A comparison of the external morphology of the membranous inner ear in elasmobranchs

Abstract

Studies on the elasmobranch inner ear have focused predominantly on a small group of sharks, particularly, carcharhinids. As a result, subsequent studies in other species have subdivided species into two main groups: those typical and those atypical of carcharhinid sharks. This study proposes a different set of inner-ear morphology groupings to those previously suggested. The inner ears from 17 species of elasmobranchs (representing both sharks and rays) are examined in this study and based on morphometric data some groups include both rays and sharks. Four groups are now proposed based predominantly on the shape and dimensions of the membranous otoconial organs, and characteristics of the semicircular canals. Evident morphological differences between the ear types belonging to the new groups include the membranes of the semicircular canals being bound to the otoconial organs in some species, while only being connected via the canal ducts in others, as well as clear variation present in saccular organ size. Previous studies examining variation in the inner ear have attributed differences to either phylogeny or functional significance. Results from this study suggest that neither phylogeny nor feeding strategy solely accounts for the morphological diversity present in the external morphology of the elasmobranch inner ear.

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

“…In sharks (A) and Latimera (B) the saccule and lagena form two sacs, whereas lungfish (C) and non-teleost actinopterygians (D-F) are characterized by a common pouch for these otolith end organs (sacculolagenar pouch). Note, however, that in other species of chondrichthyes (see especially rays and skates and Holocephali; Lisney, 2010), saccule and lagena may be less well-separated (Evangelista et al, 2010). In cartilaginous fishes the endolymphatic duct connects the inner ear to the head surface and thus to the external medium.…”

“…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.…”

“…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). In teleosts, diversity in semicircular canals is restricted to differences in canal thickness and canal radii.…”

“…Large semicircular canals are also present in the angler Lophius piscatorius (Lophiiformes) and the gray gurnard Eutrigla gurnardus (Perciformes; Figure 2E; Retzius, 1881). The functional meaning of these enlarged semicircular canals remains to be studied (see also discussion in Evangelista et al, 2010).…”

“…Most studies on macula morphology in Chondrichthyes focused on the macula neglecta in Elasmobranchii (Corwin, 1981(Corwin, , 1989Myrberg, 2001;Casper, 2011). It therefore remains unclear if cartilaginous fishes show variability not only with regard to gross inner ear morphology (Evangelista et al, 2010) and the macula neglecta but also with regard to the maculae of the otolith end organs. Moreover, to our knowledge data on the macula morphology in Holocephali is completely lacking (see Lisney, 2010).…”