Branchial arches or gill arches are a series of paired bone/cartilaginous "loops" behind the throat (pharyngeal cavity) of fish, which support the . As , all vertebrate develop , though the eventual fate of these arches varies between taxon. In all jawed fish (gnathostomes), the first arch pair (mandibular arches) develops into the jaw, the second gill arches (the hyoid arches) develop into the complex (which supports the back of the jaw and the front of the gill series), and the remaining posterior arches (simply called branchial arches) support the gills. In , a mostly terrestrial clade evolved from lobe-finned fish, many pharyngeal arch elements are lost, including the gill arches. In and , only the oral jaws and a hyoid bone remains, and in and the hyoid is simplified further to support the tongue and floor of the mouth. In mammals, the first and second pharyngeal arches also give rise to the .
Most vertebrates are aquatic animal and breathe with , where water comes in contact for gas exchange dissolved oxygen before flowing out through a series of openings () to the outside. Each gill is supported by a cartilaginous or bony gill arch,[Romer, A.S. (1949): The Vertebrate Body. W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)]
In amphibians and some primitive bony fish, the bear external gills branching out from the gill arches.[Clack, J. A. (2002): Gaining ground: the origin and evolution of tetrapods. Indiana University Press, Bloomington, Indiana. 369 pp]
Function
The branchial system is typically used for respiration and/or feeding. Many fish have modified posterior gill arches into
, often equipped with specialized
pharyngeal teeth for handling particular prey items (long, sharp teeth in carnivorous moray eels compared to broad, crushing teeth in durophagous black carp). In amphibians and reptiles, the hyoid arch is modified for similar reasons. It is often used in
buccal pumping and often plays a role in tongue protrusion for prey capture. In species with highly specialized ballistic tongue movements such as
Chameleon or some
Plethodontidae Salamander, the hyoid system is highly modified for this purpose, while it is often hypertrophied in species which use
suction feeding. Species such as snakes and monitor lizards, whose tongue has evolved into a purely sensory organ, often have very reduced hyoid systems.
Components
The primitive arrangement is 7 (possibly 8) arches, each consisting of the same series of paired (left and right) elements. order from dorsal-most (highest) to ventral-most (lowest), these elements are the pharyngobranchial, epibranchial, ceratobranchial, hypobranchial, and basibranchial. The pharyngobranchials may articulate with the
neurocranium, while the left and right basibranchials connect to each other (often fusing into a single bone). When part of the hyoid arch, the names of the bones are altered by replacing "-branchial" with "-hyal", thus "ceratobranchial" becomes "ceratohyal".
-
The Basihyals and Basibranchials lie at the midline of the lower edge of the throat. Almost all modern Chondrichthyes have a single midline basihyal, as do many , lungfish, and Tetrapodomorpha. In , the basihyal is modified into a structure known as the hyoid bone, which provides muscle attachment for the tongue, pharynx, and larynx. Basibranchials, which are most common in Osteichthyes, have the form of one or more rod-like bones projecting backwards along the midline of the throat.
-
The Ceratohyals and Ceratobranchials lie above their respective basi- components, slanting backwards and upwards. They are often the largest bony components of the gill system, as well as the most essential and abundant components. Small connecting bones known as Hypophyals or Hypobranchials may link the basi- and cerato- components, and hypobranchials in particular are common among all types of fish. Paired hypophyals are characteristic of living osteichthyans. Living chondrichthyans lack hypohyals, though several extinct forms are known to have had them.
-
The Epihyals and Epibranchials lie above their respective cerato- components, slanting forwards, upwards, and often inwards. Along with the ceratohyals and ceratobranchials, they are also essential components of the gill system, found in every fish. In filter-feeding fish, the epibranchials often host , specialized spines projecting backwards to trap plankton. The epihyal is more commonly known as the hyomandibula, which is homologous to the sound-sensitive stapes (sometimes known as the columnella) of tetrapods.
-
The Pharhyngobranchials are the most dorsal bony elements of the gill system, connecting to the upper extent of the epibranchials. Living chondrichthyans have large pharyngobranchials which lean backwards and upwards. Osteichthyans, on the other hand, have two different types of pharyngobranchials: Suprapharyngobranchials are toothless structures similar to those of chondrichthyans, while Infrapharyngobranchials often possess teeth and lean inwards and forwards, forming the roof of the throat. A hyoid equivalent of the pharyngobranchial, the Pharyngohyal, is only found in living Holocephali, also known as Chimaera.
Amniotes
Amniotes do not have
gills. The gill arches form as
during embryogenesis, and lay the basis of essential structures such as
, the
thyroid gland, the
larynx, the
columella (corresponding to the
stapes in
) and in mammals, the
Ossicles.
Studies on
also show that the
shoulder girdle also originated from gill arches.
[Brazeau et al, Fossil evidence for a pharyngeal origin of the vertebrate pectoral girdle, Nature volume 623, pages550–554 (2023)]
External links
