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Gills: Definition & Anatomy

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Instructor: Elizabeth Friedl

Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.

Gills are specialized organs that allow aquatic organisms to obtain oxygen from the water. Learn the definition and anatomy of gills; discover their types, shapes, and sizes; and read about the animals that have them. Updated: 07/05/2022

Definition of Gills

Ever tried to hold your breath underwater? You can probably do it for only a short amount of time. So how do some organisms, like fish, live underwater? They have a special organ that allows them to breathe underwater called gills. Aquatic organisms are very much like humans in that they need to breathe in oxygen and breathe out carbon dioxide. However, these aquatic organisms are able to get oxygen from the water, while we are only able to get oxygen from the air.

Gills are filaments on both sides of an animal's neck, directly behind the mouth. As the animal swims, oxygenated water, as in water with lots of oxygen, is forced over the gills. At the same time, deoxygenated blood, as in blood with little or no oxygen, flows through the gills. Because there's more oxygen in the water than in the blood, the oxygen diffuses through the gills into the animal. Carbon dioxide being carried by the animal blood also leaves through the gills during this exchange.

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Gill Anatomy & Types

Depending on the species, gills come in different sizes and shapes. Gills in bony fish look similar to a car radiator. They're made of three parts: the filaments, the arches, and the rakers. The filaments are where the exchange of oxygen and carbon dioxide actually takes place. The arches provide structural support for the gills, and finally, the rakers are appendages that run along the inside edge of the arches, preventing food from passing through the gills.

Most bony fish also have an operculum, a hard flap that covers the gills that allows the water pressure to be adjusted in the gills so the fish can breathe without forward movement.

Cartilaginous aquatic animals (sharks, skates, and rays) have gill slits, which open directly to the outside of the body. Each slit is separated by a cartilaginous gill arch. For this type of gill, the animal has to swim constantly in order to push water over the gills. This type of breathing is called ram ventilation and is used by many shark species. Some species of sharks that aren't active swimmers can switch between ram breathing and other methods of moving water over the gills or use other methods completely.

Some animals, like crabs, need to extract oxygen from water to breathe, but can also survive out of water for periods of time. To do this, a crab needs to keep its gills moist. This allows oxygen from the air to diffuse into the gills, allowing the crab to breathe.

Other fish, such as lamprey and hagfish, have gill pouches, which open to the outside through circular pores. Lamprey have seven gill pouches on each side of their head, while hagfish have anywhere from six to 14.

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