Fish Gills: Pollution's Impact And Entry Points

Fish gills are a vital organ for survival, facilitating the intake of oxygen and expulsion of carbon dioxide. Gills are also the site of gas exchange, ionic regulation, acid-base balance, and nitrogenous waste excretion. Fish absorb water through their mouths, which then passes over their gills, allowing the blood in the capillaries to pick up oxygen. However, the presence of pollutants in the water can have detrimental effects on fish gills. Water pollution can cause inflammatory processes in the distal region of the gill filaments, with epithelial hyperplasia, increased mucus production, and even gill necrosis. Various environmental pollutants, such as heavy metals, acid rain, and organic xenobiotics, have been found to affect the morphology of the gill epithelium, leading to alterations in blood ionic levels and gill enzyme activity. These disturbances can impact the health and survival of fish populations.

How does pollution affect the morphology of the gill epithelium?

The morphology of the gill epithelium is affected by pollution in several ways. Firstly, fish from polluted waters showed a chronic inflammatory process in the distal region of the gill filaments, with epithelial hyperplasia. The raker length was increased, and hypersecretion occurred, resulting in a considerable volume of alcian blue-positive mucin on the epithelium of the rakers.

The secondary epithelium of the gill, which covers the free part of the secondary lamellae, may also be affected by pollution. This epithelium has an exclusive relationship with the arterioarterial vasculature, which is actively controlled by innervated sphincters. While the secondary epithelium usually does not exhibit obvious differences between freshwater and saltwater fish, it can become modified when the absorptive functions of freshwater chloride cells are intensified.

Additionally, the primary epithelium of the gill, which surrounds the primary lamellae, contains chloride cells that vary in morphology and number depending on the environment in which the fish lives. Pollution can impact the morphology and number of these chloride cells, particularly in seawater or seawater-adapted fish, where an accessory cell is often found beside the chloride cell.

Overall, the effects of pollution on the morphology of the gill epithelium can vary depending on the species and the specific pollutants present in the water.

What are the effects of heavy metals on fish gills?

Fish gills are very sensitive to heavy metals, which can enter the fish's body through the gills and cause damage to the gills and other organs. Heavy metals can affect the oxygen-carrying capacity of the blood by influencing the levels of haemoglobin and haematocrit in different ways, depending on the type of metal, the species, and the duration of exposure.

Effects of Specific Heavy Metals on Fish Gills and Haemoglobin and Haematocrit Levels

Lead

Lead is a highly toxic, non-essential heavy metal that can accumulate in fish gills and cause harm to their physiology, behaviour, and biochemistry, as well as damage to their central nervous system, peripheral nervous system, hematopoietic system, and organs such as the liver and kidneys. Lead exposure can cause a decrease in haemoglobin and haematocrit levels, although some studies have found no significant changes or even an increase in these levels.

Mercury

Mercury is highly toxic to both humans and animals, and it can be present in fish in organic (more toxic) or inorganic forms. Mercury exposure has been found to decrease haemoglobin and haematocrit levels in several fish species, although some studies have shown an increase in these levels at lower concentrations.

Cadmium

Cadmium is a highly toxic heavy metal that can accumulate in fish and cause harm even at very low levels. Cadmium exposure has been found to decrease haemoglobin and haematocrit levels in several fish species, with the degree of decrease depending on the duration of exposure.

Chromium

Chromium, particularly hexavalent chromium, is considered one of the most hazardous heavy metals due to its high toxicity and ability to penetrate biological membranes. Chromium exposure has been found to decrease haemoglobin and haematocrit levels in several fish species, with the degree of decrease depending on the duration and concentration of exposure.

Copper and Zinc

Copper and zinc are essential trace elements for fish, but at high concentrations, they can become toxic. Copper and zinc exposure have been found to increase or decrease haemoglobin and haematocrit levels in different fish species, with the direction of change depending on the duration and concentration of exposure.

Manganese, Nickel, and Cobalt

Manganese is an essential element for fish, but at high concentrations, it can be toxic. Manganese exposure has been found to decrease haemoglobin and haematocrit levels in some fish species. Nickel exposure has also been found to decrease these levels, with nickel toxicity being particularly harmful to the respiratory system. Cobalt is another essential element that can be toxic at high concentrations, and cobalt exposure has been found to increase or decrease haemoglobin levels, depending on the duration of exposure.

Heavy metal exposure can have significant effects on fish gills and the levels of haemoglobin and haematocrit, which are important indicators of fish health. The effects vary depending on the type of metal, the species, and the duration and concentration of exposure. More research is needed to fully understand the impacts of heavy metals on fish gills and haematological parameters, and to develop strategies to mitigate the harmful effects of heavy metal pollution on aquatic ecosystems and human health.

How does water pollution affect the gill apparatus of fish?

Water pollution can have a detrimental impact on the gill apparatus of fish, which are delicate organs that enable them to breathe underwater. Fish absorb oxygen from the water through their gills, which are made up of many tiny blood vessels called capillaries.

A study on the effects of water pollution on the gill apparatus of fish, published in the Journal of Comparative Pathology, found that fish exposed to polluted water showed signs of inflammation and epithelial hyperplasia in the distal region of their gill filaments. The rakers, which are structures on the gills that help filter water, were also affected, with increased length and hypersecretion of mucus. The mucus itself was more rigid, which could potentially impact the fish's ability to breathe effectively.

Water pollution can also decrease the oxygen levels dissolved in the water, creating "dead zones" where fish and other animals cannot survive due to the lack of oxygen. This further exacerbates the problem of gill damage, as fish in these areas may experience even more difficulty breathing.

Additionally, pollutants can be absorbed by fish through their gills. Fish living in contaminated areas can absorb toxins from the water, sediments, and even the food they eat. These toxins can build up in their bodies over time, leading to health problems and even death.

Overall, water pollution poses a significant threat to the gill apparatus of fish, impairing their respiratory function and overall health. It is crucial to address water pollution and reduce toxic chemicals in the environment to protect the delicate gill structures of fish and ensure their survival.

What are the effects of acid rain on fish gills?

Fish gills are very susceptible to environmental pollutants, including heavy metals, acid rain, and organic xenobiotics. Acid rain, in particular, has been found to affect the morphology of the gill epithelium, which is the site of gas exchange, ionic regulation, acid-base balance, and nitrogenous waste excretion in fish.

The negative effects of acid rain on fish gills have been observed in several regions, including Scandinavia, Nova Scotia, Ontario, and New York. The impact of acid rain on fish populations in these areas has been significant, with declines in Atlantic salmon populations in southern Norway and the disappearance of fish populations in over a third of the more than 2,000 lakes in the same region.

The primary response of fish to acid rain appears to be reproductive failure, with several mechanisms reported, including the failure to produce and deposit viable eggs, failure of hatching, and mortality of embryos and alevins. In addition, acute mortalities of adult fish have been observed, particularly following a sudden spring thaw or heavy rain, which may be due to a decline in blood pH or loss of body sodium in response to increased external hydrogen ion concentration.

Furthermore, sublethal effects such as reduced growth and skeletal deformities have also been observed in fish inhabiting acidified lakes. Reduced growth may be an expression of sublethal stress from pH or metal toxicity, while skeletal deformities are believed to be related to a reduction in body calcium levels due to low pH.

Overall, the effects of acid rain on fish gills are far-reaching and have resulted in significant declines and disruptions in fish populations in affected regions.

How does pollution impact the ionic fluxes in fish gills?

Fish gills are the site of gas exchange, ionic regulation, acid-base balance, and nitrogenous waste excretion. The last three processes are controlled by passive and active transport of various solutes across the epithelium.

Various environmental pollutants, such as heavy metals, acid rain, and organic xenobiotics, have been found to affect the morphology of the gill epithelium. Associated with these morphological pathologies, one finds alterations in blood ionic levels, as well as gill Na, K-activated ATPase activity and ionic fluxes.

For example, in a study, specimens of Nile tilapia were collected from a polluted site in the São Paulo metropolitan area (Billings reservoir) and from a “clean” area. Fish from the polluted site showed a chronic inflammatory process in the distal region of the gill filaments, with epithelial hyperplasia.

In another example, the toxic action of nitrite on fish is incompletely understood, but it depends on a number of internal and external factors such as fish species and age, and general water quality. Nitrite ions are taken up into the fish by the chloride cells of the gills. In the blood, nitrites become bound to haemoglobin, reducing the oxygen-carrying capacity of the blood.

In summary, pollution can impact the ionic fluxes in fish gills by altering the morphology of the gill epithelium and affecting blood ionic levels, as well as the activity of enzymes involved in ionic regulation.

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