Dear Art; (and anyone else with lots of scepticism and not-quite-perfect
memories!)
This entire discussion, nearly verbatum, took place only a few months
ago. So, to complete it (again!), i attatch the relevant and very
Scholarly cap-stone messages from Mr. Avent and Mr. Rinsberg- they are
quite worth re-visiting, especially since we have come so far down the
same path.
-Ross M.
Subject: Re: Numbers (Deep sea)
Date: Thu, 11 Jun 1998 09:59:38 -0500
From: "Andrew K. Rindsberg" <[log in to unmask]>
As usual, Art, you have the power of stimulating thought. You pointed out
that the deep sea has little to eat and more room to spread out, so the
mollusks should not be as diverse as in shallow waters. Just about everyone
found these arguments to be convincing until extensive collections were
actually made in deep-sea environments. First off, the deep sea is very
large; it covers about half of the planet's surface. And, although it is
mostly a "desert" in terms of having little food, the influx of food as a
"gentle rain from heaven" is constant enough to support animals readily at
a reduced rate of metabolism. That's why the longest-lived known clam
(about 200 years) is a deepsea form.
But the deep sea is not the same everywhere. The bathyal continental slopes
have a relatively high influx of food, and they have some pretty diverse
environments, including submarine canyons, rocky areas where currents sweep
the seafloor, muddy slopes and basins, sandy areas where little is depos
ited. In some areas, submarine landslides occur at rare intervals, and
there you have a pioneer fauna that exploits the freshly deposited
sediment, and is replaced by a succession of later communities. In the
abyssal realm, there are seamounts, island slopes, and more submarine
landslides. Also, the overlying water masses may be tropical or temperate
or polar, and this makes a big difference in the rate and kind of food
influx. It makes a difference whether the submarine landslide carries mud
and sand, or tropical carbonates, into the deep sea. And then there are the
areas where upwelling currents have created extremely rich areas in the
overlying water, as off Peru. AND there are the deepsea trenches to
consider: Each one separate from all others, and therefore having a
uniquely specialized fauna, just like islands. Islands? Think, too, of
island slopes--isolated seamounts--long stretches of continental slopes,
broken up into faunal provinces. And hot springs! No, the deep sea is not
the same everywhere!
It's true that deepsea sediments have a certain sameness to the human eye;
we tend to see the same few kinds of ooze over and over again. But to the
senses of a deepsea creature, they are not the same at all. To a tiny
snail, it can make the difference between life and death to be near or far
from the chimney of a polychaete worm tube or the aperture of a crustacean
burrow. The crustaceans dig extensive burrow systems and pile the excavated
material in mounds around the burrow openings. This sediment, which was
fermenting under the surface, is now a bonanza of opportunity for some
kinds of deepsea creatures. And what about sunken driftwood, colonized by
boring bivalves? It's still a mystery how their larvae find another piece
of driftwood, so sparsely scattered on the huge seafloor. And what happens
when a really big chunk of food, such as a dead whale, hits the seafloor
with a thud? Most whales sink after death. That windfall must profoundly
change the food value of that piece of ooze for decades to come. A stretch
of seafloor may look homogeneous to us, but must be in reality a mosaic of
different food values and opportunities for the animals that live there.
The seafloor itself is not a simple surface, but a fairly thick zone of
life that goes down a few yards (meters). There are the animals that live
in the water above; those that swim or drift near the bottom; those that
live on the surface; those that burrow shallowly; those that burrow deeply.
And the major, "elite" species have others that are dependent upon them.
Nothing is simple here.
There is one more complication of deepsea life to consider, and that is the
factor of time. The deep sea realm has lasted for a very long time, and,
despite the occasional landslide, volcanic eruption, or windfall, it is
really very uneventful compared to continental environments, where a storm
can change the local environment any day. This means that deepsea animals
have had a long time to evolve in a relatively stable environment. In this
situation, animals tend to become more specialized, and as more species
develop, the number of niches increases, so even more species can split off
to fill those new ways of living. The idea is called the "time-stability
hypothesis."
Consider, say, an oak tree, and a species of bird that eats nuts, and a
species of lice that lives on the bird. Given NO time to develop--constant
interruptions and emergencies--that's all you get. Given plenty of time to
develop, you might end up with five species of oaks, all living in
different drainage basins, each with its own kind of bird that lives only
on acorns (oak nuts), and a generalist that still eats all kinds of nuts.
Given even more time, you might find that the oaks are all living together
because of changes in geography, but they are now so different that they
can't interbreed effectively. However, you now have some hybrids too, and
now you have ten more kinds of acorn-eating birds, because they have
continued to specialize. Some drill the acorns open, some crack them, some
eat in the morning and some only at night, and so on. As long as there is a
constant supply of acorns, there's no problem. And now consider the lice
that lived on the original species of bird. There are now 32 species of
bird lice, adapted to live on the 16 bird species. Half of the species live
on the bird's heads and half on their bodies. (This is not far-fetched:
there are three species of lice that can live only on humans, each in their
own niche.) This situation, which is common in tropical forests and reefs,
also occurs in the deep sea.
I must say that it is easier for Art to pose a question than it is to
answer it!
Andrew K. Rindsberg (who wrote his dissertation on deepsea burrows. In two
volumes)
Geological Survey of Alabama
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Subject: Re[2]: Numbers (Deep sea)
Date: Thu, 11 Jun 1998 12:46:42 -0400
From: Robert Avent <[log in to unmask]>
Hi Andrew and the shellers out there.
Andrew, in general much of what you said was true. You have made a lot of
generalizations and observations here and I don't really have time to respond
much. A response to just about any sentence in your first four or five
paragraphs could go on forever! But if the original query was about deep-sea
diversity, I'm not sure I read an answer. In general, the deep-sea tends to
show an increse in benthic faunal diversity with depth until one gets really
deep. I'm not sure what the species richness patterns are in molluscs. But
overall the deep-sea is REALLY diverse. There are many new bits of information
gathered since Sanders' 1968 stability-time hypothesis that have stimulated
thought on deep-sea diversity, not to mention trophic relationships, faunal
antiquities, evolution, effects of scale, zoogeography, effects of pressure and
temperature, reproductive strategies, bioturbation, disturbance, resiliency,
fragility, etc., etc.
To those of you who want to take the time to learn something about deep-sea
ecology, I refer you to: Gage, J.D. and P.A Tyler. 1991. Deep-Sea Biology.
Cambridge Univ. Press. 504 pp.
And for molluscs: Allen, J.A. 1983. The Ecology of the Deep-Sea Mollusca. In
The Mollusca, Volume 6, W.D. Russel-Hunter [ed.]. pp. 29-75. London. Academic
Press.
A fun photo tour of the deep-sea can be found in: Heezen, B.C. and C.D.
Hollister. 1971. The Face of the Deep. Oxford Univ. Press. 659pp.
If anybody takes the time to look any of these, please let me know.
Also Andrew, were you doing work on lebenspurren, or what?
Bob Avent
Minerals Management Service
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