Remarks on the inner ear of elasmobranchs and its interpretation from skeletal labyrinth morphology

Abstract

The structure and function of the craniate inner ear is reviewed, with 33 apomorphic characters of the membranous labyrinth and associated structures identified in craniates, gnathostomes, and elasmobranchs. Elasmobranchs are capable of low-frequency semi-directional phonoreception, even in the absence of any pressure-to-displacement transducer such as ear ossicles. The endolymphatic (parietal) fossa, semicircular canals, and crista (macula) neglecta are all adapted toward phonoreception. Some (but not all) of the morphological features associated with phonoreception can be inferred from the elasmobranch skeletal labyrinth. Endocranial spaces such as the skeletal labyrinth also provide suites of morphological characters that may be incorporated into phylogenetic analyses, irrespective of how closely these spaces reflect underlying soft anatomy. The skeletal labyrinths of Squalus and Notorynchus are compared using silicone endocasts and high-resolution CT-scanning. The latter procedure offers several advantages over other techniques; it is more informative, nondestructive, preserves relationships of surrounding structures, and it can be applied both to modern and fossil material.

Diversity in Fish Auditory Systems: One of the Riddles of Sensory Biology

“…An endolymphatic duct is present in all fishes (Maisey, 2001). In cartilaginous fishes ( Figure 1A) this duct is connected to the surface of the head via a small pore (endolymphatic pore), whereas it ends blindly and may be widely reduced in bony fishes (Maisey, 2001;Lisney, 2010).…”

“…Similar to the canal cristae, the macula neglecta is overlain by a gelatinous cupula and lacks otoconia or an overlying otolith. The term crista neglecta instead of macula neglecta was therefore suggested by some authors (see Maisey, 2001).…”

“…Also the amount of encapsulation and attachment of the ear to the skull differs considerably. Certain teleosts such as poeciliids show rather "free" ears with encapsulation limited to the semicircular canals (Schulz-Mirbach et al, 2011), whereas the non-teleost actinopterygian Amia calva (Popper and Northcutt, 1983) or elasmobranchs display almost full encapsulation of the ears (Maisey, 2001). In other species, attachment of one or several otolith end organs to the skull is associated with the presence of accessory hearing structures as found in the notopterid Chitala chitala, the morid Antimora rostrata, or the cichlid Etroplus maculatus Deng et al, 2011;SchulzMirbach et al, 2013).…”

“…Unlike in Holocephali and bony fishes, Elasmobranchii do not have a connection between the anterior and posterior semicircular canals (common crus); instead the anterior and horizontal canals are connected to each other via a crus ( Figure 1A; Maisey, 2001;Evangelista et al, 2010). According to Maisey (2001) the posterior canal in elasmobranchs is thus rather a circuit than a semicircular canal.…”

“…Unlike in Holocephali and bony fishes, Elasmobranchii do not have a connection between the anterior and posterior semicircular canals (common crus); instead the anterior and horizontal canals are connected to each other via a crus ( Figure 1A; Maisey, 2001;Evangelista et al, 2010). According to Maisey (2001) the posterior canal in elasmobranchs is thus rather a circuit than a semicircular canal. Within this group, species display variability in the presence/absence of the canal ducts that connect the semicircular canals to the otolith end organs and thus may differ in whether the semicircular canals are directly connected to the saccule, in the length of the endolymphatic duct, and in the size of the saccule with respect to the utricle (Evangelista et al, 2010).…”