Abstract
An astonishing diversity of inner ears and accessory hearing structures (AHS) that can enhance hearing has evolved in fishes. Inner ears mainly differ in the size of the otolith end organs, the shape and orientation of the sensory epithelia, and the orientation patterns of ciliary bundles of sensory hair cells. Despite our profound morphological knowledge of inner ear variation, two main questions remain widely unanswered. (i) What selective forces and/or constraints led to the evolution of this inner ear diversity? (ii) How is the morphological variability linked to hearing abilities? Improved hearing is mainly based on the ability of many fish species to transmit oscillations of swim bladder walls or other gas-filled bladders to the inner ears. Swim bladders may be linked to the inner ears via a chain of ossicles (in otophysans), anterior extensions (e.g. some cichlids, squirrelfishes), or the gas bladders may touch the inner ears directly (labyrinth fishes). Studies on catfishes and cichlids demonstrate that larger swim bladders and more pronounced linkages to the inner ears positively affect both auditory sensitivities and the detectable frequency range, but lack of a connection does not exclude hearing enhancement. This diversity of auditory structures and hearing abilities is one of the main riddles in fish bioacoustics research. Hearing enhancement might have evolved to facilitate intraspecific acoustic communication. A comparison of sound-producing species, however, indicates that acoustic communication is widespread in taxa lacking AHS. Eco-acoustical constraints are a more likely explanation for the diversity in fish hearing sensitivities. Low ambient noise levels may have facilitated the evolution of AHS, enabling fish to detect low-level abiotic noise and sounds from con- and heterospecifics, including predators and prey. Aquatic habitats differ in ambient noise regimes, and preliminary data indicate that hearing sensitivities of fishes vary accordingly.
Highlights
This review provides an overview of the diversity of fish inner ears and accessory hearing structures as well as auditory sensitivities
Those species which have a large bladder but no connection to the inner ears display intermediate hearing abilities. They can detect frequencies up to 3 kHz, similar to E. maculatus, but the absolute sensitivity is low and similar to S. tinanti. This indicates that the large swim bladder in H. guttatus contributes to high-frequency hearing despite the lack of a direct connection to the inner ears
Fishes have evolved an enormous diversity of inner ears and accessory hearing structures
Summary
The inner ear is the primary hearing organ in vertebrates. Typically, tetrapods (amphibians, reptiles, birds, mammals) developed thin membranes on the body surface laterally of the inner ears (tympana or eardrums) to pick up sound pressure changes in the air and transmit these pressure fluctuations via 1-3 tiny auditory ossicles to the inner ear fluids (Ladich, 2010). These calcareous structures (otoconia and/or otoliths) lag behind in movement relative to the fish in the sound field and thereby stimulate the sensory hair cells by deflecting their ciliary bundles This physically different process, namely detecting the movement of a tiny calcareous stone, means that fish are unable to detect sound pressure but particle motion instead. Air-filled cavities within the body such as swim bladders or organs for air-breathing undergo volume changes because air is much more compressible than fluids in any sound field These volume fluctuations will result in oscillations of the walls, which function similar to tympana as soon as these membranes transmit their oscillations to the inner ears and improve hearing sensitivities (Alexander, 1966). It is quite safe to assume that several taxa of modern bony fishes (teleosts) evolved structures which serve only to connect given gas-filled cavities to the inner ears mechanically (e.g., Weberian ossicles)
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