The Weberian apparatus is an anatomical structure that connects the
swim bladder to the
auditory system in
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% of ...
es belonging to the superorder
Ostariophysi. When it is fully developed in adult fish, the elements of the apparatus are sometimes collectively referred to as the Weberian ossicles. The presence of the structure is one of the most important and phylogenetically significant distinguishing characteristics of the Ostariophysi. The structure itself consists of a set of minute bones that originate from the first few
vertebrae
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...
to develop in an embryonic ostariophysan. These bones grow to physically connect the auditory system, specifically the
inner ear, to the swim bladder.
The structure acts as an amplifier of sound waves that would otherwise be only slightly perceivable by the inner ear structure alone.
Structural anatomy and function
The generalized structure of the Weberian apparatus is akin to a skeletal complex of bones and ossicles that are physically connected to the
labyrinth
In Greek mythology, the Labyrinth (, ) was an elaborate, confusing structure designed and built by the legendary artificer Daedalus for King Minos of Crete at Knossos. Its function was to hold the Minotaur, the monster eventually killed by t ...
auditory complex anteriorly and the anteriormost region of the swim bladder posteriorly. The entire structure is derived from skeletal elements of the first four
vertebrae
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...
. The involved elements include: the supraneural bones of the skull; modified
neural arch
The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
bones, specifically the paired
claustra and the
scaphia; the intercalarium and the lateral processes; the tripus; the os suspensorium from the fourth vertebra; the
parapophysis of vertebra number five including the vertebra itself, plus the vertebra's corresponding pleural rib. In addition, a structure composed of fused
neural spine
The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
s form the dorsal most part of the Weberian apparatus. Together, the structure interacts anteriorly with the
lagenar otolith set within the skull and posteriorly with the swim bladder via the pleural rib. Postero-ventrally, it is the tripus, the os suspensorium and the third rib that interact directly with the anterior chamber of the
swim bladder.
The Weberian apparatus functions by transmitting auditory signals straight from the
gas bladder, through the Weberian ossicles and then straight into the
labyrinth
In Greek mythology, the Labyrinth (, ) was an elaborate, confusing structure designed and built by the legendary artificer Daedalus for King Minos of Crete at Knossos. Its function was to hold the Minotaur, the monster eventually killed by t ...
structures of the
inner ear. The structure essentially acts as an amplifier of sound waves that would otherwise be only slightly perceivable by the inner ear structure alone. With the added function of the swim bladder as a
Resonance chamber, signals are amplified to noticeable levels.
Embryology
Embryonic analysis of Weberian apparatuses of the taxon ''
Brycon'' has shed some light on the
development
Development or developing may refer to:
Arts
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*Development (music), the process thematic material is reshaped
* Photograph ...
of the structure itself. The Weberian apparatus elements form from the fully distinguishable first five vertebrae of the individual. The supraneural starts as an element of the skull. The claustra and the scaphia develop from expanded elements of the neural arch of the first vertebra (V1). From the second vertebra (V2), the intercalarum and the vertebra's lateral process are reduced and clump together. The plural rib (R1) of the third vertebra (V3) shrinks and moves somewhat ventrally, forming the tripus from a vertebral parapophysis fusing with the pleural rib. The
os suspensorium bone of the fourth vertebra (V4) somewhat retains its shape, developing from the pleural rib of the vertebra (R2). The remaining elements of the fifth vertebra (V5), the parapophysis and the articulating rib (R3), including the vertebra itself form the posterior structure of the Weberian apparatus. The
neural spine
The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
s of the first four vertebrae fuse and compress, forming one of the major structures of the apparatus.
Study of the embryology of the Weberian apparatus has since been conducted on various other ostariophysan species, the outcomes of which have resulted in various interpretations of the development (and thus the
homology) of the structures that form the structure. Specific studies have been done on the Weberian apparatuses of a few select taxa, including ''
Danio rerio
The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family (Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ca ...
'',
''
Rhaphiodon vulpinus''
and ''
Corydoras paleatus''.
Evolutionary history
The earliest recorded incidence of a Weberian apparatus is from the fossil fish ''
Santanichthys diasii'' dating from the
Early Cretaceous of Northeastern
Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
. In the aforementioned taxon, the Weberian apparatus is fairly developed; there is a distinguishable intercalarium and a tripus which articulate with the second and third vertebrae respectively. A scaphium can be seen in at least two specimens. The neural arch of the third vertebra has already broadened, almost similar to that of modern ostariophysans. The claustrum, an element in modern apparati, is noticeably absent from the Weberian apparatus of ''S. diasii''. Only the first four vertebrae are involved in the Weberian apparatus of ''Santanichthys''; There are no signs of involvement from the elements of the fifth vertebra unlike in modern otophysans.
An important feature within the formation of the Weberian apparatus, which is a
synapomorphy
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to hav ...
of the Otocephala, is the attachment of the anterior
Pleural cavity
The pleural cavity, pleural space, or interpleural space is the potential space between the pleurae of the pleural sac that surrounds each lung. A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication b ...
(rib) to the
Swim bladder. Another crucial feature is the anterior otophysic diverticula of the swim bladder and contacting the
inner ear, seen in extant
Clupeiformes
Clupeiformes is the order of ray-finned fish that includes the herring family, Clupeidae, and the anchovy family, Engraulidae. The group includes many of the most important forage and food fish.
Clupeiformes are physostomes, which means that ...
. There is also a relationship between the interossicular ligament and the swim bladder that it originated from the swim bladder diverticulum. This was shown by comparing the fiber of the ligament and the tunica externa of the swim bladder that have the same histological composition of elastin and icthyocoll (a specific type I collagen), as established in a research by
Rui Diogo.
Etymology
The Weberian apparatus is named after the
German anatomist and physiologist
Ernst Heinrich Weber (1795 – 1878). The apparatus was first described in detail by Weber. Only four bones were identified, specifically the claustrum, scaphium, intercalarium and tripus. Together, these elements were hypothesized to play a part in auditory functions.
Over the years, other functions have been proposed and discarded.
Hydrostatic regulation was one of the early alternative suggestions for the function of the apparatus.
See also
*
Cyprinidae
References
Bibliography
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{{diversity of fish
Fish anatomy