Otolith regularities.

The masses and the area sizes of the otoliths for the utriculus, sacculus and lagena of 15 species of the Black Sea fish are analyzed. Morphometrical otolith regularities are derived and their functional and ecomorphological explanations are suggested. The otolith regularities are summarized in four otolith rules: (1) the masses of the otoliths gradually increase with the fish growth. (2) The mass ratio of the sacculus and utriculus or the sacculus and lagena otoliths does not change with the fish growth. (3) The ratio between the otolith area s and the otolith mass m is described by the exponential equation s=alpham(2/3). (4) The ratio between the otolith and macula sizes does not change with fish growth. Mathematical modeling of the otolith displacement responses to the acoustic and the instant force stimuli is performed. Based on the modeling the functional and ecomorphological explanations of the otolith regularities are suggested: (1) the greater the otolith mass, the higher the acoustic sensitivity at low frequencies and the sharper the frequency-response curve at its maximum. (2) The separation between maxima of the frequency-response curves for the saccular and lagenar otoliths remains virtually constant with the fish growth. (3) The bottom and littoral fish have better auditory capabilities than the pelagic fish. (4) The sensitivity to vestibular stimuli for greater otoliths is higher but the response is slower. The corresponding acceleration resolution for greater otoliths is higher and the range of accelerations in which the otolith organ can operate is narrower. (5) The relative vestibular sensitivities of the utriculus, sacculus and lagena otolith organs remain constant with fish growth. (6) The otolith organs of the bottom and littoral fish are tuned to different accelerations and possess different functional properties. The otolith organs of pelagic fish are adapted to a limited range of accelerations and are less sensitive to low accelerations as compared to the bottom and littoral fish.