Environmental Engineering Reference
In-Depth Information
periods. Common responses include delayed development,
formation of abnormalities, and reduced hatching. Some
chemicals can produce effects that do not become apparent
until later stages such as larvae or even adults. Most marine
animals hatch out as small planktonic larvae with little resem-
blance to the adult form that they will eventually become.
Larvae may be even more sensitive than embryonic stages of
the same organism, since embryos are protected by an outer
membrane that may reduce contaminant uptake and which
is no longer present after hatching. Larvae also must usually
swim and obtain food for themselves. Most benthic inver-
tebrates have planktonic larvae, which at a certain stage of
development must settle to the bottom to metamorphose into
a juvenile in an appropriate habitat. Larval exposures to con-
taminants can lead to impaired settlement or to delayed physi-
ological problems as juveniles or adults.
Developmental processes in later life can also be impaired
by exposure to contaminants such as metals. Growth is an
obvious and easily measured response that is frequently
traced back to reduced food intake, but even without reduced
feeding it may occur because organisms must expend energy
to defend themselves against contaminants. The more energy
needed to detoxify pollutants, the less will be available for
growth. In addition to overall body growth, molting, regen-
eration, development of calcified structures (shell and bone),
carcinogenesis (cancer), and smoltification (defined below)
are other developmental processes that take place after larval
stages and that are all sensitive to environmental contaminants
such as metals. Salmon breed and embryos hatch upstream
in shallow freshwater streams where they live for some time
before migrating down to the sea. They undergo a develop-
mental process called smoltification, which enables them to
osmoregulate and live in salt water. This is hormonally con-
trolled (by thyroid hormone) and can be impaired by a vari-
ety of contaminants. Aluminum (Al) is a normal constituent
of soil and is generally not an environmental problem because
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