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salt vs. fresh

Date: Mon, 10 Jul 2000 11:40:02 -0600
From: Charley Bay <Charleyb at Cytomation_com>
Subject: RE: Why doesn't anything live in substrate?

Charley Bay wrote:

I wanted to add one more thing, though:  There's a TON of stuff
suspended in marine systems (many kinds of salts, calcium, lots
of other compounds).  This makes the water more viscous (easier
to swim in the Mediterranean Ocean than Lake Michigan), and much
easier to support life (to a point, it's pretty salty in the Red Sea and
not a lot grows there.)  Haven't we all seen how much more viable
our freshwater snails and crustations are in a semi-brackish,
or harder-water environment?

Because of the additional compounds and additional environmental
diversity, it's much easier to sustain life and a much wider variety of
life.  If for no other reason, a freshwater critter has to deal with the
osmotic gradient because impurities within the living system (inside
the critter's body) are much higher than impurities in fresh water.
That's hard, in a physiological sense.  In the ocean, that's much
less of an issue... a human's saline content (percentage) is actually
about on par with the suspended salts in the ocean.

I've given this issue a lot of thought over the years, and I've concluded
that we CAN have things called "freshwater reefs", but they can't be
a flora/fauna match with marine reef systems.  In freshwater, we
are greatly blessed with plant diversity and greatly penalized with
substrate-bound animals.  (For physiological reasons we can move
into next, plants typically can't handle salts and most animals tend
to require salts.)

- --charley
charleyb at cytomation_com

Carmen adds (with a fair bit of help from her biology text):

I was a little confused by your comments on the experience of saltwater
organisms.  I'm not sure how viscous (or salty?) water is better able to
support life?  This doesn't seem to make sense in my mind.

Freshwater animals have more salts (of various flavors) inside them--not
quite impurities--than their water environment.  For bony fish, their gill
tissue is a site of osmotic water gain, and their body fluids are
constantly under the threat of overdilution.  To compensate, they don't
drink any extra water and they excrete a large amount of highly dilute
urine.  They've gotta flush out all that extra water to maintain the proper
internal salt concentration.  (When this system fails in your sick fish,
they bloat up.)

But living in saltwater doesn't guarantee an easy time of it.  Most bony
fishes--so my text claims--"evolved from ancestors that entered freshwater
habitats" and later "became marine but internally remained more similar to
fresh water in osmolarity."  Gee, I had no idea.  Also "Marine bony fishes
constantly lose water [through their gills] by osmosis to their
hyperosmotic surroundings.  They compensate by drinking large amounts of
seawater and using the epithelium of their gills to pump the excess salt
out of the body."  They also put out very little urine.  Seabirds have
similar issues regarding salt.

As for the notion that the marine tanks have more substrate critters, seems
to me like alot of the organisms (or eggs and larval forms) may be brought
in on live corals and anemones...?  I've seen a fair bit of teeny-teeny
creatures zipping around my vacuumed off tank water (fresh tank).  And
planaria (thin tiny flatworms) will come out of the substrate in fry tanks
if you're adding alot of protein to the water.  So things are there.  In
the past (before keeping loaches), I had found some fairly large worms
living in the substrate.  Probably rode in with a plant.  They got a cute
name and were left alone.  If you really WANT worms, I'm sure you could get
some happy ones into your tank.  But most minds would probably jump to the
fear of uncontrolled parasite introduction.

This reminds me about the freshwater clam that I used to have.  Wonderful
creature.  If anybody else keeps these guys or knows of a source for
obtaining them (haven't seen them at my LFS in years) please let me know
off list--I'd love to get some more.


Carmen C. Robinett      Berkeley, CA   (510) 642-5971