Respiration of the Bony Fish
Ardan Huck July 7, 2002

I keep mentioning bony fish as there are different classifications of fish and variances in the anatomy and physiology among them. Discus are members of the bony fish class (Osteichthyes).

Respiration is the process of getting oxygen into the blood and to the cells and getting the waste (carbon dioxide) out of the body. In the bony fish this is accomplished through the use of gills. Gill plates were the precursor of the jaw of the fish.

In water, "oxygen is 30 times more dilute than it is in air" (Dando and Burchet, 1996).

A fish uses muscles of the pharynx (part of the throat) to fill the mouth with water while at the same time closing the external gill slit and then squeezing the water through the gill membranes while opening the slit and closing the mouth. The gill membranes are made up of small filaments with small feathery structures called "lamellae." They are in two rows on each gill arch. Most fish have 4 gill arches in a gill chamber. The gill arches are made up of bones to protect and support the gills. This is where the exchange of oxygen and carbon dioxide take place as it is where many blood vessels containing red blood cells are.

The red blood cells have hemoglobin containing iron which is highly reactive to oxygen. Hemoglobin will absorb less oxygen at lower pH. As fish swim they can open their mouths and use some of the flowing water to move the oxygen-containing water through the gills. If the fish is stationary it must use only its pharynx to do this. The outside of the gills have a bony cover which helps to protect the gills. Within the gill filaments, blood flows in the opposite direction of the incoming water. This helps with the oxygenation. A fish can extract up to 80% of the oxygen contained in the water. If we reverse the flow of water, the efficiency can drop to as low as 9% (humans only extract 10 to 20% of the oxygen from the air we breathe, this is possibly due to air being less dense than water, and that fish are more efficient at getting oxygen into the blood).

In the gills there is a high oxygen content and some parasites take advantage of this.

Ammonia is also removed from the blood and excreted into the water from the gills.

Water also enters the fishes circulatory system through the gills by osmosis. Some minerals also enter the fish’s body through the gills. Fish have evolved through the ages from having more gill filaments to having less.

  • "Handbook of Tropical Aquarium Fishes" by Dr. Herbert R. Axelrod and Dr. Leonard P. Schultz, 1990
  • "World Book Encyclopedia" by World Book Inc., 1990
  • "The Complete FishKeeper" by Joseph S. Levine, 1991
  • "SeaLife A complete Guide to the Marine Environment" by Marc Dando and Michael Burchett, 1990

Internet Resources: (there are some good explanatory pictures in these links)

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