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Methylmercury In Zebrafish

Mercury is a naturally occurring heavy metal that can be transported through the atmosphere and transformed into a toxic, organic compound. Anthropogenic and natural determinants can lead to the bioaccumulation of methylmercury in the tissues of aquatic organism, leading to non-lethal affects. This chemical can also biomagnify in the tissue of organisms at higher trophic levels (Zillioux et al. , 1993). Activities caused by humans such as the burning of fossil fuels, coal along with the manufacturing of degradation soil each assist in the accumulation of mercury in the environment.

Hg can travel thousands of miles throughout the atmosphere from its original source, contaminating unaffected bodies of water where it can then be absorbed and ingested by aquatic life. Methylmercury, which is the organic form or mercury, has been found throughout many bodies of water, causing non-lethal effects in organisms as well as humans. This reactive form of mercury can then undergo several transformations in the environment, including methylation by bacteria. Methylation is most prevalent in anoxic sediments having high organic matter contents, such as in wetlands and marshes. source)

The toxicity of mercury increases when methylated, causing detrimental affects. Mercury contamination in adult and juvenile fish has been shown to disrupt vital functions including reproduction, osmoregulation, orientaion, searching for food, predator recognition, and communication (Zillioux et al. , 1993). Juvenile and adult fish can be used as biomarkers to detect methylmercury in aquatic habitats causing reproduction, behavioral, developmental and neurological disruptions. Since the occurrence of Minimata Bay in Japan, evidence has shown that fish are highly sensitive to methylmercury.

Correlation between reproductive effects shown in women during this treacherous disaster and fish seen in contaminated bodies of water indicate that methylmercury is detrimental to humans as well as aquatic organisms. Since fish are completed submerged in water, with it being their primary habitat, methylmercury accumulation is high via water and or food. Even when being excreted, it is then reabsorbed causing a continuous uptake of MeHg. Recent studies have revealed that low concentrations of methylmercury can cause inimical reproductive effects in fathead minnows (Pimephales promelas), decreasing spawning success.

Additional studies on the reproductive endocrinology of fathead minnows have shown that male fish exposed to low levels of methylmercury had a 50% decrease in testosterone and exposed females had an 80% decline in 17? -estradiol(Drevnick & Sandheinrich, 2003). MeHg is known to alter the reproduction of fathead minnows, causing feminization in males. During one experiment, males fed food contaminated with methylmercury had a lower concentration of testosterone than those of the control group. Not only were males affected, females too showed a decrease in estrogen levels concluding that MeHg heavily interferes with reproduction.

Considering that the amounts of these concentrations were giving at low dosages reinforces the idea that MeHg is extremely toxic to aquatic organisms. Vitellogenin is a protein found in female fish that accounts for most of the content in egg yolk. Vitellogenin is a major female yolk-sac precursor glycolipoprotein produced in the liver of teleost fish and oviparous animals (Danilchik & Gerhart, 1987). Methylmercury contamination formulates the expression of vitellogenin in male fish. When vitellogenin is formed within males studies have shown that it alters the transcription of genes. Vitellogenin is divided into four different components.

When studying the effects of methylmercury in relation to gene transcription, zebrafish are proven to be the most sufficient test subject. These fish are extremely accommodating when constructing research on developmental effects, due to their high rate of fertility. Not only are they cost efficient but they also easy to cultivate in a laboratory. In this current study, two groups of zebrafish were fed different environmentally relevant concentrations of methylmercury for a total of 60 days to determine if similar changes in gene transcription could be observed in zebrafish as previously observed in fathead minnows (Klaper et al. 2006).

When comparing the control group to the group contaminated with methylmercury, the protein structure was damaged. At which the concentration that was given to the zebrafish only a small dose was required to show a significant effect. In addition, because each region of the brain has different functions, different sensitivities, and different constitutive gene expression, studies of whole brain will capture only changes that occur in large regions of the brain, or that occur in multiple regions (source). Not only can methlymercury affect the genes of fish but it can also have alter the behavior.

Different studies have shown that the embryos of fish developed behavioral changes once being exposed to this toxic. Behavioral development occurs in association with the development of the nervous system and developing fishes are generally more sensitive to environmental contaminants than adults. (Weis JS, Candelmo). Occasionally, the effects are short lived, disappearing over time. Two different experiments were done by Weis (2010) to study the behaviors of diverse species from a contaminated and reference site located in New Jersey.

During one experiment, mummichogs Fundulus heteroclitus were examined for predator and prey interactions. Piles creek is located near a chemical factory, releasing organic pollutants such as Mehg contaminating aquatic life. When Weis (2010) compared Piles Creek to the reference site, the results were as expected. Piles Creek, which had higher levels of contamination, clearly presented increased affects from methylmercury in mummichogs. The sediments from Piles creek that were fed to fish from an uncontaminated region reduced activity levels.

Reduced activity levels lead to a shorten life span because of the inability to catch a prey or escape a predator. If individuals with higher concentrations of mercury are easier for predators to capture, this will facilitate the transfer of the contaminant to higher trophic levels (Weis, 2009). In another experiment Weis (2010) evaluated mummichog embryos being exposed to small concentrations of methylmercury. Prey capture ability of early larvae was impaired when they first began to feed, but after about one week after hatching the prey capture ability was comparable to controls, showing that this effect was temporary (source).

Not only were there delayed effects in foraging but methylmercury exposure also made them more vulnerable to predation. The lateral line system is found in fish and it is essential because it detects motion. Methylmercury interferes with the lateral line system, reducing its ability to detect prey. However, effects from methylmercury on the embryos were shown to be reversible. Aside to behavioral occurrences in embryos, an experiment conducted by (Samson and Shenker, 2000) revealed other tertatogenic defects.

According to Weis and Weis (1977) methylmercury contamination can cause abnormalities in the tail flexures of mummichogs. Exposure of D. Rerio embryos to sublethal concentrations of methylmercury consistently caused two types of morphological defects (Samson and Shenker, 2000). The first seen defect appeared in the fin fold of the tail. Actinotrichia are spicules, which are important to fin development in that it develops first. Collagen is the main protein which actinotrichia is composed. When embryos become contaminated, the tissues of the tail are affected.

In normally developing embryos, the median finfold is a clear, thin membrane around the entire trunk region containing actinotrichia, or unsegmented collagenous fin rays (Samson and Shenker, 2000). The second defect due to exposure of methylmercury caused tail flexures. According to Samson (2000) embryos with this defect also exhibited aberrations in the finfold. These abnormalities altered the tissues, which led to developmental deficiencies in these organisms. Severe abnormalities such as a stunted tail or flex of the tip of the tail occurred after being exposed to 20 g of methylmercury.

The nervous system is essential in fish, assisting in communication and detection of objects. Gymnotus sylvius is a weak electric fish that emits low-voltage electric pulses at a discharge rate of 20-70 Hz (Moraes, 2012). Electric organ discharges (EODs) in the weak electric fish Gymnotus sylvius are produced by the electric organ and modulated by the CNS. Moraes (2012) examined uptake of methylmercury by injecting earthworms and feeding them to weak electric fish G sylvius. The fish were then exposed to different concentrations of MeHG for 18 days and tested using a oscilloscope.

Moraes (2010) collected signals to determine the electric organ discharges. Results showed that the higher the dosage of MeHg, the higher the EOD rate. Although the MEHg concentration was not quantified in G. sylvius tissues, it likely reached the brain of the fish, leading to alterations in the neural control of the EOD rate (Moraes, 2012). The fine-tuning of the EOD rate is crucial for electrical communication among Gymnotus species, which determines reproductive and non-reproductive interactions and affects social and ecological aspects. (Gouvea Junior, Stopa, Paula & Hoshino, 2002).

When considering the affects of methylmercury on aquatic organisms, the outcome can result in non-lethal aberrations. MeHg is known to enter the environment through chemical spills or by simply burning coal. Not only is MeHg dangerous for organisms but humans as well. Many people are unaware of the detrimental occurrences that take place in our own backyard. Many experiments have been done using zebrafish and many other fish as bioassays to study the affects of this toxin. Methylmercury has been proven to cause behavioral, neurological, developmental and reproductive complications.

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