Part I
Over the years saltwater and freshwater fish have evolved differently due to the effects of the environment that they live. This difference has been evident in the weight difference seen by the whale shark and beluga sturgeon.
Water buoyancy has played a crucial role in this difference. The buoyancy effect caused by water density is higher in salt water than in fresh water. It means that fish living in salty water would require more weight to adapt to the environments than those living in fresh water. Salt-water fish have therefore developed more muscles to be able to conquer the dense seas. For instance, the giant basking shark depends on its huge liver to achieve neutral buoyancy.
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Part II
Oil does not mix with water. This is an immiscible property, which serves to protect living organisms in water. When oil and soap are mixed with water, an emulsion is formed. This is because water molecules are highly attracted to each other making it impossible to combine with these components. Likewise, oil molecules are also attracted to their molecules.
Detergents are organic compounds with polar and non-polar features. Detergents such as soap have toxic effects on freshwater biological systems. Detergents reduce the surface tension of the water thus making it easier for organic components to be absorbed by fish by a higher amount. Soaps are also harmful to the mucus layer of the fish which protects against bacteria and parasites. Phosphates found in detergents can cause algal blooms which remove toxins and thus depleting oxygen levels.
In general, detergents and oil, have adverse effects on the composition of aquatic life. They are harmful to the oxygen levels in water.
References
Lewis, M. A. (1992). The effects of mixtures and other environmental modifying factors on the toxicities of surfactants to freshwater and marine life. Water Research , 26 (8), 1013-1023.
Liu, Z. F., Gao, X. Q., Yu, J. X., Qian, X. M., Xue, G. P., Zhang, Q. Y., ... & Hong, L. (2017). Effects of different salinities on growth performance, survival, digestive enzyme activity, immune response, and muscle fatty acid composition in juvenile American shad (Alosa sapidissima). Fish physiology and biochemistry , 43 (3), 761-773.
