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International Journal of Marine Science 2014, Vol.4, No.52, 1-9
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6
MN/1000 cells in mercury (0.034 mg. L
-1
) followed
by copper (0.16 mg. L
-1
), lead (0.96 mg. L
-1
),
cadmium (14.864 mg. L
-1
) and zinc (16.444 mg. L
-1
).
Significant increase (
P
< 0.05) in BN frequency was
observed in the mussels treated with metals when
compared to control groups.
3 Discussion
The coastal and marine water ecosystems are
ecologically and economically important for fishing
and recreational activities. However, the domestic
sewage and industrial wastewaters, agricultural runoff
etc., into coastal marine ecosystems decrease the
marine water quality consequently affecting the
marine biota. Mostly the marine fish and invertebrates
have been used for studying the toxic, mutagenic, and
carcinogenic potential of pollutants as model systems
(Braunbeck et al.,
2005) because of their ability to
metabolize, concentrate and store water-borne
pollutants (Palanikumar et al.,
2012a).
In the present study, it is concluded that mortality of
test animals were positively correlated with increasing
concentration of metal as well as increasing exposure
period. As may be expected, the greater the metal
concentration, the greater is the mortality. Similarly,
reports on the mortality of marine molluscs and clam
were observed in relation to heavy metal toxicity
(Ramakritinan et al., 2012; Prato et al., 2006).
Molluscs
M. philippinarum
exposed to Cu at 48 h and
72 h and Zn at 72 h;
C. cingulata
exposed to Cd at 48
h and 96 h found partial mortalities (Ramakritinan et
al.,
2012) i.e., the lower metal concentration found to
have higher mortality (USEPA, 2002).
This may be
due to individual sensitivity of animal and health
status, while, no partial mortalities was noticed in this
study.
Primary criterion of a toxicity test is the survival
after exposure to contaminated and uncontaminated
(Control) waters (ASTM, 1990). None of the
control animals died, demonstrating that the holding
facilities and handling techniques were acceptable
for conducting such tests, as required in the
standard EPA/COE protocol where mean survival
should be 90 % (ASTM, 1990). In the present study,
100 % survival was noticed in animal exposed in
seawater only medium.
The 96 h LC
50
values for Cu, Cd and Zn for sandy
shore scavenging gastropod
Nassarius festivus
were
0.36, 1.52 and 1.76 mg. L
−1
respectively (Cheung et
al.,
2002). The calculated LC
50
values for marine
mollusc
M. philippinarum
decreased with increasing
exposure period (Figures 1a & b). The 96 h LC
50
values for
M. philippinarum
exposed to Cu, Cd, Pb,
Zn and Hg were 0.019, 0.158, 2.025, 2.823 and 0.007 mg.
L
-1
respectively under acute continuous flow-through
bioassay test methods while the LC
50
values for the
same species exposed to Cu, Cd, Pb, Zn and Hg under
acute static renewal bioassay tests were 0.023, 0.221,
2.876, 2.337 and 0.007 mg. L
-1
, respectively
(Ramakrtitinan et al.,
2012). This concludes that
M.
philippinarum
exposed to Cu, Cd and Pb found to be
higher the LC
50
values under acute static renewal
bioassay test method but it found to be less in animals
exposed to zinc under acute continuous flow-through
bioassay test method. However, no variation in LC
50
value of
M. philippinarum
exposed to Hg was noticed
both under acute static and continuous flow through
bioassay systems i.e., 0.007 mg. L
-1.
Present study
indicated that mercury was highly toxic to
M.
philippinarum
when exposed under continuous
flow-through bioassay test method while zinc found to
be least toxic metal.
Based on the LC
50
values, the order of increasing
toxicity of metals to
M. philippinarum
was Hg > Cu >
Cd > Pb> Zn. Whereas
M. edulis
embryo exposed to
mercury and copper found to be equally toxic (Martin
et al., 1981)
to and the rank of order of toxicity was
Hg and Cu > Ag > Zn > Pb > Ni > Cd > Ar > Cr > Se.
The LC
50
values of the present study may be useful in
deriving marine water quality standards in the coastal
waters of Gulf of Mannar. When compared with the
LC
50
values for copper, cadmium, lead, zinc and
mercury from other marine bivalve molluscs
(Sommanee, 1980; Martin et al.,
1981; Prato et al.,
2006). The present study shows variation in lethal
concentrations and different physico-chemical
characteristics of seawater.
Generally marine bivalves have been used as indicator
organisms of marine pollution (Bolognesi and Hayashi,
2011). The development of corrective action plan is
mainly dependent on the criterion or safe
concentration of metals that is protective to snails,