M .H.S. Santos et al. J. Exp. Mar. Biol. Ecol. 247 2000 233 –242
239
PL oxygen consumption was significantly reduced by all copper and zinc con-
17
centrations tested, except 41 ppb of zinc. For both metals, the effect was not dose dependent, and the mean reduction registered was of approximately 32. As observed
above for feeding experiments, the copper–zinc mixture did not significantly modify the PL
oxygen consumption, despite the fact that a reduction of 34.7 was observed in
17
the higher concentration tested Table 3. It is important to note that in the control group, no significant correlation could be established between oxygen consumption rate
and PL wet weight P .0.05; r 50.15; N 58.
17
4. Discussion
Acute exposure to copper and zinc reduced food consumption by Farfantepenaeus paulensis PL . Wong et al. 1993 also observed a reduction in the rate of Artemia sp.
17
nauplii consumption by PL of Metapenaeus ensis, when PL were exposed to copper 2
3
ppm, chromium 8 ppm and nickel 1 ppm. In the crab Cancer irroratus, Johns and Miller 1982, reported that larvae food ingestion was reduced by exposure to copper
10 ppb and cadmium 50 ppb. In the same species, Johns and Pechenik 1980, demonstrated that larvae exposure to crude oil also reduced food ingestion. The authors
suggested that this response was due to narcosis caused by the oil on the sense organs of the larvae, as previously proposed by Blumer et al. 1973 for other marine organisms.
In the case of metals, the physiological basis of pollutant action on the feeding behaviour is not well established. However, it is known that metals inhibit the
chemoreception in aquatic animals Sutterlin, 1974. Further, it was demonstrated that the metal effects on the nervous system could induce impairment of prey capture and
manipulation, as well as, of locomotory activity Bryan et al., 1995. Despite the fact that our results can not discard this hypothesis, they strongly suggest that the inhibition
of food ingestion induced by copper is, at least in part, due to an inhibition of the mechanisms involved in chemoreception. This assumption is based on the fact that the
feeding response induced by L-isoleucine was significantly reduced after acute exposure to copper 212 ppb and tended to be lower after acute exposure to zinc 525 ppb.
In addition to the inhibitory effect of copper and zinc on feeding behaviour, a significant reduction of PL
oxygen consumption was observed. This effect was noticed
17
in all concentrations tested, except in the lowest concentration of zinc 41 ppb. A decrease in metabolic rate induced by heavy metals has been also described for other
crustacean species. Exposure to cooper and zinc, for example, significantly reduced the metabolic rate in the shrimp Caridina rajadhari Chinnaya, 1971. A similar effect was
observed when the shrimp Palaemon serratus was exposed to cadmium and mercury Papathanassiou, 1983 and the mysid Leptomysis lingvura was contaminated with
sub-lethal concentrations of cadmium Gaudy et al., 1991. According to Bryan 1971 and Mehrle and Mayer 1985, alterations in oxygen consumption induced by metals
could be due to their effects on oxydizing enzymes.
The significant reduction of food and oxygen consumption in PL of F
. paulensis,
17
induced by copper and zinc exposure, suggests that the energy available for postlarvae
240 M
.H.S. Santos et al. J. Exp. Mar. Biol. Ecol. 247 2000 233 –242
maintenance and growth under this situation would be reduced. So, if the PL did not reduce the energy requirement to maintain other physiological processes than growth, a
reduction in the growth rate would be expected under this situation. In fact, it was observed that chronic exposure to sub-lethal concentrations of copper and zinc, singly or
in mixture, significantly reduced both the total length and weight wet and dry of F
. paulensis PL . Inhibition of growth after copper and zinc exposure has also been
17
described for other crustaceans species Saliba and Krzyz, 1976; McKeenney and Neff, 1979; White and Rainbow, 1982.
In the present study, acute exposure of F . paulensis PL
to mixtures of sub-lethal and
17
equipotent concentrations of copper and zinc did not change the feeding behaviour and the aerobic metabolism. Thus, our results, suggest an antagonism between these metals
under the experimental conditions employed here. Antagonism between metals has also been widely described in aquatic toxicology experiments using several groups of living
organisms. For example, nickel toxicity to growth of filamentous bacteria could be reduced by addition of zinc to culture medium Shuttleworth and Unz, 1991.
Antagonism between cadmium and zinc was observed in metal accumulation in the macroalga Enteromorpha prolifera Haritonidis et al., 1994. In the marine prosobranch
Monodonta turbinata, a mixture of copper and chromium caused less pronounced effects on the survival, oxygen consumption and bioaccumulation than copper and chromium
acting alone Catsiki et al., 1993. Alonso and MartinMateo 1996 have established a competition between zinc and copper for the production of metallothionein in the oyster
Ostrea edulis. Moulder 1980 observed a reduction of the lethal toxicity induced by inorganic mercury in Gammarus duebeni when in presence of sub-lethal concentrations
of copper. In fathead minnows Pimephales promelas, Parrott and Sprague 1993 also observed antagonism for some combinations of toxicants. In the other hand, according to
Klaassen and Eaton 1991 the most common result of exposure to a metal mixture is an additive effect. In fact, reciprocal effect of metals additive, synergism and antagonism
depends on the metal ion speciation and the comparative concentrations of each metal Parrott and Sprague, 1993; Wang et al., 1995. For example, in the marine algae
Phaeosactylum antagonism occurs if Cd Cu is in the range of 0.4 to 4, but synergism appeared beyond this concentration range. Zinc and copper presented antagonism when
the ratio of Zn Cu was between 1 and 20, synergism appeared out of this range Wang et al., 1995. So, the metal interaction observed in the present study is consistent with
the data presented above, since antagonism between zinc and copper was observed when Zn Cu52.5 was tested on feeding behaviour and oxygen consumption of F
. paulensis PL . The physiological basis of this antagonism cannot be explained from our results.
17
However, it seems that this interaction is consistent with the physicochemical similarity between zinc and copper, which implies in similar potential effects on metal ion
speciation, competitive interactions affecting membrane transport, or competitive interactions at sites of toxic lesions Taylor et al., 1992; Sandstead, 1995.
5. Conclusion
