Re: do fish "yawn"?

["William Vannerson" <William_Vannerson@ama-assn.org>]

>>>Also do fish yawn if they have some kind of parasite?

No.  It's a normal behavior that's not totally understood.

Another little oddity about fish is that salt water fish drink water, while fresh water fish do not.  See below.


========================================
NEC - 1991
2ND PLACE  BEST PROFESSIONAL
NAS - WET PET GAZETTE

PHYSIOLOGICAL DIFFERENCES BETWEEN FRESHWATER AND SALTWATER FISHES
BY DON JOHNSON

The topic of this article is supposed to be the
difference between freshwater and saltwater fishes.  I
can sum that up in a sentence: freshwater fishes do
not drink water; saltwater fishes must drink water.

The volume of water inside of a fish (found mostly in
the blood, as with almost all vertebrates) along with 
its concentration of organic and inorganic solubles
must be closely controlled.  These solubles consist of
nutrients and products of metabolism (organic) and
oxygen, carbon dioxide, and electrolytes including 
Na+, Cl-, HCO3- and Ca2+ (the inorganics).  Fish must
constantly strive to be in osmotic equilibrium with
their environment.  In freshwater fishes, this means
that some mechanism must be present to prevent needed
electrolytes from diffusing through cell walls and into
the water.  In marine fishes, the reverse is true.  In
both fresh and salt water, the main regulating
mechanism is the kidney but, as we'll soon see, the 
process is far more complicated in saltwater fishes.

In freshwater fishes, the kidney functions largely to
excrete water.  In the saltwater forms, the kidney 
serves largely to regulate excretion of magnesium and
sulfate ions.  The sharks, skates and rays are termed 
"hyperosmotic" and their kidneys combine both
functions.  Then too, many fishes are euryhaline,
(moving from fresh to salt water and back again). 
Examples include sticklebacks, salmon and eels, to name
only a few.  Such fishes must posses kidneys capable
of switching from water excretion to water conservation
and divalent (an element having two atoms with which
another element can combine) ion excretion, and back
again.

A marine environment forces a water loss from body
fluids due to the difference in osmotic 
concentrations.  This fluid loss comes from the skin
and gills.  The kidney loses water only to a minor 
extent since it has to produce enough urine to get rid
of metabolic wastes.

To make up for this loss of fluid, marine fishes drink
sea water.  For the handful of fishes actually studied
to determine their rate of water ingestion ( 12
species) , the rate of drinking is between 0.3 - 1.5%
of body weight per hour.  After swallowing sea water,
the fish extracts water form the brine along different
segments of its intestinal tract.  Monovalent ions are
extracted along with the water.  Time out for a 
chemistry course refresher:  Valence represents the
number of atoms of one element with which an atom of
another element may directly combine.  Every element
except the six inert elements has a valence number, a
whole number not more than eight.  An element which has
a valence of 1, such as hydrogen or chlorine, is said
to be monovalent.

Tests, using rainbow trout, show that for these fish at
least, between 60-80% of the water in the ingested 
salt solution was absorbed and 95% of the salt that was
absorbed was sodium chloride.  Later tests on flounder
showed similar results.

Now what happens to all that absorbed sodium chloride?

We saw earlier that divalent ions are excreted by the
kidneys.  NaCl (sodium chloride for those who never 
took chemistry, or who like me took it too many years
ago) is a monovalent ion.  How do saltwater fish get 
rid of it?  NaCl is eliminated across the gill
epithelial surface.  The turnover rate will vary for
different salinities of water; or more precisely, its
sodium content.

This, believe it or not, is greatly simplified
description of what happens.  Fish over the millennia
have evolved adaptations to insure their survival in
wide variety of waters.  Some of these mechanisms are
still only partly understood with new discoveries
being made regularly.

Excretion is a normal process of all living animals. 
Unfortunately for fishes living in closed systems, it 
also results in a slow poisoning of their environment.

Only a small portion of the total nitrogen excreted by
fishes appears in the urine.  The greatest amount of 
nitrogen is excreted across the gill in the form of
ammonia -- as much as 6-10 time as in all the 
nitrogenous compounds formed by the kidneys.  The gills
also serve to excrete other highly diffusible waste
products such as urea and amine or amine oxide
derivatives.  Studies dating back to 1931 have shown
that ammonia is the chief end product of nitrogen
metabolism of all aquatic animals, both freshwater and
marine; simplest to most complex organism.

The pH difference that exists between the internal and
external environments of aquatic animals is thought to
be a significant factor in determining the rate of
ammonia elimination.  For example, alkaline sea water
would tend to retard and acidic freshwater accelerate
diffusion of ammonia into the surrounding 
environment.  This should indicate to an aquarist that
there is a vital need for regular water changes 
combined with the use activated carbon to control the
amount of free ammonia in an aquarium.  Ammonia is
highly toxic.  I should not have to point out that the
excretion of ammonia is also a reason to make frequent
pH checks and, when necessary, pH adjustments in a
closed system.  The excretion of ammonia also, in
freshwater fishes, aids in sodium uptake, necessary to
maintain critical internal salt and water balance.

The cartilaginous fishes, since their earliest
evolution, have always practiced internal
fertilization.  (How do we know? Fossil male sharks
possess claspers).  To protect developing embryos, some
of which may develop for up to two years, the longest
gestation of any vertebrate, from ammonia toxicity, the
embryos convert ammonia into urea until they are
capable of living independently.

The comparatively naked eggs of telecosts, on the other
hand, dispose of ammonia by diffusing it into the 
water.  In livebearing teleosts, ammonia is passed
across fetal membranes into the mother's blood for 
eventual disposal by her gills.  Purely freshwater
cartilaginous fishes, such as the South American 
stingrays, have abandoned urea retention entirely. 
Other fishes such as the African and South American 
lungfishes and the mudskippers have ammonia or urea
conversion systems which operate depending upon 
whether the fish is on land or in water; and in the
case of the mudskippers, depending on the salinity of 
the water they're in.

There are other minor excretory products produced by
both fresh and salt water fishes but I think a 
discussion of these at this time would only be
confusing.  None of these products is toxic in itself
though any accumulation of wastes, of course, will
lead to a lowering of pH and a potentially dangerous
problem in a closed system.




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