Gills: Definition & Anatomy

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.

Why can animals like fish and crabs breathe underwater but you can't? Aquatic organisms have a special organ called gills, which allows them to breathe oxygen in the water. Some gills are even adapted to allow animals to breathe on land!


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 - water with lots of oxygen - is forced over the gills. At the same time, deoxygenated blood - blood with little or no oxygen - flows through the gills. Because there is 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.

Anatomy and Types

Depending on the species, gills come in different sizes and shapes.

Gills in bony fish look similar to a car radiator. They are 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. The rakers are appendages that run along the inside edge of the arches, preventing food from passing through the gills.

Fish gill
fish gill anatomy

Most bony fish also have an operculum, a hard flap that covers the gills. The operculum 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 are not active swimmers can switch between ram breathing and other methods of moving water over the gills or use other methods completely.

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