[Coral-List] causes of Acropora loss
Esther Peters
esther.peters at verizon.net
Sat Oct 6 12:36:25 EDT 2007
Diego, John, and "listers":
I wanted to add a correction and word of caution to this thread.
Diego, you noted
the WBD pathogen is a relatively common Vibrio species (most likely V. carchari
ae = V. harveyi), at least for A. cervicornis. We found differences
(resistance?)among organisms, being some of them affected and dying much faster
than others, in the presence of the same pathogen; moreover, disease
stopped after couple days.
and this is what has been found for white band disease type II, from
analysis of coral surface mucopolysaccharide layer samples (Ritchie,
Smith, Gil-Agudelo). But histopathological examinations were not
performed on those affected coral colonies. I found bacterial
aggregates in A. cervicornis and A. palmata colonies affected by white
band disease type I, at Tague Bay, St. Croix (where Bill Gladfelter
reported the first observations of this distinctive rapid tissue
loss), and elsewhere on Caribbean and Florida Keys reefs, using
histological techniques. Both of these studies have shortcomings,
because the "picture of disease" is incomplete. Current studies on
"white acute unknown (WAX)" or "white subacute unknown (WSAX)" disease
signs on these species and their hybrid (A. prolifera) are using
histological, microbiological, and molecular techniques on samples
collected at the same time or even on the same samples. Based on my
knowledge of these studies (I'm not doing all this work, talented
younger colleagues are, too), we are going to have more surprises
about these coral diseases.
On a personal note, I remember talking with Bill Gladfelter at the
West Indies Lab when I was starting my graduate studies and he
wondered if white band disease were not related to some environmental
change. After seeing Al Gore's graphs, the timing of when change was
detected (temperature related or something else) is certainly suspect
for contributing to the elkhorn coral's demise there. I estimated
that 95% of the elkhorn off Tague Bay had died because of the
epizootic of white band disease by 1986, last time I was there. In
type I, tissue loss did not stop until the entire colony was gone (I
think only a couple of possibly resistant elkhorn colonies were found
at Tague Bay). And the bacterial aggregates still might be a chronic
or latent agent that could lead to tissue loss over time (based on
Anderson, GG, JJ Palermo, JD Schilling, R Roth, J Heuser, SJ Hultgren,
2003, Intracellular bacterial biofilm-like pods in urinary tract
infections, Science 301:105-107). Note that James Duerden had found
them in A. cervicornis from Jamaica in his 1902 work West Indian
Madreporarian Corals. We need to keep our minds open to all sorts of
possibilities!
And that includes what Diego is thinking about the acute, complete
(100%) massive tissue loss of thickets of A. cervicornis and A.
palmata off the Rosario Islands, as being the result of a
high-temperature-related bleaching event in 2005. He had seen the A.
cervicornis alive 3 months before, and it appeared that the entire
patches had died (lost tissue) within the same time period, on the
order of a couple weeks before. John pointed out that white band
disease signs were not associated with bleaching in the 1970s to
1980s, and the linear tissue loss rate was on the order of mm per day,
thus entire colony mortality could take a few months (on the large
colonies that used to be seen on forereefs in the Caribbean). So both
mechanisms are quite plausible, as are others, potentially!
Esther Peters
John Bruno wrote:
Diego L. Gil-Agudelo, Ph.D. wrote: Please, correct me if I?m
wrong, but
diseases hardly affect 100% of the individuals of any population, as
supposedly happened with Acroporas in the 80?s
Hola Diego, You probably know a lot more that me about the etiology
of white band disease. And it is true that disease prevalence is
often less than 100%. But it can indeed be very high as it was for
Acropora spp., Diadema, etc. Outbreaks of infectious diseases that
kill 95% or more of individuals are not that uncommon in marine
systems. There are many well documented examples of comparable
urchin die offs all around the world, seagrass die offs (such as the
epidemic that nearly extirpated seagrass in New England in the
1930s), oyster diseases, etc. Kevin Lafferty has written a number of
excellent reviews on this ([1]http://www.werc.usgs.gov/chis/
lafferty.asp). And there are countless terrestrial examples; just
think of American Chestnut Blight ([2]http://www.apsnet.org/online/
feature/chestnut/).
(sorry, I was in high
school). Besides, it is not good for pathogens to do so (who will
they
infect next?).
True, but many of these examples have resulted from the introduction
of novel pathogens; the pathogen and host have no evolutionary
history. And of course pathogens, like all life forms, are not
optimized (otherwise they would have evolved wheels)-they do
sometimes wipe out their host (or nearly so-Ebola anyone) even though
this leads to their own demise. We see predators do this all the
time too. Just think of what an Acanthaster outbreak does to a coral
population.
If our data is right (and I?m sure it is), the WBD pathogen
is a relatively common Vibrio species (most likely V. carchariae = V.
harveyi), at least for A. cervicornis. We found differences
(resistance?)
among organisms, being some of them affected and dying much faster
than
others, in the presence of the same pathogen; moreover, disease
stopped
after couple days.
That is interesting, but not surprising. Remember you are working
with the 1% or so of the population (and their offspring) that
survived the epidemic. The proportion of resistant individuals is
surely much higher now that it was 30 years ago before all the
susceptibles were weeded out of the population.
This lead me to think that probably the Acropora loss
experimented during
the 1980?s was the product of bleaching due to thermal events. I
know this
decade experienced strong Ni?o events, although I?m not aware of
analysis
showing the coincidence or not of these thermal events with reports on
Acropora die offs.
That is an interesting idea that was also discussed fairly
extensively in the recent paper in JEMBE by Lesser et al. (2007).
The trouble is, as far as we know, the early-mid 1980s (when nearly
all Caribbean coral loss seems to have occurred) was a relatively
cool period according to an analysis by Barton and Casey (2005); an
often overlooked and very important paper. Additionally, the white
band symptoms were widely observed and documented-i.e., we saw it
happen and it was not bleaching, overgrowth by algae, sedimentation,
etc. It was disease.
To my mind, this is the only kind of event we know that
can produce such massive deaths of these corals in such an ample
manner, in
a large geographic scale and in a short period of time.
See my comments above. We do know that infectious diseases can
cause such rapid, devastating, and geographically widespread
population declines . Did you see what happened to crows in the
eastern US in the early 2000s? They and many other species suffered
rapid population declines due to West Nile Virus.
Diego
A GENRAL RANT, NOT DIRECTED TOWARD DIEGO: Diseases are a common and
very important ecological phenomena. Pathogens and parasites are at
least as important in regulating populations as competitors and
predators. I think in some cases (not all) human alterations to the
environment have increased coral disease severity. But we as coral
reef scientists don't need to continue to grasp at other explanations
for the Acropora die off of the 1980s. White band disease was
clearly not bleaching, it was not caused by coastal development, by
nutrient pollution, by fishing or by algae. It was/is caused by a
bacterial pathogen (as nicely shown in an excellent paper by Diego
(Gil-Agudelo et al. 2006)) and its impacts were identical next to
large urban centers and on very isolated, unfished reefs. We don't
need any convoluted explanations to understand what happened.
John B
Barton, A. D. and K. S. Casey (2005). "Climatological context for
large-scale coral bleaching." Coral Reefs 24: 536-554.
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References
1. http://www.werc.usgs.gov/chis/
2. http://www.apsnet.org/online/
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