Minnows: Pollution Resilience And Tolerance Explored

Minnows are a common name for a variety of small freshwater fish species. They are known to have a high tolerance for variable water qualities, which has helped them spread across many regions. For instance, the bluntnose minnow, a primary bait fish in North America, can withstand a wide range of water conditions. Similarly, the sheepshead minnow, a species native to estuaries, has demonstrated exceptional resilience to temperature, salinity, and oxygen extremes. However, the impact of pollution on minnow populations is complex and depends on various factors, including the specific pollutants present and the genetics of the minnow species in question. For example, large minnows in the heavily polluted Houston Ship Channel have developed resistance to pollution through genetic selection, while Killifish, also known as mud minnows, have evolved genetic mutations that allow them to survive in severely polluted waters.

Sheepshead minnows are used for toxicity testing

Sheepshead minnows (Cyprinodon variegatus) are widely used for toxicity testing. They are the most widely used vertebrate for estuarine toxicity testing. They are also used as a surrogate species in assessing contaminant risk to two endangered cyprinodontids.

Sheepshead minnows are more tolerant of temperature, salinity, and hypoxic extremes than any known teleost species. They are osmoregulators at ambient salinities between 0 and 70% and can survive salinity > 180 per thousand for extended periods. They are also more tolerant than other fish species of natural and anthropogenic abiotic challenges. Their exceptional tolerance of abiotic challenges is also apparent in their atypical response to toxicant exposure.

The US EPA aquatic toxicity database AQUIRE lists 153 different toxicants for which sheepshead minnows have been tested since about 1970. For more than 50% of these toxicants, sheepshead minnows are the only species tested. For 63% of the toxicants, sheepshead minnows proved to be the most tolerant (i.e., greatest LC50 value). In contrast, they are the most sensitive species for only 20% of the toxicants and are never the most sensitive when at least two other species are tested.

The American Society for Testing Materials (ASTM) early life-stage toxicity test with sheepshead minnows investigates the effects of toxicants on the survival, growth, and development of the fish, from the embryonic stage through hatching and growth to the juvenile stage.

Sheepshead minnows are tolerant of temperature, salinity and hypoxia

Sheepshead minnows (Cyprinodon variegatus), also known as sheepshead pupfish, are native to the eastern coasts of North and Central America, from Cape Cod to the Yucatan peninsula in Mexico. They are found in brackish water in bays, inlets, lagoons, salt marshes, and similar locations with little wave action and sandy or muddy bottoms.

Sheepshead minnows are highly tolerant of temperature, salinity, and hypoxia. They are eurytolerant fish, capable of enduring extreme temperatures and inhabiting a diverse range of environments. They adjust to varying temperatures by regulating the activity of enzymatic antioxidants in their bodies, such as catalase, superoxide dismutase, and glutathione peroxidase. This ability to manage oxidative stress in extreme temperatures contributes to their remarkable temperature tolerance.

In terms of salinity, sheepshead minnows can osmoregulate across a broad range of salinities, from 0 to over 180 per thousand. They are commonly found in environments with varying salinity, including brackish water and hypersaline conditions. This adaptability to different salinity levels showcases their high salinity tolerance.

Sheepshead minnows also possess a notable tolerance to hypoxia, or low oxygen conditions. They have been observed to survive in hypoxic environments with oxygen levels as low as 0.22 mg/l (EC50, equilibrium). This resilience to hypoxia further highlights the sheepshead minnow's ability to thrive in challenging environmental conditions.

The sheepshead minnow's exceptional tolerance to temperature, salinity, and hypoxia makes them well-suited for estuarine environments, which often exhibit fluctuations in these conditions. Their tolerance to these abiotic factors also makes them valuable organisms for toxicity testing and research.

Bluntnose minnows are tolerant of variable water qualities

Bluntnose minnows (Pimephales notatus) are a species of small cyprinid freshwater fish, primarily found in the eastern half of North America. They are very common and may be the most prevalent freshwater fish in the Eastern US. They are also found in Canada, from southwestern Quebec in the east to southeastern Manitoba in the west.

Bluntnose minnows are known for their high tolerance for variable water qualities, which has helped them spread throughout many regions. They can be found in a wide variety of water habitats, including lakes, rivers, ponds, streams, swamps, and springs. They have a preference for shallow, clear water with sandy or gravelly bottoms. Their habitats can range from hard-bottomed pools to headwater bogs.

Bluntnose minnows are small, typically growing to an average length of 5-6.5 cm, with a maximum length of around 11 cm. They have a rounded head, a blunt snout that overhangs the mouth, and a dark lateral line running from the opercle to the base of the tail, ending in a large black spot. Their colouring is pale olive above the lateral line and silvery below it, with silvery-blue scales near the line.

These minnows are omnivores, feeding on aquatic insects, algae, diatoms, aquatic insect larvae, and small crustaceans called entomostracans. They occasionally eat fish eggs or small fish. They spawn from early spring to midsummer, attaching their eggs under stones in depressions they have dug. During mating, males develop darker heads and bluish bodies, along with three rows of tubercles on their heads.

Killifish, or mud minnows, are usually sensitive to pollution

Killifish, also known as mud minnows or mummichog, are usually sensitive to pollution. They are abundant in brackish waterways and marshes along the Atlantic coast of the USA and Canada. Due to their sensitivity to pollution, they are often used as an indicator species to gauge the health of aquatic ecosystems.

However, killifish populations in four severely polluted East Coast estuaries have been found to possess genetic mutations that make them tolerant of lethal doses of industrial pollution. These mutations deactivate or turn off a molecular pathway that would typically cause cellular damage due to chemical exposure, allowing them to survive in polluted environments. The researchers sequenced the entire genomes of these fish and compared them to the genomes of killifish in unpolluted waters nearby each site. They found that the mutations were rare among fish living in clean waters, likely because they do not provide a survival advantage in those environments.

While the killifish's ability to survive in polluted waters is impressive, it may come at a cost. In adapting to their new environment, the pollution-tolerant fish lost a small amount of their genetic diversity, which could make them less able to cope with future environmental changes. This loss of genetic variability could make it more difficult for killifish to adapt to further stressors.

The discovery of pollution-tolerant killifish provides valuable insights into the genetic differences that influence resilience to environmental toxins and has implications for understanding the impact of pollution on both animal and human health.

Gulf killifish from polluted sites are most resistant to pollution

Killifish, sometimes called mud minnows or mummichog, are abundant in the brackish waterways and marshes along the Atlantic coast. They are typically very sensitive to pollution. However, Gulf Killifish from polluted sites have been found to be extremely resistant to pollution.

The Houston Ship Channel, where the Gulf Killifish live, is in close proximity to the northwest coast of the Gulf of Mexico. Decades of industrial activity have contaminated the channel with two kinds of aromatic hydrocarbons ("polycyclic" and "halogenated"), which can cause the hearts of developing animals to become deformed. Yet, Gulf Killifish appear to be resistant to these lethal chemicals.

To understand why, Elias Oziolor, a postdoctoral researcher, identified 12 sites along a pollution gradient, from relatively "clean" sites near Galveston Island to heavily polluted sections of the Houston Ship Channel. Oziolor then used seine nets to catch killifish at these sites, spawned them in a lab, and tested their embryos against various concentrations of toxicity. The results showed that Gulf Killifish from polluted sites in the upper ship channel were 1,000 times more resistant to toxicity than fish from Galveston.

The genetic resistance of the Gulf Killifish is attributed to the introgressive hybridization of their genes with those of the Atlantic Killifish, a process known as adaptive introgression. The Atlantic Killifish, native to the heavily polluted rivers of the East Coast, were likely brought to the Houston Ship Channel by shipping activities. The massive population sizes of the Gulf Killifish further contribute to their genetic diversity, providing a broader range of genetic material for evolution.

The discovery of the Gulf Killifish's resistance to pollution offers valuable insights into understanding how chemical pollutants affect people and animals. Additionally, it provides a framework for predicting the resilience of other species to rapid environmental changes.

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