Excretory System of Salt Water Fish

Excretion vs. Elimination

• In scientific jargon, "elimination" is pooping: the removal of solid waste comprising undigested foods and digestive byproducts. Fish do that much like humans. Meanwhile, "excretion" refers specifically to removing the waste products of metabolism. Peeing is one way of doing that, and saltwater fish also excrete waste through their gills and skin.

Nitrogenous Waste

• When a fish eats protein, part of it goes unused (like the peel of an orange) and becomes waste. As Dave McShaffrey, Professor of Biology at Marietta College in Ohio explains on the college website, "when proteins are converted to carbohydrates to provide energy, the amino group is removed and must be dealt with." In saltwater fish, this nitrogenous (i.e., nitrogen-rich) waste is usually converted to either ammonia or urea---one of the main excretory products of saltwater fish. Ammonia is easier to produce, but urea is less toxic, requires less water and gets rid of twice as much nitrogen. The word "urine" is related to "urea."

The Kidneys

• Fish have kidneys, just like we do, and, like us, the kidney is the core of a fish's excretory system. All vertebrates, including fish, have two kidneys. The kidneys' three primary functions are to maintain bodily fluid levels, collect and excrete waste, and maintain pH.

The Excretory Role of the Kidneys

• Ironically, despite living in the ocean surrounded by water, saltwater fish must conserve water. This is because most saltwater fish have lower salt concentrations in their bodies than does the seawater itself, which causes the fish to lose water through osmosis. Their kidneys stabilize this trend by concentrating bodily salts and excreting them in nitrogenous urine, keeping salt levels balanced, which promotes homeostasis. ("Homeostasis" refers to an organism's internal chemistry being properly balanced.)

The Gills

• Saltwater fish have gills, which have evolved to excrete much of the excess salt from the seawater they drink. Saltwater fish have to drink a lot of seawater in the first place because of the water they lose through osmosis.

  Their gills also play an important role in excreting nitrogenous waste---mainly in the form of ammonia. Ammonia is as toxic to fish as it is to humans, but is flushed away in the seawater, where it is converted into other chemical forms through the nitrogen cycle. Humans who keep fish in aquariums must regularly clean and filter the aquarium water.