Excretory System of Saltwater Fish

Excretion means getting rid of the waste products of metabolism---or all of the chemical reactions that happen in a living body. Saltwater fish have similar excretory systems to other fish and even mammals. In fact, in common with all other vertebrates, the kidney is the primary organ for regulating excretion in fish. However, saltwater fish face an extra excretory challenge: what to do with all that extra salt.

  1. Excreted Substances

    • In saltwater fish the main substances that are excreted include carbon dioxide (CO2) from respiration; nitrogenous waste as a result of converting proteins into carbohydrates for fuel; and salts from the surrounding water. All are excreted in different ways, though some organs in the fish are able to deal with more than one substance.

    Urine and Salt

    • Because of their habitat, saltwater fish take in a lot of salt via their food and by constantly drinking salty water. According to the Division of Animal Resources at the University of Illinois, if saltwater fish didn't excrete this salt they would risk losing much of their body water by osmosis through the skin and gills. Saltwater fish are efficient at removing salt ions from their body, partly by creating small amounts of highly concentrated urine containing very little water.

    Ammonia

    • Whereas in mammals nitrogenous waste is excreted as urea, the chief ingredient in urine, in saltwater fish much nitrogenous waste takes the form of ammonia. Ammonia is more poisonous than urea. However, any ammonia excreted by the fish is quickly diluted by the surrounding seawater. This is why fish kept in tanks are at risk of suffocating unless the water is changed or refreshed.

    Gills

    • Most of the nitrogenous waste excreted by saltwater fish is through the gills---primarily ammonia. The gills also contain cells that allow the release of sodium and chloride--in other words, salt. However, while a fish's gills excrete ammonia, CO2 and salt, the gills also allow the loss of a lot of water. Thus the fish has to intake salt water continuously. To fish, drinking and "breathing" are effectively the same thing, as they obtain their oxygen by drinking in water past the gills.

    The Kidney

    • Like all vertebrates, fish have a pair of kidneys. One of the main functions of the kidney is to regulate excretion and water levels in the body. It acts like a balancing device, making sure that the body has the right ratio of water to salt and other chemicals. The kidneys also produce nitrogenous waste, which in turn is excreted through urea in fish urine.