RE: Do fish drink water?
Area: Zoology
Posted By: Joseph Agro, SME from AT and T
Date: Tue Nov 19 17:59:17 1996
Message:
Web research seems to indicate that most fish do drink plenty of water,
some continuously. Some fish absorb water through their skin and/or gills,
and may excrete water that way, too. Apparently, fish need to maintain a
fairly high concentration of salt, so how a fish deals with this need
depends on whether it is a saltwater or freshwater fish. Most saltwater
fish get enough or too much salt, and so preventing water loss is their
concern. Freshwater fish are short on salt, and since water dilutes sodium,
they have many mechanisms for excreting water. It is interesting to note
that kidneys process water for excretion, so some saltwater fish have
dysfunctional kidneys or are missing kidneys to help prevent water loss.
The four "water/salt" strategies are listed below:
------
Admin Note: The problem with fish vs. watery environment has do with
the process of osmosis, defined as 'net movement of water
molecules through a semipermeable membrane from a dilute solution to a more
concentrated solution. The 'membrane' is the 'skin' or any other part of the
fish that separates it from the water. If the concentration of salts and other
solutes in the fish is greater than the outside, watery world, the fish
takes on water (natural attempt to 'dilute the fish to what it's
like outside' - bad for the fish). In scientific terms the fish is hypertonic to its
surroundings (freshwater fish, e.g.). If a fish is hypotonic to
its environment (marine fish), then the seawater has a higher solute
concentration than the fish; the fish loses water to its surroundings
(also bad for the fish). Kidneys function to overcome the effects of osmosis
so fishX can stay either more or less concentrated than its environment.
------
1. Agnathans - hagfishes
There are basically four different strategies of regulation of
internal water and total solute concentrations used by
fish. This depends, in part, on the environment in which they live.
The first osmoregulatory strategy (control of the excretion of
'osmotic' compounds such as sodium, chloride, urea..) is that used
by the hagfishes (Agnatha,
Myxiniformes) which are slimy eel-like animals that are found only in
deep- water marine habitats. Actually they have a very simple method,
their body fluids have basically the same total salt concentration as
sea water and they are the only vertebrates with this characteristic
(Moyle and Cech, 1982). In other words they are isotonic (equal
concentrations) with their environment and there is no osmotic
gradient by which fluids or salts can be lost.
2. Elasmobranchs - sharks, skates and rays
The second strategy is that employed by marine elasmobranchs (sharks,
skates and rays). Although their body fluid has a lower concentration
(hypoosmotic) of salt than sea water (about 1/3 of sea water) they
have developed a strategy to overcome this. Instead of passing urea
(which is mostly composed of organic salts) out of their bodies, it
is put into their blood stream, effectively raising the concentration
to that of sea water! Even with this method, they must still
eliminate excess sodium (Na+) and chloride (Cl-) ions (Moyle and
Cech, 1982). This is performed by a special gland known as the rectal
gland, which concentrates Na+ and Cl- ions into a solution that is
passed out of the body (Gordon, 1977). The coelacanth also uses this
mechanism.
3. Teleosts - freshwater fish
Freshwater fish (teleosts) have the exact opposite problem, their
body fluids (1/3 the concentration of sea water) have a greater
concentration than their surrounding environment (hyperosmotic). As a
result they are constantly taking on water by diffusion through their
skin and, to a much larger extent, through the thin membranes of
their gills. Therefore, to maintain the high concentration of their
body fluids, they must continuously excrete the excess water they
have absorbed. This is accomplished by highly efficient kidneys which
produce a very dilute urine (Moyle and Cech, 1982). The only problem
with such a high rate of urine production is that a loss of salts
and other solutes is unavoidable. Salts, mostly Na+ and Cl-, are
also lost by diffusion through gill membranes. Some of these can be
replaced by ions contained in food but by far the most common method
is through the movement of a substance against an osmotic gradient
through the use of energy. This usually involves the exchange of one
substance for another. In the case of freshwater fish, Na+ ions are
taken from the water and ammonia ions are taken from the fish and
they are exchanged. This effectively rids the fish of ammonia.
Chloride ions are exchanged for carbonate ions which helps in
maintaining the pH of the body fluids.
4. Marine teleosts
Marine fish (teleosts) have the exact opposite problem to that
encountered by freshwater teleosts. Their body fluids are, again, 1/3
of that of sea water but this time they are in sea water so their
body fluids are hypoosmotic to their environment. As a result they
will tend to lose water by osmosis to the environment through their
skin but mostly through their gills. Consequently, they have
developed mechanisms and behaviour to compensate for this water loss.
Firstly, the kidneys of marine teleosts are modified in such a way
that very little water is extracted from the blood, some species even
lack certain kidney structures and can't eliminate water (Gordon,
1977; Moyle and Cech, 1982). This results in a reduction in the loss
of water by the production of urine. However, water is still being
lost by the gills and this cannot be stopped, so the only method left
is to somehow replace the water as quickly as it is lost. Marine
teleosts accomplish this by actually drinking water, the most
reliable drinking rates reported in the literature range from 3-10
ml/(kg hr) (Gordon, 1977). However, drinking water by itself cannot
solve the problem, a complex series of events must first occur in the
digestive tract. These events are not yet well understood but it is
known that most of the water is absorbed as are the monovalent ions
Na+ and Cl- (they are, after all, drinking salt water!), while the
divalent ions (such as magnesium and sulfates) are excreted by the
kidneys (Gordon, 1977). Sodium (Na+) and chloride (Cl-) also move by
diffusion into the body through the gills. Therefore, Na+ and Cl-
ions will accumulate in the body of the fish and must be eliminated,
this is accomplished by special cells in the gills called chloride
cells, which me these ions out of the body by active transport (Moyle
and Cech, 1982; Gordon, 1977).
(Thanks to E. Lund - Rummler-Brache Corp. - Investigator - For providing
this info)
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