On the origin of susceptibility to kinetotic swimming behaviour in fish: a parabolic aircraft flight study

Humans taking part in parabolic aircraft flights (PAFs) may suffer from motion sickness (SMS, a kinetosis; it comprises a dynamic and a static component). It has been argued that the so-called static variety of SMS during PAFs might be based on asymmetric statoliths (i.e., differently weighed statoliths on the right and the left side of the head), with asymmetric inputs to the brain being disclosed in microgravity. Since it has been repeatedly shown earlier that some fish of a given batch reveal a kinetotic behaviour during PAFs (especially so-called spinning movements and looping responses), we investigated whether fish swimming kinetotically in microgravity have a pronounced inner ear otolith asymmetry. Therefore, the swimming behaviour of larval cichlid fish was video-recorded during PAFs and subsequently, size and asymmetry (size difference between the left and the right side) of inner ear otoliths were determined. The asymmetry of utricular otoliths of kinetotic samples was found to be significantly higher than that of normally behaving experimental specimens. Regarding the asymmetry of saccular otoliths of the two groups, statistically different results were not obtained. The findings strongly support the earlier theoretical concept, according to which otolith asymmetry causes (static) SMS.