---------- Forwarded message ----------
From: Douglas Fenner
Date: Thu, Mar 17, 2011 at 2:32 PM
Subject: Re: [Coral-List] Coral Immortality
To: coral-list@coral.aoml.noaa.gov
I think the author of the original study does us a favor, showing us that
what looks like a dead reef to us, does not mean people killed it. It can
happen completely naturally. Makes sense to me, conditions can get bad
naturally- if a dry coastline with a lot of exposed soil (like a desert) started
having rain in huge rainstorms that washed off lots of mud, it could kill nearby
reefs. Long before humans were doing anything.
Gene is right, science does have fashion sometimes, something new is
discovered and if it is exciting enough, people rush to work on it, and start
interpreting everything in terms of it, and can forget that it doesn't explain
everything.
Gene makes some comments about corals perhaps dying of old age and maybe we
just happen to be around when a group of geriatric corals are naturally dying.
But my impression is that for a lot of corals, they continue to grow and have no
set specific maximum size or age. Adult human size is pretty uniform, we grow
until we reach an adult size, then stop growing. And human ages are pretty well
limited too, people don't on rare occasions just go on living to 150, 200, or
500 years old. But some corals certainly do. We recently documented a Porites
lutea in American Samoa with a girth of 41 meters (Brown et al. 2009). Far as I
know the largest girth yet documented. A Porites lobata was documented in
Taiwan that is 12 meters tall (Soong et al. 1999). That's a lot of coral. And
I know of no reports where living, healthy coral, reaches a maximum size and
stops growing. Some fish do, such as some surgeonfish (e.g., Ochavillo et al
2011). Corals seem to be more like some trees, like Redwoods. Redwoods as far
as I know have no maximum size or age, they don't stop growing, they just keep
growing until something gets them, like the soil underneath them gives way, or
strong winds blow them over, or disease kills them, etc. Vertebrates typically
have maximum sizes and stop growing. Not all species of corals get to the huge
sizes I mentioned, those are the champions, and only a few species of Porites
get to those sizes, and I'd bet Montastrea annularis complex corals in the
Carribean gets to huge sizes at times. Most individuals of those same species
are vastly smaller, and won't make it to that size. But they grow every year as
long as they live, and they die at a wide variety of ages, not at a specific
size or age. But it is also clearly true that different species of corals grow
to different typical sizes and maximum sizes. There are some small
zooxanthellate corals in the Caribbean like Favia fragum, also called "golfball
coral" due to its typical small size. Scolymia cubensis doesn't get very big
either, and Siderastrea radians is usually small. The champion small colonial
zooxanthellate coral as far as I know is Stylaraea punctata, an Indo-Pacific
coral that is often less than 1 cm diameter and a really big one might reach 2
cm..
The original geology paper that stimulated this discussion reports on
reefs near shore in the Great Barrier Reef, which live in very high sediment
environments. Like any reef, when they reach the surface, that upper surface
can no longer grow. Corals don't grow well in air. These reefs stopped growing
on their slopes as well, perhaps because of the sedimentary conditions, and some
restarted later. I work in American Samoa, on a relatively young (only 1.5
million years old) island, which has lots of fringing reefs. The reefs have
reef flats that have reached the surface, and indeed although there is coral
cover on the reef flat currently, it is not high cover, and you can see why,
when the lowest tides of the year come, corals that are exposed to air too long
in the hot sun don't survive. The reef slopes, however, are in relatively clear
water and a low sediment environment (at least outside the harbor!). Those
slopes have fairly decent coral cover (about 30% average), large and small
colonies, coral cover has not been going down in the last few years, very little
macroalgae, lots of coralline algae and what looks to this non-geologist like a
reef slope that is growing geologically as well as biologically. My guess is
that many islands and atolls around the Pacific have reefs that are growing
geologically on their slopes, but I am certainly guessing and not a qualified
geologist.
For a longer time span perspective, consider Eniwetok Atoll in the
Marshall Islands. When it was drilled, the carbonate reef rock was about 1200 m
thick, and on top of volcanic rock, fitting with Darwin's theory. The reef rock
just above the volcanic rock was about 65 million years old. The reef at the
top is at the surface, with a few islands in the atoll that stick up a couple
meters or so above the water. So, the reef there began growing 65 million years
ago, and it has kept growing enough for the top to still be at the surface, even
though if it stopped growing and didn't restart, it would have subsided until it
was deep enough not to have enough light to grow back up to the surface. The
Pacific is dotted with atolls that have done the same sort of thing, and the
Indian O. has quite a few too. So over the time span of tens of millions of
years, there are lots of reefs that have managed to "keep up." Which is not to
say they may not have stopped growing and started again several times or even
many times. But an awful lot of them must have started again after each stop or
there would be few left at the surface, I think.
On an even longer time scale, there are long periods in the geological
record in which there are no fossil reefs anywhere. Veron (2008) correlates
these with periods when water conditions were bad, primarily chemically. But
you might have to hang around for 100 million years or so to see such a hiatus.
All of which does not say that some reefs don't naturally stop growing
from time to time.
I recommend the popular article Gene's post pointed to, and the comments
posted below it, all interesting.
Cheers, Doug
Brown, D. P., Basch, L., Barshis, D., Foresman, Z., Fenner, D., Goldberg, J.
2009.American Samoa’s island of giants: massive Porites colonies at Ta’u island.
Coral Reefs 28: 735.
Soong K, Chen CA, Chang J-C (1999) A very large poritid colony at Green Island,
Taiwan. Coral Reefs 18:42
Ochavillo, D., Tofaeono, S., Sabater, M., and Trip, E.L. 2011. Population
structure of Ctenochaetus striatus (Acanthuridae) in Tutuila, American Samoa:
the use of size-at-age data in multi-scale population size surveys. Fisheries
Research 107: 14-21.
Veron, JEN. 2008. Mass extinctions and ocean acidification: biological
constraints on geological dilemmas. Coral Reefs 27: 459-472.
Douglas Fenner, Ph.D.
Coral Reef Monitoring Ecologist
Dept Marine & Wildlife Resources
American Samoa
Mailing address:
PO Box 3730
Pago Pago, AS 96799
USA
work phone 684 633 4456
Faster ice melt, higher sea levels
Ice loss on Greenland and Antarctica is accelerating at three times the rate of
mountain ice loss. If it continues, this melting will dominate sea-level rise
this century.
Eric Rignot at the University of California, Irvine, and his colleagues compared
calculations based on 18 years' worth of data on climate and ice discharge with
8 years' worth of data from the Gravity Recovery and Climate Experiment, which
uses satellite measurements to assess ice mass. The authors estimate that the
rate of loss is increasing by around 36..3 gigatonnes of ice a year, with a
cumulative loss of 475 gigatonnes in 2006.
http://www.nature.com/nature/journal/v471/n7338/full/471268b.html?WT.ec_id=NATURE-20110317
