[Coral-List] [Fwd: Re: Heat; light, food, and bleaching, continued]

[Esther Peters]

Dear Tom, Pedro, Charles, Andrea, Shashank, and All,

Tom, I'm glad you wrote in at this time, because it reminded me I had
wanted to provide a few comments earlier but could not get to answering
the e-mails then!

Yes, more research is needed on carbon and energy metabolism, but I hope
we can also use the tools from histology in addition to physiology,
biochemistry, microbiology, ecology, and systematics.  As many of you
know, my interest is in understanding the structure of the coral's cells
and tissues in relation to its function (histology), or as Tom noted in
an earlier e-mail about the Goreau, Goreau, and Yonge paper, functional
anatomy.  Visual images with light and electron microscopy of what is
happening to tissue structure help us understand grossly visible
reactions (e.g., bleaching) and aid in interpretation of quantifiable
measurements of molecules (e.g., respiration, carbon).

Experimental and field observations on the histology/histopathology of
scleractinia since about 1900 have confirmed the importance of mucus
production by these organisms, the variable nature and biochemistry of
mucus production by different species, and the changes that occur in
mucus production when the coral is exposed to levels of sedimentation
and turbidity above those to which it was adapted (or other stressors).
These changes include biochemical composition, initial increases in
mucus production, and loss of mucocytes with continued stress.  This
loss is also probably related to changes in nutrient intake, not only
carbon contributed from the zooxanthellae (which might be shaded under
these conditions and not producing as much for the coral host) but also
interference with obtaining zooplankton or bacterial carbon sources.
The result is tissue atrophy and can lead to death of the polyps.  The
significance of vibrios that kill zooxanthellae and other changes that
occur in the microbial community on the mucus layer of corals is just
now being realized, which are also closely associated with mucus
changes, in more than one way.  (And then there are those coccidia I
found that are somehow associated with bleaching, but everyone seems to
forget about them!)

    But I digress...the niche partitioning based on surface-to-volume
ratios of polyps brought to light another aspect of ecological
limitations which has directed small polyp vs. large polyp studies and
what we suspected that they could eat or how well they could get rid of
sediment, but more information is necessary.  In Andrea's study of
Montipora capitata, a small-polyped coral that can obtain all its energy
requirements from zooplankton when bleached, the free edges of the
polyp's mesenteries possess a cnidoglandular band (part of the
mesenterial filament) that in some areas contains numerous large
cnidocytes producing the stinging cells known as nematocysts.  These
nematocysts are at least 80 micrometers long with extensively coiled and
spiny tubules, indicating they can aid in the capture of zooplankton.
The mesenterial filaments are convoluted and can be extruded through the
mouth to capture zooplankton.  The cnidoglandular bands also have
numerous granular gland cells, which contain acidophilic granules,
likely zymogens, digestive enzyme precursors.  The epidermal cells also
may not produce much mucus (this species is susceptible to sedimentation
in Kanahoe Bay, I understand).  So this species is indeed capable of
heterotrophy.

Conversely, the genus Porites has only small (maybe 10-20 micron)
nematocysts in its tentacles and mesenterial filaments.  It has larger
nematocysts in its gastrodermis, but they don't appear to be used in
food gathering (although more observations are needed on this!).  And
the fascinating chromophore cells of this genus are probably related to
its "more" autotrophic mode.  In addition, Porites epithelia are loaded
with mucocytes and they produce copious amounts of mucus, even when not
under sediment stress!  Perhaps they rely more on particulates and
bacteria growing on their mucus for carbon?

Yes, much more is to be learned about the Scleractinia, and histology
should be included in more studies to help interpret the functional results.

Esther Peters


Thomas Goreau wrote:

 > Estimado Pedro,
 >
 > Thanks for your comments and reinvigorating this discussion, I'm just
 > back from coral reef restoration projects in Micronesia and Polynesia
 > and I had not seen many of the comments below. You are absolutely
 > right that little is known and much more needs to be done on both
 > carbon and energy metabolism in both bleached and normal corals, but
 > this has never been a fad issue with funding agencies.
 >
 > My parent's first work on the fate of C-14 incorporated by
 > zooxanthella photosynthesis in the early 1950s showed that the coral
 > retained very little of the released carbon, almost all went into
 > mucus release. Later biochemical work (Trench and Muscatine, Cooksey
 > & Cooksey, Battey & Patton, etc.) confirmed that almost all
 > zooxanthellae release to the host were precursor biochemicals used in
 > mucus synthesis. The loss of these precursors causes a much greater
 > energy drain on bleached corals that must release mucus to deal with
 > sedimentation and other stresses, and almost certainly changes the
 > amount and chemical composition of mucus and the cost to the coral of
 > producing it.
 >
 > Since corals are specialized for different types of zooplankton prey
 > (something we have long known, but which you won't find in the "peer"
 > "reviewed" literature), their ability to make up loss of this will
 > depend on their nutrition from predation. The heavy loss of mucus in
 > bleaching must impose severe limitations on coral and their nutrition
 > and environment must affect their ability to replace this. There is
 > very little good work on this, but Andrea Grottoli and colleague's
 > work is an important start.  A recent paper in Nature on the
 > biochemical utilization of mucus release of corals by Wild et al. was
 > done on corals that were releasing exceptional levels of mucus
 > because they were in the first stages of severe bleaching, but this
 > abnormal condition was not noticed by the authors, the reviewers, or
 > the editors, who refused to publish our comment pointing this out!
 >
 > In the late 1980s Ray Hayes and I took samples of coral mucus from
 > bleached and unbleached portions of corals in Jamaica and Cayman,
 > which were analyzed for the microbial community by Kim Ritchie and
 > Garriet Smith. We found large amounts of a previously unknown Vibrio
 > species on the mucus of bleached Montastrea annularis and Porites or
 > Acropora (as I remember, I don't have it here) that did not occur on
 > mucus of the unbleached portions of the same colonies. In 1991 we
 > found large amounts of the same Vibrio found in the Caribbean on the
 > mucus of bleached Acropora, Porites, and Montastrea in the South
 > Pacific that again did not occur on the mucus from unbleached potions
 > of the same colony. We published a couple of papers on this in the
 > microbiological literature about 15 years ago. We assumed the Vibrio
 > was an opportunist specializing on the mucus of changed composition,
 > but were not able to follow through with biochemical measurements.
 > Several types of Vibrio has been known to be abundant on coral mucus
 > since the pioneering work of Hugh Ducklow and Ralph Mitchell in the
 > early 1970s.
 >
 > Later Rosenberg, Loya, Ben Haim, Kushmaro and colleagues found Vibrio
 > associated with bleached corals and claimed it to be the cause of
 > bleaching. We suspected (incorrectly) at the time it was probably the
 > likely opportunist we had found rather than the cause of bleaching.
 > Later work with James Cervino, Ray Hayes, Garriet Smith, Shawn and
 > Sara Poulson, Rob Martinez, Fabiano Thompson, and myself have found
 > another set of Vibrios closely related to the pathogenic shellfish
 > poisoning Vibrios which cause Yellow Band Disease in Caribbean
 > Monstastreas and in several other Pacific genera, and we have shown
 > that this is not bleaching at all in that there is no expulsion of
 > the algae. Instead the bacteria attack the zooxanthellae in situ and
 > cause programmed cell death (apoptosis). Ben Haim and Rosenberg have
 > since confirmed that the same takes place with their different Vibrio
 > strains. So those Vibrios are a zooxanthella (not coral) disease, not
 > bleaching, and quite separate from the opportunistic Vibrios on the
 > mucus.
 >
 > Furthermore the ways in which carbon and energy are partitioned will
 > differ between corals with different trophic pathways (an issue non
 > long known to coral researchers but essentially absent from the
 > published literature except for a paper by Jim Porter on niche
 > partitioning by surface to volume ratios of polyps in the early 1970s
 > and Grottoli et al's more recent work). Zac Forsman just showed me
 > his experiments in Honolulu two days ago with the same species
 > (Porites compressa and Montipora capitata)  that Andrea Grottoli
 > worked on, and he has found (working I think with Cindy Hunter and
 > Bob Richmond) that they also respond very differently to light stress
 > and the speed of water circulation. Clearly there is much more to be
 > learned!
 >
 > Saludos,
 > Tom
 >