Branchial arch

Gill arches supporting the gills in a pike

Branchial arches, or gill arches, are a series of bony "loops" present in fish, which support the gills. As gills are the primitive condition of vertebrates, all vertebrate embryos develop pharyngeal arches, though the eventual fate of these arches varies between taxa. In jawed fish, the first arch (the mandibular arch) develops into the jaws. The second gill arch (the hyoid arch) develops into the hyomandibular complex, which support the back of the jaw and the front of the gill series. The remaining posterior arches (simply called branchial arches) support the gills. In amphibians and reptiles, many elements are lost including the gill arches, resulting in only the oral jaws and a hyoid apparatus remaining. In mammals and birds, the hyoid is simplified further.

All basal vertebrates breathe with gills. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the esophagus to the exterior. Each gill is supported by a cartilaginous or bony gill arch.^[1] Bony fish (osteichthyans) have four pairs of arches, cartilaginous fish (chondrichthyans) have five to seven pairs, and primitive jawless fish ("agnathans") have up to seven. The vertebrate ancestor no doubt had more arches, as some of their chordate relatives have more than 50 pairs of gills.^[2]

In amphibians and some primitive bony fish, the larvae bear external gills, branching off from the gill arches.^[3] These are reduced in adulthood, their function taken over by the gills proper in fish and by lungs in most amphibians. Some neotenic amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution of tetrapods.^[4]

^ Scott, Thomas (1996). Concise encyclopedia biology. Walter de Gruyter. p. 542. ISBN 978-3-11-010661-9.
^ Romer, A.S. (1949): The Vertebrate Body. W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)
^ Szarski, Henryk (1957). "The Origin of the Larva and Metamorphosis in Amphibia". The American Naturalist. Essex Institute. 91 (860): 287. doi:10.1086/281990. JSTOR 2458911. S2CID 85231736.
^ Clack, J. A. (2002): Gaining ground: the origin and evolution of tetrapods. Indiana University Press, Bloomington, Indiana. 369 pp

[1] Scott, Thomas (1996). Concise encyclopedia biology. Walter de Gruyter. p. 542. ISBN 978-3-11-010661-9.

[Romer-2] Romer, A.S. (1949): The Vertebrate Body. W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)

[3] Szarski, Henryk (1957). "The Origin of the Larva and Metamorphosis in Amphibia". The American Naturalist. Essex Institute. 91 (860): 287. doi:10.1086/281990. JSTOR 2458911. S2CID 85231736.

[Gaining_ground-4] Clack, J. A. (2002): Gaining ground: the origin and evolution of tetrapods. Indiana University Press, Bloomington, Indiana. 369 pp

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