Spiracles () are openings on the surface of some

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

s, which usually lead to

respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...

The spiracle is a small hole behind each eye that opens to the mouth in some

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% ...

. In the jawless fish, the first

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...

opening immediately behind the mouth is essentially similar to the other gill opening. With the

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

of the jaw in the early jawed vertebrates, this gill slit was caught between the forward gill-rod (now functioning as the jaw) and the next rod, the hyomandibular bone, supporting the jaw hinge and anchoring the jaw to the

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, t ...

proper. The gill opening was closed off from below, the remaining opening was small and hole-like, and is termed a spiracle. In many species of sharks and all rays the spiracle is responsible for the intake of water into the buccal space before being expelled from the gills. The spiracle is often located towards the top of the animal allowing breathing even while the animal is mostly buried under sediments. As sharks adapted a faster moving lifestyle some became obligate ram ventilators, breathing exclusively by forcing water through their gills by swimming, among these are requiem sharks and

hammerhead shark The hammerhead sharks are a group of sharks that form the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a cephalofoil. Most hammerhe ...

s which have lost their spiracles In elasmobranchs (sharks and rays) the spiracle bears a small

pseudobranch The pseudobranch, also pseudobranchia is the reduced first gill arch of a fish (on the inner surface of the opercle, near the junction of the preopercle) as well as a reduced "false" gill in some gastropods. In teleost fish, the pseudobranchs are ...

that resembles a gill in structure, but only receives blood already oxygenated by the true gills. The function of the pseudobranch is unknown, but it is believed that it supplies highly oxygenated blood to the optic choroid and retina and may have baroreceptor (pressure) and thermoregulation functions. It may also be a site of oxygen chemoreception. Chimaeras lack spiracles, using gill opercula for buccal pumping instead. Bony fish have similar gill opercula but the basalmost ray-finned fish bichirs use their spiracles for inhaling air into their lungs, this leads to speculation this may be the original air breathing mechanism ancestral to all bony fish and

tetrapod Tetrapods (; ) are four-limb (anatomy), limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant taxon, extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (p ...

s. Coelacanths have closed off spiracles which may be a product of their deepwater lifestyle and loss of air breathing lungs. Bichirs as a whole may more closely resemble the common ancestor of lobe-finned fish and bony fish as a whole than coelacanths due to their deepwater adaptations. Acipenseriformes including sturgeons and paddlefish have small seemingly vestigial spiracles much like coelacanths further reduced in

Holostei Holostei is a group of ray-finned bony fish. It is divided into two major clades, the Halecomorphi, represented by a single living species, the bowfin ('' Amia calva''), as well as the Ginglymodi, the sole living representatives being the gars ...

and completely absent in Teleostei, the clade containing 96% of all extant species of fish. In tetrapods the spiracle seems to have developed first into the otic notch of early tetrapods where it was still used in respiration and incapable of sensing sound, and then into the ear of modern tetrapods which by the Eustachian tube remains connected to the buccal cavity. The spiracle is still found in all cartilaginous fish except requiem sharks,

s, and chimaeras, and is found in some primitive bony fishes ( coelacanth, sturgeon, paddlefish and bichirs). It is also seen as an otic notch in the skull of the extinct labyrinthodonts, and is thought to be associated with the ear opening in amniotes and

frog A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" '' Triadobatrachus'' is ...

s. Romer, A.S. (1949): ''The Vertebrate Body.'' W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970) Blowholes in cetaceans are also sometimes referred to as spiracles, but they are not homologous with the spiracles of fish, having instead developed from the trachea. In cetaceans and other mammals, the organs homologous with the spiracles of fish are the ears.

References

{{reflist Respiratory system Vertebrate anatomy