Subterranean Fishes of the World

4469

2019

Journal of Comparative Physiology A

Both the lateral line and the inner ear contribute to near-field dipole source detection in fish. The precise roles these two sensory modalities provide in extracting information about the flow field remain of interest. In this study, evoked potentials (EP, 30–200 Hz) for blind Mexican cavefish were measured in response to a dipole source. Greatest sensitivity was observed at the lower and upper ends of the tested frequency range. To evaluate the relative contributions of the lateral line and inner ear, we measured the effects of neomycin on EP response characteristics at 40 Hz, and used the vital dye DASPEI to verify neuromast ablation. Neomycin increased the latency of the EP response up until 60 min post-treatment. DASPEI results confirmed that neuromast hair cell death was significant in treated fish over this timeframe. These results indicate that the inner ear, whether it is sound pressure or particle motion detection, makes a significant contribution to the dipole-induced EP in blind cavefish at near-field low frequencies where the lateral line contribution would be expected to be strongest. The results from this study imply that under some circumstances, lateral line function could be complemented by the inner ear. Keywords Evoked potentials · Fish senses · Dipole stimulus · Aminoglycoside antibiotics Introduction Complex water movement and pressure waves caused by abiotic and biotic factors, such as currents, swimming motions, surface waves, and sound can offer aquatic vertebrates a wealth of information about their environment (Engelmann et al. 2000). Fish have developed a variety of mechanosensory systems associated with the lateral line and inner ear to detect hydrodynamic and acoustic stimuli. These sensory modalities are based on the mechano-electrical transduction capabilities of the hair cell. Morphologically, the hair cell structure is quite similar across these systems and it is the difference in the way which the hair cell sensors physically couple with the outside world that determines their response and what they detect (Coombs and Montgomery 2005; Webb et al. 2008). The lateral line system is composed of rows of individual organs called neuromasts located either on the surface of the skin (superficial) or sub-dermally (canal). Superficial neuromasts (SN) are most sensitive to the unidirectional and low-frequency flow of water across the body of the fish (< 70 Hz; e.g., < 50 Hz Weeg and Bass (2002); < 36 Hz Kroese and Schellart (1992); < 70 Hz Munz (1985)), while the canal-embedded neuromasts (CN) can detect minute higher frequency signals (< 200 Hz; e.g., < 93 Hz Kroese and Schellart (1992); < 200 Hz Munz (1985)) by filtering out the effects of low-frequency flow disturbance (Engelmann et al. 2000). The subject of this study is the Mexican blind cavefish, Astyanax mexicanus (De Filippi 1853), that has been shown to have relatively low-frequency receptors (SN 10 Hz; CNs 50 Hz; Montgomery et al. (2001)) and behavioural attraction to a vibrating stimulus at 35 Hz (Yoshizawa et al. 2014). By comparison, the hair cells of the inner ear organs are primarily sensitive to angular and linear accelerations of the whole fish (Popper and Fay 2011). This is because they are clustered into sensory macula deep within the end organs that make up the semi-circular canals and otolith (saculae, utriculae, and lagenae) organs of the inner ear. It is saculae and utriculae otolithic organs that are of most relevance to the direct detection of motion of the fish driven by an acoustic source. Within the otolith chamber, a single dense calcareous otolith is positioned on a sensory * Craig A. Radford c.radford@auckand.ac.nz 1 Leigh Marine Laboratory, Institute of Marine Science, University of

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