TheWeberian apparatus is an anatomical structure that connects theswim bladder to theauditory system infishes belonging to the superorderOstariophysi. When it is fully developed in adult fish, the elements of the apparatus are sometimes collectively referred to as theWeberian ossicles orWeber's 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 fewvertebrae to develop in an embryonic ostariophysan. These bones grow to physically connect the auditory system, specifically theinner ear, to the swim bladder.[1][2] The structure acts as an amplifier of sound waves that would otherwise be only slightly perceivable by the inner ear structure alone.
The generalized structure of the Weberian apparatus is akin to a skeletal complex of bones and ossicles that are physically connected to thelabyrinth auditory complex anteriorly and the anteriormost region of the swim bladder posteriorly. The entire structure is derived from skeletal elements of the first fourvertebrae. The involved elements include: the supraneural bones of the skull; modifiedneural arch bones, specifically the pairedclaustra and thescaphia; the intercalarium and the lateral processes; the tripus; the os suspensorium from the fourth vertebra; theparapophysis of vertebra number five including the vertebra itself, plus the vertebra's corresponding pleural rib. In addition, a structure composed of fusedneural spines form the dorsal most part of the Weberian apparatus. Together, the structure interacts anteriorly with thelagenar 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 theswim bladder.[1]
The Weberian apparatus functions by transmitting auditory signals straight from thegas bladder, through the Weberian ossicles and then straight into thelabyrinth structures of theinner 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 aResonance chamber, signals are amplified to noticeable levels.[2]
Embryonic analysis of Weberian apparatuses of the taxonBrycon has shed some light on thedevelopment 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. Theos 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. Theneural spines of the first four vertebrae fuse and compress, forming one of the major structures of the apparatus.[1][2]
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 thehomology) of the structures that form the structure. Specific studies have been done on the Weberian apparatuses of a few select taxa, includingDanio rerio,[3]Rhaphiodon vulpinus[4] andCorydoras paleatus.[5]
The earliest recorded incidence of a Weberian apparatus is from the fossil fishSantanichthys diasii dating from theEarly Cretaceous of NortheasternBrazil. 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 ofS. diasii. Only the first four vertebrae are involved in the Weberian apparatus ofSantanichthys; There are no signs of involvement from the elements of the fifth vertebra unlike in modern otophysans.[6] An important feature within the formation of the Weberian apparatus, which is asynapomorphy of the Otocephala, is the attachment of the anteriorPleural cavity (rib) to theSwim bladder. Another crucial feature is the anterior otophysic diverticula of the swim bladder and contacting theinner ear, seen in extantClupeiformes. 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 byRui Diogo.[7]
The Weberian apparatus is named after theGerman anatomist and physiologistErnst 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.[8] 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.[9]
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