Fishes' Eye Lenses Indicate Exposure to Harmful Methylmercury Threatening Animal Reproduction and Health

Mercury contamination is a major concern for human health worldwide, particularly for fetuses and young children. Methylmercury, produced when mercury ends up in rivers and lakes, can damage children's brain development and cause problems such as speech problems and muscle weakness in adults who primarily eat seafood. Methylmercury also risks the health and reproduction of fish and other wildlife.

People, animals, and birds can all be exposed to methylmercury by consuming fish and shellfish. Researchers have been working for a long time to figure out how and when fish take in mercury. This information is important for evaluating the risk of mercury contamination in different water bodies and environments and evaluating policy changes to reduce mercury emissions.

Scientists have used fish ear stones, called otoliths, for decades to learn about fish growth, migration, diet, and the timing of their exposure to certain pollutants. Otoliths are small calcium carbonate structures about the size of a pea found in the inner ear of fish, which help with hearing and balance. They can also provide insights into how climate change is affecting fish.

Contamination of Methylmercury

However, some pollutants, including mercury, are not incorporated into otoliths. Instead, they bind very tightly to tissues that contain sulfur, such as muscle tissue. That's why muscle tissue has been used to assess mercury contamination. In a recent study published in Environmental Science & Technology Letters, researchers describe a new way of understanding a fish's lifetime exposure to mercury by measuring it in the fish's eyes. This research is opening up new opportunities for understanding the long-term exposure of fish to this toxic substance.

Scientists study mercury uptake in fish by measuring the amount that accumulates in the whole fish or the filets (i.e., muscle tissue). This method tells them how much mercury the fish has accumulated over its lifetime, but it doesn't give them specific information about when the fish was exposed; there is no timestamp.

Mercury levels can vary greatly within a single species of fish. For instance, from 1991 to 2010, monitors from the US government found that mercury levels in cod averaged 0.111 parts per million but could be as high as 0.989 parts per million, a difference of nine times. This suggests that in addition to changes in mercury emissions over time, a particular fish's movements and diet can significantly influence its exposure.

In the study, scientists suggest a new method that combines measurements of otolith aging and mercury in the lenses of a fish's eyes to determine the ages of fish based on mercury concentrations in their eyes. Fish eye lenses are made of pure protein and high in sulfur content, so they readily take mercury directly from the water or the fish's diet. Methylmercury, including eye lenses, seems to be preferentially taken up in certain organs. In high quantities, it may taint fish vision.

Analyzing Fish Lense Otolith

The approach begins with the well-established method of aging a fish using its otolith. As a fish cultivates and ages, its otoliths count annual calcium carbonate coatings. Researchers can estimate the fish's age and growth rates by measuring the distance between the yearly growth layers, called annuli, similar to how foresters date trees by counting the growth rings in their trunks. The team also knows that a fish's eye grows at a rate proportional to its otolith's growth.

So in their analysis, scientists apply the proportional distance found in the fish's otolith to its eye lens. For the specific species they studied, the round goby (Neogobius melanostomus), there is a strong linear relationship between these two measurements. As the eye lens grows and accumulates mercury, they can pinpoint when the fish was exposed using this correlation with the otolith. And because the fish's eye lens grows in layers throughout its life, it can track the chronology of its lifetime exposure.

With this new method, the team begins to trace the chronology of a fish's lifetime exposure to mercury. Scientists can also investigate how life history events, such as migration and diet changes, or temporal events, like low dissolved oxygen levels in the water at certain times of the year, may affect a fish's mercury levels. The advantage of this method is that it provides information for individual fish, which is important just like it is for humans. Different individual fish have different abilities to catch prey and handle or tolerate stress, all of which can impact their growth and exposure to mercury.

Hypoxic Waters

And having the data about mercury vulnerability for all ages of a single fish can assist in lessening the necessity to accumulate large pieces of many fish at all generations, which is the conventional form scientists have examined how a fish's orientation alters over its lifetime. This new method may also help them to understand the impact of climate change on mercury exposure. As water temperatures rise, rivers, lakes, estuaries, and oceans are losing some of their dissolved oxygen, a process called deoxygenation, which is a major stressor for aquatic life.

When the oxygen in a pond or bay falls below 2 milligrams per liter, compared to normal levels of 5 to 8 milligrams, that body of water is considered hypoxic, and hypoxic conditions can be linked to higher concentrations of methylmercury. This loss of oxygen can be exacerbated by nutrient pollution, such as from agricultural or urban runoff, but it can also occur in the open ocean far from continents due to warming. Increasing hypoxia could undermine recent global efforts to reduce mercury emissions by making the mercury already in lakes and oceans more readily taken up by fish.

However, depending on the individual and species, fish can react differently to hypoxia. The current research, funded by the National Science Foundation, examines how fish eye lenses, combined with otoliths, can help them understand the impact of mercury exposure versus diet and hypoxia. Scientists increasingly realize that different parts of an organism's body can function as records of the past. For the team, eye lenses and otoliths serve as important tools for understanding the hidden lives of individual fish.

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