[Coral-List] Coral immortality

Thu Mar 24 23:17:50 EDT 2011

Apologies for the length of this one, don't read if you're not interested.

   The Lough (2008) paper referred to in her message documents that in 43 coral 
cores of over a century each, there were no signs of slowing of growth.  
Skeleton extension slightly increased over the 100 years, skeleton density 
decreased, they balanced each other and so total calcification did not change.  
So the lack of decrease in thickness of annual bands in X rays of coral 
skeletons has been documented, unlike in trees and clams where the rings get 
thinner with increasing age (thanks, Kelley K.!).
    The live tissues of coral colonies consist of repeating units we call 
polyps, which are connected together by tissue, and also have connecting 
digestive cavities and connecting nervous systems.  Thus, all the polyps of a 
colony and the tissue between them, is all genetically identical, and all 
derives from one original egg that became a larva, settled, and formed the first 
polyp.  Although each polyp has all the parts needed for independent existence 
(and if the coral is broken can continue to live), all the polyps are connected 
and coordinated as a single organism.  So if you find a colony with fuzzy polyps 
or tentacles out, and you touch one area, tentacles and polyps will not only 
contract there, but farther away as well as the impulses travel through the 
colony nerve net.  A whole colony is also all one sex or all the other sex or 
all both sexes.  So it behaves largely as a single individual.  The word 
“colony” is somewhat an unfortunate term, since a coral is not like a colony of 
humans, where the humans are not connected by tissue nor are they genetically 
identical.  In fact, they often have conflicting interests, which is quite 
different from genetically identical, connected tissues that are all part of one 
organism.
     If you are a single individual, it is usually not in your best interest to 
have one part of you kill another part of you.  (cancer does that, and 
“programmed cell death” is a normal part of growth of some organisms that is not 
bad for the individual, but my impression is that it is essentially cell 
suicide, not one part of the organism killing another)  It would not be in the 
interest of the whole coral for one polyp to kill another.  The paper by Darke & 
Barnes (1993) documents that in species of Porites that grow large, once they 
get past a certain size their surface develops lumps.  At the top of lumps, new 
corallites (the skeleton structure or “cup” that the polyp is attached to) are 
formed.  As new corallites are formed, older corallites grow at an angle toward 
the surface of the side of the lump, forming a fan like structure in the 
skeleton.  Slowly as the coral grows, they end up at the bottom of the sides of 
the lump, in the crack between two lumps.  At that point they are squeezed 
together, first having a thin wall between the corallites, then the corallite 
itself being squeezed until it no longer exists.  A single corallite lasts about 
5 years.  One presumes that the polyp in the corallite has a similar sort of 
thing happen, the authors speculate that the polyp is resorbed at the crack 
between lumps.  Porites skeletons are porous (hence the name) and a layer about 
5 mm thick from the surface down has tissue filling the pores.  What exactly 
happens to the cells in the polyp tissues down in those pores at the bottom of 
the cracks, we don’t know yet, the Darke & Barnes (1993) study was strictly a 
skeleton study.  But it is normal for an organism to produce new cells during 
its life and have old cells die.  Humans produce new skin cells, gut cells and 
blood cells as old ones die, though we don’t produce new nerve cells or muscle 
cells, I believe.  But we remain the same organism for our entire life, 
genetically the same (other than some cells that get somatic mutations), and 
continuous cell lines originating from that first egg (a point made in the 
Potts, 1985 paper referred to in Janice Lough’s message).  Same true in a 
massive Porites coral 1000 years old.  Not only is the oldest part of the 
skeleton 1000 years old, but the living tissue is an organism that is 1000 years 
old, even if no one cell in the organism is 1000 years old (though some might 
conceivably be, it seems unlikely).  In other corals, there is no evidence of 
polyps being squeezed like they do in massive Porites, so this is not a common 
thing.  Living polyps on the surface of corals do not sit on dead polyps just 
underneath them, they sit directly on skeleton that is empty.  One of the other 
articles Janice Lough pointed to, reported that the layer of tissue in the holes 
in the skeleton of Porites is thickest at the top of a massive Porites, and gets 
thinner down at the sides.  It also increases in thickness with age.  That is 
likely because skeletal formation doesn’t keep up with tissue growth, due to 
geometrical constraints of a large hemisphere (Barnes, 1973).
      Most species of corals have an attachment site, where boring organisms can 
enter, up through whatever the coral attached to.  As was pointed out (thanks 
David W.!), most boring organisms can’t gain easy access through the living 
surface of a coral (exceptions I can think of being boring clams, fan and 
feather duster worms).  But they can through the base, and commonly do.  Many 
corals have boring sponges and/or filamentous algae in at least parts of their 
skeleton.  But I agree that the boring organisms rarely kill the coral, though 
they may help it to fragment (and thus asexually reproduce).  (One type of coral 
that is completely surrounded by tissue is the mushroom corals or Fungiids that 
are not attached.  There are also quite a lot of corals that when small, can be 
unattached fragments, or begin as attached to a small piece of rubble, but 
remain unattached and grow tissue on all sides.  Waves roll them around so no 
one side is smothered.  These are called “coralliths” much as coralline algae 
nodules that are similar are called “rhodoliths.”  If a corallith grows large it 
may become stable, die on the bottom, and look like any other coral.  In 
another, related situation, it is possible for fungi to bore from the reef rock 
a coral is attached to, up into the coral skeleton and then even up into the 
coral tissue.  That then is a disease, one that Work et al. (2008) documented.  
In the corals I’ve seen it in, I don’t see any of the coral dead.
     Corals also commonly have partial mortality.  Often something kills part of 
the coral, a small or large spot.  If the spot is small enough, the tissue may 
be able to cover it over quickly and the coral will look like new soon.  If the 
dead spot is too large, it may never be able to cover it over with live tissue 
again.  A single coral can have a long history of damaged areas, partial 
recovering and so on.  Each time an area is recovered, it is recovered by tissue 
derived from the living colony, which is thus genetically identical with the 
original tissue and a continuous cell line from the original egg that founded 
the colony.  A colony often gets other things growing on a dead area, algae 
first, but then lots of other things, can easily include other corals.  If those 
other corals cover part of the dead area, then that is not the same as the 
original coral’s tissues, and it will not fuse with the originally coral’s 
tissues even if it is the same species.  The one exception is if the second 
coral is actually a fragment of the original coral.  The dead area could divide 
two areas of the original coral where tissue survives.  If those islands of 
tissue grow and encounter each other again, they are genetically identical and 
can fuse together again.  The different sorts of regrowth are responsible for 
coral skeletons that have a “hiatus” that Janice Lough refers to, when that area 
of the coral was dead, but then above it there is more skeleton where the coral 
was alive, the same coral.  How can that be, a dead coral can’t start a new 
growth.  But the coral survived in some other part of the colony, and recovered 
the area where the core was taken.
     Mike Risk points out that the mushroom-like shape of many corals, with a 
dead column holding the live part up, is produced by bioerosion, and there are 
many papers documenting that.  He also says “Bioerosion almost never kills 
corals-but is often responsible for spreading them, as in that Acropora work 
done years ago by Verena Tunnicliffe, my ex-student.  And yes, long cores often 
seem free of obvious bioerosion. This is a self-fulfilling prophecy: bioeroded 
ones don't extract properly.”  Thanks, Mike!  I note that for small colonies up 
to maybe a half meter or so, the whole colony is often collected and then cut 
into slices by a saw, producing the X ray figures in many publications.  That’s 
not feasible with larger colonies.
     I continue to think that the species of Porites that get really large have 
no programmed limit to their size or age, and keep growing until something kills 
them.  The living tissue is the same organism that has lived for up to 1000 
years or more.  Same may also be true of clones of some fragmenting corals like 
Acropora cervicornis and A. muricata (=formosa), but will be harder to 
document.  But for small coral species, their maximum size and/or age is likely 
or surely programmed, no colonies of those species ever get any larger.
          Cheers, Doug

Barnes, D.J.  1973.  Growth in colonial scleractinians.  Bulletin of Marine 
Science 23: 280-298.

Work, T. M., Aeby, G. S., Stanton, F. G., and Fenner, D.  2008.  Overgrowth of 
fungi (endolithic hypermycosis) associated with multifocal to diffuse distinct 
amorphous dark discoloration of corals in the Indo-Pacific.  Coral Reefs 27: 
663.

________________________________
From: David Weinstein <dweinstein at rsmas.miami.edu>
To: Arthur Schultz <fish2live at acsalaska.net>
Cc: coral-list at coral.aoml.noaa.gov
Sent: Thu, March 24, 2011 2:54:05 AM
Subject: Re: [Coral-List] Coral immortality

Just a small note to point out regarding bioerosion.  Bioerosion is a
complex process which should not necessarily be blamed for coral death.
Very few boring organisms are able to bore into any part of the reef
structure covered by living tissue.  An analogy to this is termites
(although most coral borers do so for habitat creation not
substance).  Many LIVING trees have defences from termites by
secreting anti-feedant chemicals (in the same way corals have their mucus
layers)..
       Corals that grow in such a way that they expand radially without
exposing a dead skeletal base seem to be more protected from initial
"attach" of macroborers.  Therefore, the boring organisms can not be accused
for the direct death of a coral colony overall.  However, various
disturbances (coral bleaching, diseases, physical impacts) that can kill a
part of the coral surface do create the opportunity for marcoboring
organisms to begin infesting the overall colonial structure.
      It is worth pointing out that grazors (such as parrot fish and
urchins) do cause physical damage to living tissue was they search for
food.  When environmental conditions greatly favor grazing organisms, the
reefs can be significantly, and quickly destroyed.  See Reaka-Kudla et al.,
(1996) for examples of this in the Galapagos.
      Bioeroders are important components to coral ecosystems, acting as
"contractors" that re-work building material and facilitate new population
settlement.  It is usually only when natural environmental conditions are
affected by stresses that the bioerosion process becomes potentially
destructive to the living reef.

David Weinstein
---
Marine Geology and Geophysics
Rosenstiel School of Marine and Atmospheric Science
University of Miami
4600 Rickenbacker Causeway
Miami Fl 33149
http://www.rsmas.miami.edu/users/dweinstein/

On Wed, Mar 23, 2011 at 7:21 PM, Arthur Schultz <fish2live at acsalaska.net>wrote:

> I'm so glad that people are talking about corals again! Even better,
> somebody finally expanded the discussion to the rest of the world's corals.
> The hermatypic slant at times feels a bit limited in scope.
>
> Owen, your mention of bioerosion points toward many of my own observations
> and hypotheses about the life cycles of Alaskan Stylasters. They are stony
> too, though they are Hydrozoans, and they clearly suffer from some erosive
> process. They also recolonize eroded portions of the corallum.
>
> I am curious about coral life spans from the perspective of an acceptable
> level of fishing pressure. There was a minor explosion of attention to
> deepwater Alaskan corals about 8-9 years ago, and some environmental groups
> were making claims of extreme longevity. That got me started reading the
> literature and looking for any discernible clues in all the pieces brought
> up by fishing boats.
>
> While I admittedly have very little hard evidence, I've come to the
> conclusion that corals are both mortal and immortal, depending on how you
> frame the debate. The individual polyps certainly must die, inferred from
> the numerous Stylaster pieces where the remains of polyps partially protrude
> from new growth. Whether they died from senescence, disease or predation is
> an open question, but for me there is an even bigger question: Is the living
> tissue the same thing that built the earlier polyp?
>
> There seems to be a presumption that a discrete piece of tissue builds a
> discrete corallum. To the contrary, look at Andrews et al, Hydrobiologia
> 471(1) pp 101-110 which indicates multiple settlement events on a Primnoa
> fan. As an analogy, nobody who lived in Paris in 1800 is still alive, but
> Paris remains as a mostly healthy and expanding organism of sorts. Andrews'
> corallum could have been recolonized by its own creator after an attack or
> episode of disease. It could just as easily have been recolonized by a
> genetically compatible relative who lived nearby. For an example of
> neighborly harmony, I think of a paper with the distinction of Absolute Best
> Title Ever, "Behind Anemone Lines" Ayre & Grosberg, Animal Behaviour 70:
> 97-110, which shows anemones to be tribal little beasts.
>
> OK, so a coral could be more like a city than a single clonal creature, and
> you may not even realize it by doing DNA analysis. How old is it then? Do
> you even have an idea of which part you should be aging? This becomes a huge
> dilemma, because you need to keep in mind that Alaskan fisheries are well
> managed for the health of the species, but we're pretty eager to scoop up
> the individuals.
>
> On the road to bioerosion, we should also touch on growth rates. Most
> animals (excepting Costco-fed Americans) have a fairly asymptotic growth
> pattern. It's fairly easy to find evidence of asymptotic growth for lots of
> animals, but I know of no evidence that corals follow this pattern. Indeed,
> I could argue the contrary, that these are animals that grow ever more
> mouths as conditions allow. If corals have asymptotic growth, isn't it
> conceivable that the asymptote would graph as a vertical line (with respect
> to time) rather than the traditional horizontal? I'd like someone to show me
> that corals add less mass as they age.
>
> If corals were truly immortal it seems unlikely that there would be fossils
> of extinct species. Clearly they will eventually die. While there are
> undoubtedly many things that kill them, one big factor is bioerosion. It's
> pretty ordinary to find a Stylaster fan being undermined by an eroding base.
> It's equally common to find other parts chewed up and eroding away; some
> Stylasters take on wild grotesque shapes as they age. Sooner or later
> though, some insult will intrude on the integrity of a primary support and
> the party will be over. Some corals like our giant Alaskan Primnoas may set
> themselves up for bioerosion indirectly, as they seem to have a penchant for
> attaching themselves to clusters of giant barnacles. Since corals tend to
> show a preference for growth up and away from the base, it may well be
> inattention to foundation accretion that eventually dooms many of them.
>
> Now, to make the bioerosion hypothesis more intriguing I can add some
> facts/myths about global warming. We're all worked up about the aragonite
> saturation state changing in the world's oceans, but I was told several
> years back (yes, "told"—no documents in my possession to verify this) that
> the aragonite horizon in the Aleutians is already up around 120 meters. All
> these eroding Stylasters that I've seen come from 300-800 meters or more, so
> either the shallow Aleutian aragonite horizon is a myth or Aleutian
> Stylasters are already fighting an enhanced erosion rate. I do have a list
> of aragonitic vs calcitic Alaskan Stylasters.
>
> In the end, it seems that immortal or not, a great many corals have life
> spans that are determined by factors other than senescence but the tools
> you'd use to determine the answer are going to be misused or misunderstood.
> DNA and isotope analysis may both miss any history of previous occupants.
>
> As a final oddity, my rarest piece of Stylaster coral—a single piece, of a
> species described from a single piece—has no discernible base. It has no up
> or down or any evidence of its orientation as it sat on the bottom. It was a
> healthy pre-spawner when I found it. A lost base is not necessarily a death
> sentence.
>
>
> Art Schultz
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> Coral-List at coral.aoml.noaa.gov
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