Weberian apparatus
Encyclopedia
The Weberian apparatus is an anatomical structure that connects the swim bladder to the auditory system
in fish
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 to develop in an embryonic ostariophysan. These bones grow to physically connect the auditory system, specifically the inner ear
to the swim bladder.
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 involved elements include:
The supraneural bones of the skull; modified neural arch bones, specifically the paired claustra
and the scaphia
; The intercalarium and the lateral processes; The tripus; The os suspensorium from the fourth vertebrae; The parapophysis of vertebrae #5 including the vertebrae itself, plus the vertebrae's corresponding pleural rib. In addition, a structure composed of fused neural spines form the dorsalmost 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 3rd 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
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 resonating chamber, signals are amplified to noticeable levels.
of Weberian apparati of the taxon Brycon
has shed some light on the development 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 vertebrae (V1). From the second vertebrae (V2), the intercalarum and the vertebrae's lateral process are reduced and clump together. The plural rib (R1) of the third vertebrae (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 vertebrae (V4) somewhat retains its shape, developing from the pleural rib of the vertebrae (R2). The remaining elements of the fifth vertebrae (V5), the parapophysis and the articulating rib (R3), including the vertebrae itself form the posterior structure of the Weberian apparatus. The neural spines 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 results 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 apparati of a few select taxa, including Danio rerio
, Rhaphiodon vulpinus and Corydoras paleatus
.
of Northeastern Brazil
. 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 vertebrae 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 vertebrae unlike in modern otophysans.
The Weberian apparatus is named after the German anatomist and physiologist Ernst Heinrich Weber
(1795 – 1878). Historically, 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 since been proposed and discarded. Hydrostatic regulation was one of the early alternative suggestions for the function of the apparatus.
Inner ear
The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...
in fish
Fish
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...
es belonging to the Superorder Ostariophysi
Ostariophysi
Ostariophysi is the second-largest superorder of fish. Members of this superorder are called ostariophysians. This diverse group contains almost 8,000 species, about 28% of known fish species in the world and 68% of freshwater species, and are present on all major continents except Antarctica...
. 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 to develop in an embryonic ostariophysan. These bones grow to physically connect the auditory system, specifically the inner ear
Inner ear
The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...
to the swim bladder.
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 labyrinthLabyrinth
In Greek mythology, the Labyrinth was an elaborate structure designed and built by the legendary artificer Daedalus for King Minos of Crete at Knossos...
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 involved elements include:
The supraneural bones of the skull; modified neural arch bones, specifically the paired claustra
Claustrum
The claustrum, which is suspected to be present in all mammals, is a fairly thin vertical curved sheet of subcortical gray matter...
and the scaphia
Scaphium
Scaphium is a genus of 6 species of plants in family Sterculiaceae, including* Scaphium longiflorum, Ridl.* Scaphium macropodium, Buem....
; The intercalarium and the lateral processes; The tripus; The os suspensorium from the fourth vertebrae; The parapophysis of vertebrae #5 including the vertebrae itself, plus the vertebrae's corresponding pleural rib. In addition, a structure composed of fused neural spines form the dorsalmost part of the Weberian apparatus. Together, the structure interacts anteriorly with the lagenar otolith
Otolith
An otolith, , also called statoconium or otoconium is a structure in the saccule or utricle of the inner ear, specifically in the vestibular labyrinth of vertebrates. The saccule and utricle, in turn, together make the otolith organs. They are sensitive to gravity and linear acceleration...
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 3rd 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
Gas bladder
The swim bladder, gas bladder, fish maw or air bladder is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming...
, through the Weberian ossicles and then straight into the labyrinth
Labyrinth
In Greek mythology, the Labyrinth was an elaborate structure designed and built by the legendary artificer Daedalus for King Minos of Crete at Knossos...
structures of the inner ear
Inner ear
The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...
. 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 resonating chamber, signals are amplified to noticeable levels.
Embryology
Embryonic analysisOntogeny
Ontogeny is the origin and the development of an organism – for example: from the fertilized egg to mature form. It covers in essence, the study of an organism's lifespan...
of Weberian apparati of the taxon Brycon
Brycon
Brycon is a genus of fish from the family Characidae. Despite being unrelated to true trout, they are sometimes called South American trout. They reach a maximum length of depending on the exact species involved, and the largest are among the largest species in the Characidae family...
has shed some light on the development 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 vertebrae (V1). From the second vertebrae (V2), the intercalarum and the vertebrae's lateral process are reduced and clump together. The plural rib (R1) of the third vertebrae (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 vertebrae (V4) somewhat retains its shape, developing from the pleural rib of the vertebrae (R2). The remaining elements of the fifth vertebrae (V5), the parapophysis and the articulating rib (R3), including the vertebrae itself form the posterior structure of the Weberian apparatus. The neural spines 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 results of which have resulted in various interpretations of the development (and thus the homology
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
) of the structures that form the structure. Specific studies have been done on the Weberian apparati of a few select taxa, including Danio rerio
Danio rerio
The zebrafish, Danio rerio, is a tropical freshwater fish belonging to the minnow family of order Cypriniformes. It is a popular aquarium fish, frequently sold under the trade name zebra danio, and is an important vertebrate model organism in scientific research.-Taxonomy:The zebrafish are...
, Rhaphiodon vulpinus and Corydoras paleatus
Corydoras paleatus
Corydoras paleatus is a species of catfish of the family Callichthyidae. Its common names include peppered cat, pepper cory or salt & pepper cory. It originates from the lower Paraná River basin and coastal rivers in Uruguay and Brazil.-Description:This fish species reaches about...
.
Evolutionary history
The earliest recorded incidence of a Weberian apparatus is from the fossil fish Santanichthys diasii dating from the Early CretaceousEarly Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
of Northeastern Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. 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 vertebrae 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 vertebrae unlike in modern otophysans.
Etymology
Ernst Heinrich Weber
Ernst Heinrich Weber was a German physician who is considered one of the founders of experimental psychology.Weber studied medicine at Wittenberg University...
(1795 – 1878). Historically, 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 since been proposed and discarded. Hydrostatic regulation was one of the early alternative suggestions for the function of the apparatus.