Choana
Encyclopedia
Choana is the posterior nasal aperture.
The choanae are separated by the vomer
.
and the nasopharynx
.
It is therefore not a structure but a space bounded as follows:
a, and they could as well be called Choanata (they are also the only ones with a vomeronasal organ
, which has an embryonic origin from the olfactory structure).
These internal nasal passages evolved while the vertebrates still lived in water. At this point they already needed to gulp air to get enough oxygen, and rather than open their jaws each time to do this, those mutants who acquired small openings to breathe through were more successful at living in the new environment.
do not have choanae, instead they have two pairs of external nostrils: each with two tubes whose frontal openings lie close to the upper jaw, and the posterior openings further behind near the eyes. Whether choanae of tetrapods are homologous to the posterior nostrils or not has been debated. Reasons for dispute have been that the posterior nostril in its evolution into choanae would have to switch position relative to other anatomical features, i.e. a nerve. Recent paleontologiclal findings support homology: a 400-million-year-old fossil lobe-finned fish called Kenichthys
campbelli has something between a choana and the external nostrils seen on other fish, which makes it look like it has a cleft palate or cleft lip. The reason seems to be that the posterior opening of the external nostrils has migrated into the mouth for some reason.
A similar evolution has taken place in lungfish. Here the inner nostrils have generally been accepted as homologous to the posterior nostrils, but the homology to true choanae as internal nostrils has been a matter of controversy. The fossil lungfish Diabolepis
shows an intermediate stage between posterior and interior nostril and supports the independent origin of internal nostrils in the lungfish.
Tetrapods are also equipped with a lacrimal duct, or tear duct. How it evolved is not known, but it has an internal connection with the choana. It is possible that the choana started as a natural crack between maxilla and premaxilla because of an incomplete fusion in air-breathing animals. If this gap got wider and deeper with time, the frontal part of it would have to fuse together to avoid weakening the upper jaw, creating a small opening on the upper lip. Some more migrating, and this gap would meet the anterior pair of the external nasal openings. The posterior pair of the openings was then free to form the lacrimal duct if a migration caused them to come in contact with the eyes.
Some fossil species are said to have both conventional external nostrils and a choana, but only more fossils will give a real answer to how the choanas evolved.
has internal nostrils too. These seem to have a different origin than those of the tetrapods, and lungfish have no tear duct either.
Hagfish
es have a single internal nostril that opens inside the mouth cavity, while Chimaera
e have open canals that leads water from their external nostrils into their mouth and through their gills.
The choanae are separated by the vomer
Vomer
The vomer is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones.-Biology:...
.
Boundaries
It is the opening between the nasal cavityNasal cavity
The nasal cavity is a large air filled space above and behind the nose in the middle of the face.- Function :The nasal cavity conditions the air to be received by the other areas of the respiratory tract...
and the nasopharynx
Nasopharynx
The nasopharynx is the uppermost part of the pharynx. It extends from the base of the skull to the upper surface of the soft palate; it differs from the oral and laryngeal parts of the pharynx in that its cavity always remains patent .-Lateral:On its lateral wall is the pharyngeal ostium of the...
.
It is therefore not a structure but a space bounded as follows:
- anteriorly and inferiorly by the horizontal plate of palatine bonePalatine boneThe palatine bone is a bone in many species of the animal kingdom, commonly termed the palatum .-Human anatomy:...
, - superiorly and posteriorly by the sphenoid boneSphenoid boneThe sphenoid bone is an unpaired bone situated at the base of the skull in front of the temporal bone and basilar part of the occipital bone.The sphenoid bone is one of the seven bones that articulate to form the orbit...
- laterally by the medial pterygoid plateMedial pterygoid plateThe medial pterygoid plate of the sphenoid is narrower and longer than the lateral pterygoid plate; it curves lateralward at its lower extremity into a hook-like process, the pterygoid hamulus, around which the tendon of the Tensor veli palatini glides.The lateral surface of this plate forms part...
s.
Choanae in different animals
The only animals with choana are the tetrapodTetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
a, and they could as well be called Choanata (they are also the only ones with a vomeronasal organ
Vomeronasal organ
The vomeronasal organ , or Jacobson's organ, is an auxiliary olfactory sense organ that is found in many animals. It was discovered by Frederik Ruysch and later by Ludwig Jacobson in 1813....
, which has an embryonic origin from the olfactory structure).
These internal nasal passages evolved while the vertebrates still lived in water. At this point they already needed to gulp air to get enough oxygen, and rather than open their jaws each time to do this, those mutants who acquired small openings to breathe through were more successful at living in the new environment.
Fish
FishFish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate 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...
do not have choanae, instead they have two pairs of external nostrils: each with two tubes whose frontal openings lie close to the upper jaw, and the posterior openings further behind near the eyes. Whether choanae of tetrapods are homologous to the posterior nostrils or not has been debated. Reasons for dispute have been that the posterior nostril in its evolution into choanae would have to switch position relative to other anatomical features, i.e. a nerve. Recent paleontologiclal findings support homology: a 400-million-year-old fossil lobe-finned fish called Kenichthys
Kenichthys
Kenichthys was an ancient tetrapodomorph, a lineage that included tetrapods. It was one of the most primitive tetrapodomorphs. This specific genus is important because it contains a primitive choana on its upper lip, giving it a cleft palate-like structure...
campbelli has something between a choana and the external nostrils seen on other fish, which makes it look like it has a cleft palate or cleft lip. The reason seems to be that the posterior opening of the external nostrils has migrated into the mouth for some reason.
A similar evolution has taken place in lungfish. Here the inner nostrils have generally been accepted as homologous to the posterior nostrils, but the homology to true choanae as internal nostrils has been a matter of controversy. The fossil lungfish Diabolepis
Diabolepis
Diabolepis is an extinct genus of sarcopterygian fish related to lungfishes which lived in the Early Devonian period of South China...
shows an intermediate stage between posterior and interior nostril and supports the independent origin of internal nostrils in the lungfish.
Tetrapods
Similar migration is still seen in the tetrapod embryo, and can cause a baby to be born with a cleft palate. Why it should migrate is a mystery, since the nostrils would be useless as a breathing device before their final position inside the mouth. They could also already breathe air through their spiracles.Tetrapods are also equipped with a lacrimal duct, or tear duct. How it evolved is not known, but it has an internal connection with the choana. It is possible that the choana started as a natural crack between maxilla and premaxilla because of an incomplete fusion in air-breathing animals. If this gap got wider and deeper with time, the frontal part of it would have to fuse together to avoid weakening the upper jaw, creating a small opening on the upper lip. Some more migrating, and this gap would meet the anterior pair of the external nasal openings. The posterior pair of the openings was then free to form the lacrimal duct if a migration caused them to come in contact with the eyes.
Choanae analogues in other animals and fossils
This would not have been the first time the jaws evolved some sort of opening. For instance, snakes have evolved a cleft in the lower jaw, allowing them to stick out their tongues without having to open the jaw. For an animal living in water, the formation of a paired cleft on the upper jaw would be quite logical. Terrestrial vertebrates would in any case need a way to breathe without needing to open their jaws each time.Some fossil species are said to have both conventional external nostrils and a choana, but only more fossils will give a real answer to how the choanas evolved.
Lungfish and hagfishes
In addition to tetrapods, the lungfishLungfish
Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...
has internal nostrils too. These seem to have a different origin than those of the tetrapods, and lungfish have no tear duct either.
Hagfish
Hagfish
Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...
es have a single internal nostril that opens inside the mouth cavity, while Chimaera
Chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish , spookfish , or rabbitfishes...
e have open canals that leads water from their external nostrils into their mouth and through their gills.