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and stimulation of phagocytosis; (2) absence of cytotoxicity and normal phagocytosis;
(3) low cytotoxicity and dramatic inhibition of phagocytosis; and (4) relatively high cyto-
toxicity accompanied by a drastic inhibition of phagocytosis (Brousseau et al. 2000; Sauvé
et al. 2002). It seems that cadmium and zinc, like organic and inorganic mercury, have the
potential to induce a stimulation of phagocytosis in some bivalve species (Brousseau et al.
2000; Sauvé et al. 2002). Stimulation of phagocytosis at low concentrations of metals has
also been observed in rodents, reptiles, birds, fish, cattles, whales, and primates (Dieter et
al. 1983; Voccia et al. 1994; De Guise et al. 1996b). However, no stimulation of phagocytosis
was observed in the clam
Mya arenaria
exposed
in vivo
to low concentrations of both forms
of mercury (Fournier et al. 2001).
It is noteworthy that the
in vitro
toxicity of CH
3
HgCl and HgCl
2
followed similar patterns
in some species of bivalves (
Cyrtodaria siliqua, Mya truncata, Elliptio complanata, Dreissena
polymorpha
) (Sauvé et al. 2002). Organic mercury is more toxic than the inorganic form for
several wildlife species (Fournier et al. 2000b), including bivalves (Brousseau et al. 2000).
For example, a study with
Mya arenaria
exposed
in vivo
at different concentrations of mer-
cury showed that doses of 10
-6
M of HgCl
2
did not induce any difference in cell viabil-
ity by the 28th day of exposure, whereas CH
3
HgCl was found to be cytotoxic at 10
-6
M
from the 14th day of exposure (Fournier et al. 2001). In addition, phagocytosis was signifi-
cantly decreased after 28 days of exposure to 10
-6
M of HgCl
2
, whereas a suppression of
phagocytosis was induced by methyl mercury at 10
-6
M after 7 days of exposure. Results
clearly show a preponderance of the bioaccumulation of CH
3
HgCl versus HgCl
2
(Watras
and Bloom 1992; Inza et al. 1997). Comparison of
in vitro
toxicity thresholds of organic and
inorganic mercury suggests that some bivalve species may metabolize or absorb mercury
by different processes and therefore do not demonstrate the same response or relative
sensitivity for each form of mercury (Sauvé et al. 2002).
The lipid and protein contents of the digestive gland were measured in clams exposed
in vivo
to different concentrations of organic and inorganic mercury. No differences in the
contents of lipids and proteins were observed for the different treatments except for dead
animals exposed to higher concentrations (Fournier et al. 2001). However, phagocytosis
was decreased. This reinforced the fact that phagocytosis is a sensitive biomarker in the
context of
in vivo
exposures to different mercury compounds.
6.2.4 Oxidative Burst and Lysozymes
Production of oxygen derivatives (mainly superoxide radicals (ROS)) by hemocytes plays
an important role in killing foreign agents in invertebrates (Adema et al. 1991). Exposures
of invertebrates to high concentrations of trace metals inhibit their production (Anderson
et al. 1992). However, at concentrations measured in the environment there would be no
effect on this parameter (Anderson et al. 1992; Coles et al. 1995). It is interesting to note that
exposures to high concentrations of fluoranthene stimulate the production of superoxide
radicals in the blue mussel
Mytilus edulis
(Coles et al. 1994). These opposite results may be
due to a specific interaction between the contaminant involved (either metal or organic
compound) and the target such as loss of membrane permeability and the production
of oxygen derivatives or defense mechanisms against oxidative stress induced (Pipe and
Coles 1995).
Several contaminant chemicals affect the release and activity of enzymes such as lyso-
zyme. Results vary depending on the type of enzyme and the level of exposure considered.
Toxic stress induced after exposure can result in many cases in an increased release and
activity of certain enzymes and/or reduce the activities of others (Pickwell and Steinert
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