The lateral line system is not necessary for rheotaxis in the Mexican blind cavefish (Astyanax fasciatus).

Fish resist being swept downstream by swimming against a current. Mexican blind cavefish (Astyanax fasciatus) exhibit this innate behavior, rheotaxis, without the aid of vision, but it has been debated whether this ability requires sensing flow with the lateral line system. We tested the role of the lateral line by comparing swimming in a flow chamber in a group of cavefish with a compromised lateral line with a control group. Consistent with previous studies, we found that cavefish orient toward flow and more frequently swim upstream at a higher flow speed. We found that these responses to flow were indistinguishable between fish with compromised and functioning lateral line systems. Rheotaxis was also unaltered by exposing fish to varying degrees of turbulence. These results suggest that the sensing of flow is unnecessary for rheotaxis in cavefish. It appears that tactile stimuli provide a sufficient means of executing this behavior in fish and that rheotaxis may not be a major function of the lateral line system.

Gentamicin is ototoxic to all hair cells in the fish lateral line system.

Hair cells of the lateral line system in fish may differ in their susceptibility to damage by aminoglycoside antibiotics. Gentamicin has been reported to damage hair cells within canal neuromasts, but not those within superficial neuromasts. This finding, based on SEM imaging, indicates a distinction in the physiology of hair cells between the two classes of neuromast. Studies concerned with the individual roles of canal and superficial neuromasts in behavior have taken advantage of this effect in an attempt to selectively disable canal neuromasts without affecting superficial neuromast function. Here we present an experimental test of the hypothesis that canal neuromasts are more vulnerable to gentamicin than superficial neuromasts. We measured the effect of gentamicin exposure on hair cells using vital stains (DASPEI and FM1-43) in the neuromasts of Mexican blind cave fish (Astyanaxfasciatus) and zebrafish (Daniorerio). Contrary to the findings of prior studies that used SEM, gentamicin significantly reduced dye uptake by hair cells of both canal and superficial neuromasts in both species. Therefore, lateral line hair cells of both neuromast types are vulnerable to gentamicin ototoxicity. These findings argue for a re-evaluation of the results of studies that have used gentamicin to differentiate the roles of the two classes of neuromast in fish behavior.

The morphology and mechanical sensitivity of lateral line receptors in zebrafish larvae (Danio rerio).

The lateral line system of fish and amphibians detects water flow with receptors on the surface of the body. Although differences in the shape of these receptors, called neuromasts, are known to influence their mechanics, it is unclear how neuromast morphology affects the sensitivity of the lateral line system. We examined the functional consequences of morphological variation by measuring the dimensions of superficial neuromasts in zebrafish larvae (Danio rerio) and mathematically modeling their mechanics. These measurements used a novel morphometric technique that recorded landmarks in three dimensions at a microscopic scale. The mathematical model predicted mechanical sensitivity as the ratio of neuromast deflection to flow velocity for a range of stimulus frequencies. These predictions suggest that variation in morphology within this species generates a greater than 30-fold range in the amplitude of sensitivity and more than a 200-fold range of variation in cut-off frequency. Most of this variation was generated by differences in neuromast height that do not correlate with body position. Our results suggest that natural variation in cupular height within a species is capable of generating large differences in their mechanical filtering and dynamic range.