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Soil & Water Conservation Society of Metro Halifax (SWCSMH)
Over half of all vertebrates are fishes. The
most diverse and successful vertebrate group, they provided the
evolutionary base for invasion of land by amphibians. In many ways
amphibians, the first terrestrial vertebrates, can be viewed as
transitional-fish out of water.
The story of vertebrate evolution
started in the ancient seas of the Cambrian Period (590-505 million
years ago), when the first backboned animals appeared. 1
Almost all the fishes that live today (21, 000 species) are bony fishes.
Figure 1: The major fish groups. 1
From whale sharks that are 18 metres long to
tiny cichlids no larger than one's fingernail, fishes vary considerably
in size, shape, colour, and appearance. Some live in freezing Arctic
seas, others in warm freshwater lakes, and still others spend a lot of
time out of water entirely. However varied, all fishes have important
characteristics in common.
- Gills: Fish are
water-dwelling creatures and must extract oxygen for their metabolism
from oxygen gas dissolved in the water around them. They do this by
swallowing a great deal of water. The water passes over fine filaments
of tissue rich in blood vessels, called gills, in the back of the
mouth, and then back out of the body through slits in the side of the
throat. The blood moves opposite the flow of water.
All fishes have an internal skeleton with a backbone surrounding the
spinal cord, although it may not necessarily be made of bone. The brain
is fully encased within a protective box, the skull or cranium, made of
bone or cartilage.
- Single-loop blood circulation:
Blood is pumped from the heart to the gills. From the gills the
oxygenated blood passes to the rest of the body, then returns to the
heart. The heart is a muscular tube-pump made of four chambers that
contract in sequence.
- Nutritional deficiencies:
Fishes are unable to synthesize the aromatic amino acids and must
consume then in their diet. This inability has been inherited by all
their vertebrate descendants.
Normal lakes that have minimal levels of nutrients are said to be enriched, or oligotrophic. An oligotrophic lake has clear water and supports small populations of aquatic organisms. Eutrophication is the enrichment of water by nutrients; a lake that is enriched is said to be eutrophic. The water in a eutrophic lake is cloudy and usually resembles pea soup because of the presence of of vast numbers of algae and cyanobacteria that are supported by the nutrients.
Although eutrophic lakes contain
large populations of aquatic animals, different kinds of organisms
predominate there than in oligotrophic lakes (cf. Figure 2 below).
In an unenriched lake there is
a higher concentration of dissolved oxygen. In eutrophic lakes, on the
other hand, the deeper, colder levels of water are depleted of
dissolved oxygen because of the high BOD caused by decomposition on the
lake floor. Therefore, the natural fish population may be replaced by
warm-water fishes, such as catfish and carp, that can tolerate lesser
amounts of dissolved oxygen (cf. Figure 2 below).
Over vast periods of time,
oligotrophic lakes and slow-moving streams and rivers become eutrophic
naturally. As natural eutrophication occurs, these bodies of water are
slowly enriched and grow shallower from the immense number of dead
organisms that have settled in the sediments over a long period.
Gradually, plants such as water lilies and cattails take root in the
nutrient-rich sediments and begin to fill the shallow waters, forming a
Eutrophication can be
markedly accelerated by human activities, and it results from the
enrichment of water by inorganic plant and algal nutrients- most
commonly in sewage and fertilizer runoff.
Figure 2: A comparison of the features of an oligotrophic lake and a eutrophic lake 2
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