Impact of the dams in rivers
back around 480 million years or so
– in other words, from the middle
Ordovician Period – were the first
fishlike creatures. Although they
were very different from their modern-day equivalents, they nevertheless form the basal stock from
which not just fish but all other vertebrates subsequently evolved. The
earliest fish were jawless, a feature
still retained by some of today’s
aquatic vertebrates such as the hagfish and lampreys. However, unlike
hagfish and lampreys, many of the
early species had bony body plates
on their bodies, and many of the
later types were not eel-like in appearance. Fish with true jaws
(known as acanthodians) did not
evolve for another 40-50 million
years.
Characteristics of Fish:
Although there are exceptions, it is
possible to list the characteristics
that, together, allow us to recognize an animal as a fish. They are
listed below:
A braincase and limb (fin) skeleton consisting of cartilage or
bone
Fins, usually – but not invariably
– with spines or rays
Breathing through outward-
Solving the problems
directed gills covered by an
operculum (gill cover), which
results in an external slit like aperture or a series of gill slits
Bodies usually, but not always,
covered in scales
A swim bladder used in buoyancy control (there are exceptions,
as in sharks)
A sensory organ known as the
lateral line running in a head-totail direction or another series of
sensory pits (again, as in sharks)
Cold-blooded (poikilothermic) –
in other words, the body temperature matches that of the
environment (again, there are
some significant exceptions,
such as tuna, which can raise
their body temperature well
above that of the surrounding
water)
Aquatic Ecosystems:
Aquatic environments are divided
into freshwater and marine ecosystems, both of which have diverse
habitats. Freshwater ecosystems are
classified according to water flow.
Lotic ecosystems include brooks,
streams, and rivers in which water
is flowing. Attached aquatic plants
and algae provide some of the primary production of streams. How-
Steelhead migration
ever, most of the primary production for lotic ecosystems comes
from surrounding terrestrial habitats as runoff during r