METALS - Marine Pollution: What Everyone Needs to Know - page 89

to determine changes in metabolic rates. Reduced respiration
can be a response to reduced food intake as a way of conserving
resources and energy. Metals (Cu, Pb, Zn, and Cd) also reduce
the photosynthesis by the symbiotic algal cells (zooxanthellae)
that live within coral animals and which are responsible for much
of the nutrition of these animals. Exposure to metals, especially
copper, can impair osmoregulation, the ability to maintain inter-
nal salt concentrations. Maintaining a constant internal chemical
environment is particularly important in animals living in fluctu-
ating environments such as estuaries. Some animals, called osmo-
regulators, maintain their body fluids at concentrations different
from the surrounding water and must actively regulate salts. In
the salty ocean, they drink seawater to offset water loss due to
osmosis and then excrete the excess salt from the gills. This has
an energy cost. If the animal moves to a lower salinity part of an
estuary, it finds itself in an opposite environment—one where it
will absorb water through osmosis, and where it must work to
retain its salts and excrete this excess water. Exposure to contami-
nants can disrupt osmoregulation, which is performed primarily
by enzymes in the gills.
Exposure to contaminants can alter reproduction. One par-
ticular concern is that very low levels of some environmental
chemicals can interfere with the endocrine system, which is
known as endocrine disruption. Metals and other contami-
nants can also directly affect gamete production, mating, and
fertilization. These various stages of the reproductive process
are clearly connected to one another. However, since most
marine organisms normally produce enormous numbers of
embryos, it is not clear how much reproductive impairment it
takes to reduce the population size, which is rare.
One clear example of effects at the population level is that
of TBT (tributyltin) on dog whelk snails. This chemical was
formerly a very popular and effective component of antifoul-
ing paints used on boats. The first indication that there might
be a problem with this effective antifoulant was observations
in France by Alzieu and colleagues that oysters living near