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Why
protect those gills_
Today's
Watershed Radio program talks about fish that hunker down
behind large rocks to protect themselves from silt that
is suspended in the water after a heavy rainfall. But
why would those fish in the river worry about the silt
rubbing against their gills_ To understand the behavior
of these fish, let's learn some more about these gills
and how fish use them to breathe.
Just
like any other animal, fish need to breathe; they need
to take in oxygen and get rid of carbon dioxide. But how
do fish breathe under water_ First, they take in water
through their mouths and pump it over their gills by opening
and closing their mouths. Most fish have a covering over
their gills called an operculum. When fish open
their mouths, the operculum closes, drawing water in to
the mouth. As fish close their mouths, the operculum opens
and water now crosses over the gills.
For
some fish, such as tuna, the pumping system is less efficient,
so they have to swim constantly to keep water moving over
their gills. By swimming quickly, at least 65 centimeters
(about 2.1 feet) per second, the fish can force enough
water over their gills. In addition, the surface area
of tuna fish's gills is up to 30 times larger than that
of other fish to make up for their reduced pumping efficiency.
Picture:
Tuna fish have to keep swimming to keep oxygen-rich water
flowing over their gills. Picture
courtesy of
NOAA/Danilo
Cedrone 
To
the original photo.
Now
that the fish have taken in the water, they need to take
the oxygen out of the water and into their blood. This
is where the structure of the gills becomes important.
Fish gills themselves are made of fine filaments attached
to a flexible skeletal arch. The filaments contain rows
of thin, disc-shaped membranes called lamellae, which
in turn contain capillaries. Blood circulates through
these capillaries and it is the hemoglobin in the blood
where oxygen and carbon dioxide are exchanged. The oxygen
in the water passes the gills and gets into the blood
where it attaches to the hemoglobin. The carbon dioxide
that was attached to the hemoglobin leaves the blood,
passes the gills, and enters the water. Now that oxygen
and carbon dioxide have been exchanged, the freshly oxygenated
blood travels on to the fish's circulatory system and
the deoxygenated water passes out the operculum.
This
extremely efficient process removes up to 85% of the available
oxygen in the water. Because water contains less available
oxygen than air at sea level, it is very important for
gills to be as efficient as possible, maximizing a limited
resource. Some fish, such as trout, require more oxygen
than other species. This is why they prefer northern streams
with cooler waters, since colder water holds more oxygen
than warmer water.
In
polluted waters gills become less efficient as the lamellae
fuse together and reduce the amount of oxygen and carbon
dioxide that can be exchanged. Similar things happen to
mammals when their lungs are exposed to pollution. The
alveoli in the lungs, which exchange gases, fuse together,
resulting in the condition known as emphysema.
So
if gills are so great, why don't they work on land_ There
are two main reasons. First, not only are gills efficient
in exchanging gases like oxygen and carbon dioxide, they
are also efficient in exchanging water. This isn't a problem
if the animal already lives in the water, but animals
on land would lose too much internal moisture through
their gills. Second, the extremely fine structure of gills
makes them very fragile. A water environment keeps the
gills buoyant, but on land these delicate appendages would
collapse under their own weight. A terrestrial animal
with gills would suffocate even in the rich oxygen atmosphere.
References
and further reading
Information
in the Watershed Radio program comes from the Susquehanna
River Guide.
MI-NET:
Fish Facts
Learn more about fish with
MI-NET: Marine Institute Networked Education Topics
from the Fisheries and Marine Institute of Memorial University
of Newfoundland. Their section
on different
organs of the fish
has images of gills from healthy fish and fish that have
suffered from pollution.
Several
other Watershed Radio programs feature fish.
