unless live rock from the area is in the
aquarium or the worm was introduced some other way, coral
tissue loss due to errant polychaetes is probably not happening.
Table 1. Common sessile benthic competitors
capable of causing damage to corals, resulting in bleaching or tissue loss and a white
appearance. Mode: primary means listed first; c = chemical, p = physical. Common aquarium
representatives are not an exhaustive list.
Figure 16.
Figure 17.
Figures 16, 17. While many think Hermadice
carunculata only feeds on stony corals, I witnessed them feeding on sponges and
gorgonians, as well. Fortunately, these worms are uncommon in aquaria and they are only
found in the tropical western Atlantic and Caribbean.
Figure 18.
Figure 19.
Figure 20. Hermadice feeding
on Colpophyllia natans.
Photo: James Wiseman
Figure 18,
19. While seemingly impossible, Hermadice is
capable of swallowing entire branches of Acropora
cervicornis. These white areas devoid of tissue
are often mistaken as a disease. Figure 18 photo by
Dr. Andrew Bruckner.
Figure 40. A
Hermadice sp. Fireworm feeding on a bleached
patch of C. natans. Photo: James Wiseman
Other common coral
predators include the nudibranchs and corallivorous gastropods,
most of which are probably specialists one or a few types
of coral, at most. Therefore, the presence of white corals
from many varied genera in the aquarium are probably not attributable
to nudibranchs. Sea stars can feed on corals, and there is
some amount of anecdotal evidence that the small white stars
that reproduce in aquariums (Asterina sp.) may consume
some amount of coral tissue. Some may be opportunistic scavengers,
others may avoid corals entirely. There are several highly
corallivorous stars, the most infamous being Acanthasterplanci, the crown-of-thorns starfish. These animals
can consume entire table corals at a time, with thousands
or even millions moving across and consuming corals on reefs
when they occur in plagues. Interestingly, the primary predators
of these stars are whelks, giant gastropods whose numbers
have been significantly reduced over the past century for
the curio trade. Also, effective defenders of corals are commensal
crabs of the genera Trapezius and Tetralis.
These tiny crabs inhabiting branching corals prevent sea star
attack by either nipping off the "thorns" of Acanthaster,
or snipping off their tube feet (Pratchett , Vyotopil and
Parks 2000). Fortunately, Acanthaster and other large
stars like Culcita sp. are not found in the aquarium
trade and will not be a problem for aquarists. Crabs and shrimp
may also dine on coral tissue, as will some urchins. Most
urchin grazing is likely incidental and none are exclusively
corallivorous. Many other organisms may bore into corals,
through living tissue, and reside there. Generally, this does
not harm the coral, but local areas of whitening near the
bore hole may be present. A list of some known coral predators
and borers is given in Appendix 1.
Figure 21.
Barnacles and worms (Spirobranchus gigantea
pictured here) are common boring organisms in living
corals. Many others exist, but they usually do little
harm to the colony as a whole. Note the local whitening
of coral tissue near the barnacle.
Figure 22,
23. These small nudibranchs cause the bleaching-like
appearance of Montipora sp. They seem to be
almost specific for Montipora predation. Photos:
Tracy Gray
Figure 27. While appearing
like fish bites, these marks were left by corallivorous
snails, Coralliophila abbreviata.
Figure 37. This gorgonian
eating snail, Cyphoma gibbosum, is very beautiful.
While not preying on stony corals, it is a significant
consumer of seafans and other gorgonians, leaving
exposed axial skeleton with no tissue in its wake.
Figure 38. A Drupella
sp. snail is a common corallivore of Pacific
stony corals.
Competition is
another means by which corals may become whitened by bleaching
and/or tissue loss. This includes direct physical competition
and competition at a distance, usually by means of chemical
toxins produced by various organisms. These toxins and noxins
are also known as secondary metabolites or allelopathic substances.
Aquarium Corals (Borneman 2001) covers this subject
in some depth, but virtually all groups of benthic invertebrates
and plants contain members that may produce compounds that
can result in "white" corals through bleaching and/or
tissue loss. A list of some of these is found in Table 1,
with notable aquarium representatives highlighted. More direct
competition occurs when corals use cnidocyst-laden tentacles
and mesenteries, mesenterial extrusion, sweeper tentacles
and other means to injure nearby and competing species. I
use corals as an example, but there are many organisms with
various direct and indirect competitive methods that can produce
these effects. Sponges, tunicates, hydroids, algae and virtually
all benthic life must somehow compete and may emerge completely
or partially victorious, leaving their effects on the coral.
Figure 24. The Diploria
strigosa in front is in direct competition with
the Montastraeafaveolata behind it.
The Diploria appears to be the stronger competitor,
and the tentacles reaching the nearby Montastraea
tissue have created a white zone that appears bleached
and may soon show tissue loss. This type of marginal
effect is also where disease lines tend to form, and
so the two could be easily confused.
Figure 25.
Algae can also be strong competitors. A bare zone
exists around this Halimeda sp. Ordinarily,
one would assume that sediments or other damage created
a dead spot on the coral colony, onto which Halimeda
merely settled and grew. However, the white area indicates
ongoing effects, and is almost certainly due to the
allelopathic effects of metabolites produced by Halimeda.
This algae, while having defenses, is much less toxic
than many.
Photo: Deborah Lang
Figure 26. Sponges can certainly cause tissue
loss or bleaching from their production of secondary metabolites. Here, only minor local
effects are seen as the sponge tissue contacts coral tissue. Photo by James Wiseman.
Figure 34. Sweeper tentacles on a Euphyllia
ancora are structures used by many corals to inflict damage on competing colonies.
Figure 35. Mesenterial filaments on a Hydnophorarigida will, upon retracting, have killed the entire contact region on the Stylophorapistillata. These structures can deploy and work very quickly, leaving dead tissue
in their wake and an exposed white skeleton.
Recession, Starvation, and Senescence
A "catch all" phrase used to describe the progressive and
usually slow loss of tissue is recession. Generally, receding tissue on corals show poor
polyp extension and perhaps abnormal coloration (pale or otherwise), along with an area of
bare corallites where polyps have died. This type of loss is usually slow enough that
white areas are not seen because algae and other organisms colonize the skeleton at a rate
that matches the tissue loss. Starvation is probably involved in many cases and certainly
direct or intentional starvation can produce the same signs. Areas of recession tend to be
at the older areas of a colony; near the base, within the center of foliose plates, etc.
The reason recession often begins in these areas may be due to several factors: First,
corals infill their skeletons as they grow, and older areas of a colony may not maintain
good contact with other younger polyps near the growing margins. As such, they may become
isolated and not be able to communicate with other polyps through the gastrovascular
canals, losing a food source and perhaps other advantages to a colonial lifestyle. Second,
these regions tend to have reduced water flow and light capture area, along with being
protected from potential prey capture opportunities. As such, respiration and
photosynthesis can be dramatically reduced and result in the local weakening or death of
those polyps. Third, coral polyps are now known to show senescence. In simpler, if not
somewhat incorrect, terminology, they are dying of old age. Any of these reasons can
potentially cause areas of skeleton to be exposed, resulting in a patchy or even band-like
white area that may be mistaken for any number of other problems. It bears repeating that
it is very difficult to know the true reasons for these similar looking manifestations.
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Others
There are many other things that can cause a
white appearance in corals. Some of these were covered as agents of bleaching in the last
article. However, some may also cause tissue loss in addition to, or in place of, simple
loss of zooxanthellae. For example, chemicals, heat, radiation, and others can cause the
direct death of coral tissue inclusive or exclusive of the loss of symbiotic algae. Of
particular note to aquarists are accidental introductions (and from tragic stories I have
heard, sometimes intentional ones by angry store employees, customers, and ex-relationship
members) of things like bleach, pesticides, paints, and other aerosols. It should,
therefore, be little wonder that the list of potential agents of tissue loss that result
in white areas on corals is almost up to the imagination. There are only so many ways for
a coral to die, and unfortunately the signs of death are almost invariably areas of
visible or exposed white skeleton beneath, or lacking, healthy, pigmented, overlying
tissue.
Figure 33. This Acropora has been hit
by cyanide fishermen to collect small aquarium fish living in the branches. Toxic
chemicals can bleach of kill coral tissue, leaving the characteristic white affected
areas.
In the next article, I will describe some
coral diseases that are collectively known as "white syndromes." All too often,
aquarists seeing signs that may resemble these diseases jump to the conclusion that a
disease is, in fact, present. I hope this article, and the previous one, will help readers
to assess each case carefully and with good observations. There are so many potential
reasons for a coral to present visible white areas, and only by determining what the
problem actually is, and also what it isn’t, can steps occur to minimize further loss
and perhaps aid recovery
References:
Abbott, R.E. 1979. "Ecological processes
affecting the reef coral populations at the East Flower Garden Bank, northwest Gulf of
Mexico." Ph. D. Dissertation. Texas A&M Univ., College Station, TX 154 pp.
Borneman, Eric H. 2001. Aquarium Corals.
Microcosm, T.F.H., Neptune City. 464 pp.
Bruckner, A.W. and R.J. Bruckner. In press.
"Coral predation by Sparisoma viride and lack of relationship with coral
disease." Proc 9th Intern. Coral Reef Symp
Bruckner, A.W. , R.J. Bruckner and P.
Sollins. 2000. "Parrotfish predation on live coral: “spot biting” and
“focused biting”." Coral Reefs. 19:50.
Bruckner, A.W. and R.J. Bruckner. 1998a. "Destruction of coral by Sparisoma
viride." Coral Reefs. 17:350.
Bruckner, A.W. and R.J. Bruckner. 1998b. "Rapid wasting disease:
pathogen or predator." Science. 279. 2023-2025.
Bruckner, A.W. and R.J. Bruckner. 23 July, 1998c. "Rapid-Wasting
“Disease”: Coral predation by stoplight parrotfish." Reef Encounters.
pp.18-22.
Cervino, J., T.Goreau, G. Smith, K. DeMeyer, I. Nagelkerken and R. Hayes
1997. "Fast spreading new Caribbean coral disease." Reef Encounter
22:16-18.
Pratchett, M., Vytopil, E., and Parks. P. 2000.
"Coral crabs influence the feeding patterns of crown-of-thorns starfish." Coral
Reefs 19: 36.
