An opening formed embryonically from the first gill slit and preserved in sharks, rays, sturgeons and others. The opening, located behind the eyes, is used for the passage of water into the oral cavity.

Fish spiracles are a pair of openings just behind the fish's eyes that allow them to take in oxygenated water from above without having to pass it through the gills. The spiracles open in the fish's mouth, where the water passes through the gills for gas exchange and is expelled from the body.

The spiracle is a small opening behind each eye, which in some fishes extends into the mouth. In jawless fish, the first gill opening just behind the mouth is essentially the same as the other gill opening. During the evolution of the jaw in early vertebrates, this gill opening was sandwiched between the anterior gill bar (which now functions as the jaw) and the next bar, the hyoid bone, which supports the jaw joint and attaches the jaw as such to the skull. The gill hole was closed at the bottom, and the remaining opening was small and hole-like and is called a puffer.

In many shark species and all rays, the spiracles are responsible for bringing water into the cheek space before it is expelled from the gills. The spiracle is often located closer to the top of the animal, allowing it to breathe even when it is mostly buried underwater. As sharks have adapted to a faster lifestyle, some have developed forced ventilation of the lungs, breathing solely by forcing water through the gills as they swim, including rakeheads and hammerheads, which have lost their spiracles.

Elasmobranchs (sharks and rays) have a small pseudobranch that resembles a gill but only receives blood that has already been oxygenated by the true gill. The function of the pseudobranch is unknown, but it is thought to supply oxygenated blood to the optic nerve vasculature and retina, and may have baroreceptor (pressure) and thermoregulatory functions. They may also be the site of oxygen chemoreception.

Acipenseriformes, including sturgeons and paddlefish, small, apparently rudimentary spiracles very similar to those of the latacanthropods, even rarer in the holovar and completely absent in the bony fishes, a clade containing 96% of all existing fish species.

In quadrupeds, the spiracle appears to have evolved first into the ear notch of early quadrupeds, where it was still used for breathing and was incapable of perceiving sound, and then into the ear of modern quadrupeds, which remains connected to the cheek cavity by the eustachian tube.

Spiracles are still found in all cartilaginous fishes except rake sharks, hammerhead sharks and chimaeras, and in some primitive bony fishes (latacanthus, sturgeon, paddlefish and bichyrus). It is also considered to be an otic notch on the skull of extinct labyrinthodonts and is thought to be related to the ear hole in amniotes and frogs.

Cetacean blowholes, sometimes called spiracles, are not homologous to fish spiracles, but evolved from the trachea. In cetaceans and other mammals, the organs homologous to fish spiracles are the ears.