Dreissena polymorpha: Copper and its Effects on the Zebra Mussel
Zebra mussels, Dreissena polymorpha, are a highly invasive species of bivalves which have invaded many aquatic environments. Originally introduced in the U.S. through the great lakes region, this mussel has been increasing its range and continues to be found in new environments. Despite its small size, these organisms have a high reproductive rate and capability to filter feed large amounts, making zebra mussels very invasive to many aquatic environment. Because of their detrimental effect on marine environments, there is a great need to understand and develop different ways in which zebra mussel populations can be controlled, and how certain chemicals affect them. Although there have been a plethora of studies regarding the control and effects of chemicals on Dreissenid mussels, the particular chemical described in this paper is copper and its affect on Dreissenid mussels.
Copper can be seen to be prevalent in many metals, particularly in antifouling paints and coatings used on ships (Clayton et al., 2000). Because of its toxicity to marine life, mollusks in particular, copper can have a diverse effect on marine organisms such as decreased growth rate, reproductive impairment, enzyme inhibition, reductions or alterations in protein synthesis, and disruptions of ATP synthesis and Ca2+ homeostasis (Clayton et al. 2000). Because this chemical can have such a toxic effect on marine organisms, it is important to understand how this chemical affects the natural processes of mollusks if used as a control agent, or when non-target species are affected.
Respiratory
One effect which copper has on Dreissenid mussels and other mollusks is the impairment of respiratory pigments. Many mollusks and some arthropods contain respiratory pigments called hemocyanin. Hemocyanin is a protein which uses two copper atoms to bind to an oxygen molecule for respiration purposes (Sherwood 2005). When elevated levels of copper exist in the environment, impairment of this system occurs (Spicer & Weber 1991).
Heat Shock Proteins
As copper levels in an aquatic environment begin to increase, organisms in that environment begin to become stressed. As the stress level rises in Dreissenid mussels, the transcription of certain stress related proteins such as heat shock proteins (Hsp) become more prevalent (Clayton et al. 2000). Clayton et al. (2000) studied the effect copper had on the concentrations of Hsp 70 and 60 in zebra mussels. Through their study, they discovered that as copper levels were increased in the system, Hsp 70 concentrations continued to increase while Hsp 60 increased, but then fell back down to its initial concentration. This could be attributed to many factors such as the organism conserving energy, or simply the prevention of its use due to high amounts of copper.
Copper Toxicity and Temperature
Another interesting relationship which copper has on zebra mussels is its overall effect and toxicity at varying temperatures. In a study conducted by Rao & Khan (2000), the overall toxicity of copper was found to increase as environmental temperatures increased. Although it is difficult to pinpoint the exact cause for this, there a few explanations. One possible explanation could be due to an increased sensitivity of aerobic metabolism because of the presence of copper. This relationship could simply be due to the fact that at higher temperatures, the bioavailability of copper is higher which can increase its affect on cellular processes of organisms. Another possibility could be from a change in respiratory and or circulatory rates to support the changes in the cellular metabolism of the organism (Rao & Khan 2000).
Metallothioneins
Metallothioneins are low molecular weight proteins which are highly conserved in organisms and are used to bind high quantities of metals via cysteine thiolate linkages (Lafontaine et al. 2000). In a study conducted by High et al. (1997), metallotheioneins were found to strongly correlate with levels of copper, suggesting that these proteins play a role in the organisms stress response to elevated levels of copper in their environment.
Conclusion
As mentioned earlier, the affect of copper on zebra mussels is important for understanding its potential use as a control chemical. Copper is a very popular chemical in marine paints due to its ability to hinder the attachment capabilities of organisms such as mussels. If the use of copper were to increase as a mode of control for quagga mussels, it is imparative that its overall presence in the environment and the effect it has on other marine life is well studied. It is also important that rising temeperatures due to global warming are also taken into consideration. As seen through Roa and Khan (2000), as temperatures increase so does the toxicity level of copper to zebra mussels. This can have a large impact on Dreissenid mussels, as well as other species in the environment.
Yet another long term implication of the affect copper has on these mussels is how it effects them when other chemicals or stressors are present. Copper increases stress levels of quagga mussels as well as disrupts the mussel's respiratory pathway, but the presence of other chemicals may reduce and or enhance this.
On the brighter side, these highly invasive speices can pose as beneficial tool to monitoring pollutants. Because these organisms have distinct reactions from Cu in their environment, we can meausre the concentrations of Hsp's or Metallothioneinsin their tissue to guage the amount of copper and other heavy metals present in the environment and use of Dreissenid mussels as biomarkers. However, in order to use Hsp and Metallothioneins as biomarkers in Dreissenid mussels, we must also have a firm understanding of what else effects these molecular pathways.
Clayton, M.E., Steinmann, R., Fent, K. (2000) Different expression patterns of heat shock proteins hsp 60 and hap 70 in zebra mussels (Dreissena polymorpha) exposed to copper and tributyltin. Aquatic Toxicology, 47, 213-226.
High, K.A., Barthet, V.J., McLaren, J.W., Blais, J.S. (1997) Characterization of metallothionein-like proteins from zebra mussels (Dreissena polymorpha). Environmental Toxicology and Chemistry, 16, 6, 1111-1118.
Lafontaine, Y., Gagne, F., Blaise, C., Costan, G., Gagnon, P., Chan, H.M. (2000) Biomarkers in zebra mussels (Dreissena polymorpha) for the assessment and monitoring of water quality of the St. Lawrence River (Canada). Aquatic Toxicology, 50, 51-71.
Rao, P.D.G.V., Khan, M.A.Q. (2000) Zebra mussels: enhancement of copper toxicity by high temperature and its relationship and metabolism. Water Environment Research, 72, 2, 175-178.
Sherwood, L., Klandork, H., Yancey, P.H. Animal physiology: from genes to oranisms. California: Brooks/Cole, 2005.
Spicer, J.L., and Weber, R.E. (1991) Respiratory Impairment in Crustaceans and Molluscs Due to Exposure to Heavy Metals. Comp. Bio chem. Physiol, 100C, 339.
This page was developed as part of the course at the University of Washington: Integrative Environmental Physiology