Inner Ear Otolith Asymmetry in Late-Larval Cichlid Fish (Oreochromis mossambicus, Perciformes) Showing Kinetotic Behaviour Under Diminished Gravity

Ralf Anken,Miriam Knie,Reinhard Hilbig

doi:10.1038/s41598-017-15927-z

Ralf Anken, Miriam Knie + Show 1 more

Open Access

https://doi.org/10.1038/s41598-017-15927-z

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Abstract

The inner ears of all vertebrates are designed to perceive auditory and vestibular inputs. Although a tremendous diversity in the inner ear can be found even among bony fishes, the morphologies of the utricle and of the semicircular canals are rather conservative among vertebrates. Fish show kinetoses under reduced gravity (spinning movements and looping responses) and are regarded model organisms concerning the performance of the otolithic organs. Otoliths can be analysed easily because they are compact, in contrast to the otoconial masses of other vertebrates. Here, late-larval Oreochromis mossambicus were subjected to 0.0001 × g and 0.04 × g aboard a sounding rocket, their behaviour was observed and morphometrical analyses on otoliths were carried out. Fish swimming kinetotically at 0.0001 × g had a higher asymmetry of utricular otoliths (gravity perception) but not of saccular otoliths (hearing process) than specimens behaving normally at this gravity level (p = 0.0055). Also, asymmetries of lapilli in animals swimming normally at 0.0001 × g were lower than asymmetries in specimens swimming normally at 0.04 × g (p = 0.06). This supports the “otolith asymmetry hypothesis”, an explanation for the susceptibility to kinetosis, particularly concerning the utricular otoliths. It would be interesting to identify processes generating asymmetric otoliths, also in regard to human motion sickness.

Highlights

There is a tremendous diversity among the inner ears of vertebrates[22] and even among bony fishes[22,23], but some morphological features are rather conservative
Since the occurrence of a kinetosis can be observed in the form of so-called looping responses and spinning movements in fish[25,26,27,28], we have shown earlier using fish as vertebrate model system that such individual animals, which exhibit looping responses and spinning movements at the transfer from 1 × g Earth gravity to diminished gravity during parabolic aircraft flights (0.04 × g, Low Quality Microgravity, LQM) had, statistically, a higher otolith asymmetry as compared to fish swimming normally at experimental conditions[29]
In the course of an experiment under extremely low gravity, i.e., 0.000001 × g (High Quality Microgravity, HQM, ZARM drop-tower at Bremen, Germany), we found out that the ratio of late-larval cichlid fish of a given clutch being susceptible to kinetosis strongly depends on the level of environmental gravity[27]: Kinetosis was observed in more than 90% of the individuals subjected to 0.000001 × g which lead to the conclusion[28], that this level of environmental gravity is not sufficient to be used by the experimental animals as a cue for spatial orientation

Summary

Introduction

There is a tremendous diversity among the inner ears of vertebrates[22] and even among bony fishes[22,23], but some morphological features are rather conservative. Since the occurrence of a kinetosis can be observed in the form of so-called looping responses and spinning movements in fish[25,26,27,28], we have shown earlier using fish (late-larval cichlid fish Oreochromis mossambicus) as vertebrate model system that such individual animals, which exhibit looping responses and spinning movements at the transfer from 1 × g Earth gravity to diminished gravity during parabolic aircraft flights (0.04 × g, Low Quality Microgravity, LQM) had, statistically, a higher otolith asymmetry as compared to fish swimming normally at experimental conditions[29]. Our results clearly support the hypothesis that otolith asymmetry is a major source of kinetosis susceptibility in fish

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