Spiracles on Sharks and Rays

Small Breathing Openings Just Behind a Fish's Eyes

Stingray Spiracles Image / Jennifer Kennedy
Cownose ray, showing spiracles located just behind eyes. Jennifer Kennedy, Licensed to About.com

Spiracles are breathing openings found on the surface of some animals, including insects and cartilaginous fish such as sharks and rays.

Spiracles on Sharks and Rays

Spiracles are breathing openings found in some sharks and all rays. They are a pair of openings just behind the fish's eyes that allow it to draw oxygenated water in from above, without having to bring it in through the gills. The spiracles open into the mouth and the water is passed over the gills for gas exchange and out of the body.

The spiracles aid the fish in breathing even when it is lying on the ocean bottom or buried in the sand. Spiracles are absent in the requiem shark, hammerhead shark, and chimaeras.

Examples of Spiracles in Sharks and Rays

Southern stingrays are a sand-dwelling species that uses its spiracles to breathe when it is lying on the ocean bottom. Spiracles behind the ray's eyes draw in water, which is passed over the gills and expelled from its gills on the ray's underside.

Skates and rays may use the spiracles as their primary way of breathing, bringing oxygenated water into the gill chamber where oxygen is exchanged for carbon dioxide.

Angelsharks are large, flat-bodied sharks that bury themselves in the sand and breathe through their spiracles. They lie in wait, camouflaged, for fish, crustaceans, and mollusks and then lunge to strike and kill them with their jaws. By pumping water in through the spiracles and out through the gills, they are able to absorb oxygen from it and eliminate carbon dioxide without constantly swimming, as more mobile sharks must do.

Evolution of Spiracles in Fish

Spiracles likely evolved from gill openings. In primitive jawless fish, they were simply the first gill opening behind the mouth. As the jaw evolved, this gill opening ended up separated from the rest as the jaw developed out of the structures between it and the other gill openings.

It then remained as a small, hole-like opening in most cartilaginous fish. It is useful for those types of rays who bury themselves in the ocean bottom as it allows them to breathe without the gills being exposed.

Primitive bony fish with spiracles include the sturgeon, paddle fish, bichirs, and coelacanth. It's also thought that spiracles are associated with the hearing organs of frogs and some other amphibians.

Insect Spiracles

Insects also have spiracles, which allow air to move into their tracheal system, which they use to exchange oxygen and carbon dioxide with the air instead of having lungs. Gases are exchanged in moist tracheole cells. Insects open and close their spiracles through muscle contractions.

Other Spiracles

The blowhole of the whale is also sometimes called a spiracle in older texts. Whales use their blowholes to bring in air and dispel carbon dioxide when they surface. Whales have lungs like other mammals rather than gills like fish. They have to breathe air, not water.